# Waypoint error - is it additive?

## Recommended Posts

Who's point are you talking about? The point in the example given in the OP is that the errors ARE exactly 20 feet.

No, it wasn't. The OP says " If both GPSrs had an error of plus or minus 20 ft ". The GPSr cannot tell you how far off the real GZ you are, it can only tell you the EPE. The statement in the OP says that the GPSrs were showing an EPE of 20 feet.

I think you're confusing actual error with statistical or estimated error. Neither the hider nor the seeker can possibly know (not from their handhelds anyway) how far off they really are (at any given time - coordinate readings can fluctuate quite a bit in only a short time), which would be required for any calculations regarding the actual error. If you're still talking actual error though, then you simply have two sets of coordinates which are 40 feet from each other, so yeah, in that case the error is 40 feet. Kind of a no-brainer and it has no relevance to GPS usage.

Edited by dfx
Who's point are you talking about? The point in the example given in the OP is that the errors ARE exactly 20 feet.
No, it wasn't. The OP says " If both GPSrs had an error of plus or minus 20 ft ".
As the person that asked the question, I can tell you what the point was. You'll just have to trust me on that one. The quote you took out of context was about the way it was described to me when I started caching. Later on in my post, in order to ask my question, I went to the trouble to draw a diagram and define exactly what "worst case" was. In my definition of worst case, I said "20 feet", not "plus or minus 20 feet". The question that WAS asked in the OP was about what happens in that kind of circumstance. It had nothing to do with the way that EPE works, or how you should look for a cache when near the coordinates.

The GPSr cannot tell you how far off the real GZ you are, it can only tell you the EPE.
Off topic.

The statement in the OP says that the GPSrs were showing an EPE of 20 feet.
No, it did not.

I think you're confusing actual error with statistical or estimated error. Neither the hider nor the seeker can possibly know (not from their handhelds anyway) how far off they really are (at any given time - coordinate readings can fluctuate quite a bit in only a short time), which would be required for any calculations regarding the actual error.
I think you're confusing the actual question asked with a question that you thought was asked, but wasn't. As I said before, that's probably my fault for not being more clear. And I'm sorry for the confusion. But to continue to insist that I was asking the EPE question when I've said I wasn't doesn't make sense.

If you're still talking actual error though, then you simply have two sets of coordinates which are 40 feet from each other, so yeah, in that case the error is 40 feet. Kind of a no-brainer and it has no relevance to GPS usage.
I thought it was a no brainer too, until I was told otherwise. As I mentioned in the OP I read in the forums that someone claimed this no brainer wasn't true at all but didn't back it up with an explanation. I started this thread to try and find the explanation. So far I've found none, and you and I agree about the answer to the question I DID ask.

Most people's eyes will glaze over when you mention statistics, and if you have to do some trigonometry as well, then you might as well forget about trying to explain it.

The point is that there is no such thing as a hider taking a reading that is 20 ft off and then the finder's GPSr being another 20 ft off in the same direction. In the whole history of geocaching this has happened maybe 10 times.* What happens just as often is that the hider was 28 ft off and the finder was 17 feet off in a direction 56 degrees from the direction of the hider's error. Just think about all the ways the total error can come out to 40 ft. The likelihood that both GPSrs are 20 ft off in the same direction is negligable. However, the story is a simple one for a newbie to picture. It involves no math other than addition. So we tell the story to newbies and they start to search a wider area when their GPSr show they are close to the posted coordinates. They find more caches and they are less likely to complain about "bad" coordinates. You don't have to understand statistics or trigonometry to go geocaching. You don't have to understand how GPS works and what the sources of error are. You do need to understand that the cache may not be exactly where your GPSr zeros out and that sometimes it can be 40 ft away.

*92.8% of statistics are made up. Just trying to point out this is very rare.

Most people's eyes will glaze over when you mention statistics, and if you have to do some trigonometry as well, then you might as well forget about trying to explain it.

The point is that there is no such thing as a hider taking a reading that is 20 ft off and then the finder's GPSr being another 20 ft off in the same direction. In the whole history of geocaching this has happened maybe 10 times.* What happens just as often is that the hider was 28 ft off and the finder was 17 feet off in a direction 56 degrees from the direction of the hider's error. Just think about all the ways the total error can come out to 40 ft. The likelihood that both GPSrs are 20 ft off in the same direction is negligable. However, the story is a simple one for a newbie to picture. It involves no math other than addition. So we tell the story to newbies and they start to search a wider area when their GPSr show they are close to the posted coordinates. They find more caches and they are less likely to complain about "bad" coordinates. You don't have to understand statistics or trigonometry to go geocaching. You don't have to understand how GPS works and what the sources of error are. You do need to understand that the cache may not be exactly where your GPSr zeros out and that sometimes it can be 40 ft away.

*92.8% of statistics are made up. Just trying to point out this is very rare.

Et tu Toz?

The numbers in my example were just given as an example. You gave a very similar example in post number 3, and in fact you used the same numbers. However, you did the same thing in that post that you're doing now, and that dfx and others have done. You're trying to explain how numbers that are provided as givens in the question might not be really true.

If someone says they like chocolate ice cream and asks you how it's made, why would you tell them that most people like vanilla, that the chances of someone liking chocolate are less likely, and therefore explain to them how to make vanilla instead?

You've just got to accept that the GPSrs in the question are reporting the incorrect positions stated in order for me to ask the question.

If I really want to understand the science of EPE I would (and did. The additional information was definitely educational, even if it didn't address the question asked). But the science of EPE isn't what was asked.

Let me rephrase my comments, since you seem to have missed my point.

Yes, what was asked was "Given that the hider is 20 feet off, and the finder is 20 feet off, is the worst case that the finder is looking 40 feet away from where the cache is hidden."

My answer stands: yes, given what you have given, your conclusion is correct. But it's a stupid question, since the hider is never exactly 20 feet off, and the finder is never exactly 20 feet off, and if they were, it's highly unlikely that they would both be 20 feet off in exactly opposite directions.

An answer can be correct and still be absolutely useless. The 40 foot error is an example of that. You are starting with a ridiculous set of premises to reach a stupid conclusion.

I thought it was a no brainer too, until I was told otherwise. As I mentioned in the OP I read in the forums that someone claimed this no brainer wasn't true at all but didn't back it up with an explanation. I started this thread to try and find the explanation.

That's because your question isn't relevant to geocaching or GPS usage in general, nor does it match the forum post you came across, which does deal with GPS usage.

The forum post you mention answers the valid geocaching question: if the hider's GPS showed an accuracy of 20 feet when he took the coordinates (or alternatively, if the posted cache coordinates are actually 20 feet away from the actual GZ), and my GPS shows an accuracy of 20 feet when I try to find the cache, should I expect to find the cache up to 40 feet away? The answer is no, and this is what everybody has been trying to explain here.

Your question is something totally different and has no relevance to geocaching. Let me give you a real-world example:

Hider goes and puts the cache in place, then takes the coordinates. His coordinates end up 20 feet away from the actual location. The EPE shown on his GPS is irrelevant because the coordinates are what they are, and in this example they're 20 feet off.

Now seeker goes and tries to find the cache. He follows his GPS around until it shows him a distance of 0 feet. At this point the GPS also shows him an accuracy of 20 feet. He knows (or rather, assumes) that the hider's coordinates could be up to 20 feet off. Should he now expect the cache to be up to 40 feet away from his current location? Again, the answer is no, because of this: now he stands there for a minute and then checks his GPS again, and now it shows him a distance of 10 feet to GZ. Again he follows the GPS around until it shows him 0 feet. Lather rinse repeat. At some point he may end up in a spot exactly 40 feet away from the cache, but most of the time he will be closer than that, and occasionally might be further away too.

This is the "waypoint error" that geocachers deal with, it's also what the forum post you came across tried to explain, and it's why this kind of waypoint error is not additive. If you simply have location A, then location B 20 feet in one direction and then location C 20 feet in the opposite direction, then yeah, B and C will be 40 feet apart. Duh. That's got nothing to do with GPS (in)accuracies though.

Edited by dfx
If I really want to understand the science of EPE I would (and did. The additional information was definitely educational, even if it didn't address the question asked). But the science of EPE isn't what was asked.
Let me rephrase my comments, since you seem to have missed my point.

Yes, what was asked was "Given that the hider is 20 feet off, and the finder is 20 feet off, is the worst case that the finder is looking 40 feet away from where the cache is hidden."

My answer stands: yes, given what you have given, your conclusion is correct. But it's a stupid question, since the hider is never exactly 20 feet off, and the finder is never exactly 20 feet off, and if they were, it's highly unlikely that they would both be 20 feet off in exactly opposite directions.

An answer can be correct and still be absolutely useless. The 40 foot error is an example of that. You are starting with a ridiculous set of premises to reach a stupid conclusion.

What is more stupid? A post that asks a question that is a complete no brainer, just to verify that there's not something else missing in the way things work... or someone that keeps posting to that thread the same answer a question that was never asked, and then insisting that the original question was stupid?

I'm finding out that there may or may not be stupid questions, but there are most definitely stupid answers.

Your OP says "some time in the past someone in the forums stated that this just wasn't true but they didn't explain why at the time. " You don't actually point out who said your explanation wasn't true. I believe what you read in the past were explanations much like those given here, that if two GPSr were each 20 ft. off, while the maximum error might be 20, the average error would be much less. For your example the maximum error would be 40 feet.

The rest of this thread has been an explanation of why caches are sometimes found 40 feet away from where you zero out, but the true reason can not be explained by the simple story we tell newbies. A better discussion might be whether this is a valid reason to abandoned the story. I think the story is told because it is simple for anyone to understand (even if it isn't true) and therefore is a reasonable way to get people to start looking in a bigger search area and becoming more successful at finding caches.

I thought it was a no brainer too, until I was told otherwise. As I mentioned in the OP I read in the forums that someone claimed this no brainer wasn't true at all but didn't back it up with an explanation. I started this thread to try and find the explanation.
That's because your question isn't relevant to geocaching or GPS usage in general, nor does it match the forum post you came across, which does deal with GPS usage.

The forum post you mention answers the valid geocaching question: if the hider's GPS showed an accuracy of 20 feet when he took the coordinates (or alternatively, if the posted cache coordinates are actually 20 feet away from the actual GZ), and my GPS shows an accuracy of 20 feet when I try to find the cache, should I expect to find the cache up to 40 feet away? The answer is no, and this is what everybody has been trying to explain here.

Your question is something totally different and has no relevance to geocaching. Let me give you a real-world example:

Hider goes and puts the cache in place, then takes the coordinates. His coordinates end up 20 feet away from the actual location. The EPE shown on his GPS is irrelevant because the coordinates are what they are, and in this example they're 20 feet off.

Now seeker goes and tries to find the cache. He follows his GPS around until it shows him a distance of 0 feet. At this point the GPS also shows him an accuracy of 20 feet. He knows (or rather, assumes) that the hider's coordinates could be up to 20 feet off. Should he now expect the cache to be up to 40 feet away from his current location? Again, the answer is no, because of this: now he stands there for a minute and then checks his GPS again, and now it shows him a distance of 10 feet to GZ. Again he follows the GPS around until it shows him 0 feet. Lather rinse repeat. At some point he may end up in a spot exactly 40 feet away from the cache, but most of the time he will be closer than that, and occasionally might be further away too.

This is the "waypoint error" that geocachers deal with, it's also what the forum post you came across tried to explain, and it's why this kind of waypoint error is not additive.

Wow, you've found the thread that I'm referencing in my OP? I couldn't find it, maybe you could post a link. I'd love to see it. I didn't link to it, so how do you know which one I am remembering? And if it does deal with GPS usage, big deal. Whatever it said, it led me to ask the question I DID ask.

The underlined question above is also NOT the question I asked. There's a HUGE difference between "Should he expect it to be" and "In worst case is it possible"!!!

If you simply have location A, then location B 20 feet in one direction and then location C 20 feet in the opposite direction, then yeah, B and C will be 40 feet apart. Duh.
YES!!! Ding ding ding!!! You're finally answering the question that I did ask. And I agree with your answer. I agreed with it from the start, I just wanted to ask and see if there was somehow something counter intuitive that I didn't know about. Duh.

That's got nothing to do with GPS (in)accuracies though.
It has nothing to do with EPE. It has everything to do with 2 GPSrs giving 2 inaccurate readings.

I think you're confusing the actual question asked with a question that you thought was asked, but wasn't.

If that's true, then it's easy to understand how this confusion occurred. Here's your original statement:

If both GPSrs had an error of plus or minus 20 ft, then when searching for a cache your GPS might tell you you're at GZ but you might be anywhere from GZ to 40 feet away.

If you were assuming both GPSr's had actual errors that were exactly 20 feet off, then you should have worded it that way. Instead, you described their errors as "plus or minus 20 ft." Not exactly 20 feet, but rather +/- 20 feet. Describing the error in this fashion generally implies that it is an estimated error, as in the actual error will be between 0 and 20 feet a certain percentage of the time (e.g., 90 percent).

It has nothing to do with EPE. It has everything to do with 2 GPSrs giving 2 inaccurate readings.

No it doesn't. It's a tautalogy: 20 feet plus 20 feet equals 40 feet. It's so obvious that there is no question there. If you introduce 2 GPS receivers into the problem, and want to know how far apart the readings are, then it has everything to do with EPE.

I think the reason people keep answering a question you didn't ask, is because the question you did ask, has nothing to do with geocaching. Your original question boiled down to "Is 20 plus 20 equal to 40?" Since the answer to that question is obvious, people assume you must have meant something else.

Your OP says "some time in the past someone in the forums stated that this just wasn't true but they didn't explain why at the time. " You don't actually point out who said your explanation wasn't true. I believe what you read in the past were explanations much like those given here, that if two GPSr were each 20 ft. off, while the maximum error might be 20, the average error would be much less. For your example the maximum error would be 40 feet.
I don't know which thread I read. And I admit I could be remembering it wrong, but I wanted clarity on the subject so I asked the question I thought I remembered.

The rest of this thread has been an explanation of why caches are sometimes found 40 feet away from where you zero out, but the true reason can not be explained by the simple story we tell newbies. A better discussion might be whether this is a valid reason to abandoned the story. I think the story is told because it is simple for anyone to understand (even if it isn't true) and therefore is a reasonable way to get people to start looking in a bigger search area and becoming more successful at finding caches.
Correct. Many posters answered a question they thought I asked, and the information was good to read. I learned a few things. But I tried to keep steering the thread back on topic and explain what my actual question was. However, people kept steering it back off and telling me what I really must have meant to ask. I guess I can understand the first time this happened. Perhaps they were trying to clarify with me. But after the 3rd and 4th time I restated the intent of the thread, for people to continue to insist that I really was asking about EPE js mind boggling.
I think you're confusing the actual question asked with a question that you thought was asked, but wasn't.
If that's true, then it's easy to understand how this confusion occurred. Here's your original statement:
If both GPSrs had an error of plus or minus 20 ft, then when searching for a cache your GPS might tell you you're at GZ but you might be anywhere from GZ to 40 feet away.
If you were assuming both GPSr's had actual errors that were exactly 20 feet off, then you should have worded it that way. Instead, you described their errors as "plus or minus 20 ft." Not exactly 20 feet, but rather +/- 20 feet. Describing the error in this fashion generally implies that it is an estimated error, as in the actual error will be between 0 and 20 feet a certain percentage of the time (e.g., 90 percent).

The original statement you quoted was background of what I was told when I first started. The last paragraph in the OP contains the question that does NOT have anything to do with EPE.

It has nothing to do with EPE. It has everything to do with 2 GPSrs giving 2 inaccurate readings.
No it doesn't. It's a tautalogy: 20 feet plus 20 feet equals 40 feet. It's so obvious that there is no question there. If you introduce 2 GPS receivers into the problem, and want to know how far apart the readings are, then it has everything to do with EPE.

I think the reason people keep answering a question you didn't ask, is because the question you did ask, has nothing to do with geocaching. Your original question boiled down to "Is 20 plus 20 equal to 40?" Since the answer to that question is obvious, people assume you must have meant something else.

It seemed obvious to me too. It seemed like a no brainer. 20 plus 20 equals 40. So therefore the cache could be anywhere from 0 to 40 feet away.

Something else I read one day led me to believe that this no brainer, obvious as heck answer, might not be true. So I thought I'd ask the question here and verify that the idea I had in my head about how it works is correct or not. Lots of people agree that the no brainer, obvious as heck answer, is correct. And I thank you for being one of them.

Lots of other people keep answering a question about EPE. I thank them too, as I've learned a few things. But I also point out to them that this information does not address the no brainer, obvious as heck, question.

If you'd like to tell me again the answer to the EPE question, or tell me how useless my question was to begin with, you're welcome to. But both have been done several times already.

By the way, have you ever added resistor values when they're wired in parallel? 20 + 20 doesn't equal 40 then. I didn't want to assume I knew everything about how error values added, so I asked the question just in case.

The original statement you quoted was background of what I was told when I first started. The last paragraph in the OP contains the question that does NOT have anything to do with EPE.

OK, now I think you're just screwing around with us.

Is what I've explained above true for worst case error in a pair of GPS readings?

Long answer: All of the stuff posted above about EPE's, and statistics.

There's just no way to answer your question without getting in to all that stuff. I don't understand why that seems to irritate you so badly. Everyone is posting relevant, helpful answers to the question you actually asked, as opposed to the question you seem to think you have asked.

I suppose I'm as dumb as Mushtang, because I thought he'd asked the question that he also thought he'd asked!

Edited by Happy Humphrey
Wow, you've found the thread that I'm referencing in my OP? I couldn't find it, maybe you could post a link. I'd love to see it. I didn't link to it, so how do you know which one I am remembering? And if it does deal with GPS usage, big deal. Whatever it said, it led me to ask the question I DID ask.

The one thread, or rather post, that I remember that I think you came across is this: http://forums.Ground...1

(took me some searching to find)

I agreed with it from the start, I just wanted to ask and see if there was somehow something counter intuitive that I didn't know about.

The counter-intuitive part that you're missing is that it's not how it works in real life. Your example from the OP is purely academic, purely theoretical and purely mathematical and doesn't help to understand GPS inaccuracies, thus has no real-life value.

It seemed obvious to me too. It seemed like a no brainer. 20 plus 20 equals 40. So therefore the cache could be anywhere from 0 to 40 feet away.

But that's exactly what it doesn't mean. If the cache is 20 feet off the coordinates and some other set of coordinates is 20 feet off in the other direction, then the distance is 40 feet. But you say "could be between 0 and 40", so that's not what you're asking. Even if you don't want to admit it, at least one of the "20" in your equation is the EPE, and that's why your conclusion is not correct. If it wasn't the EPE, then the distance would be 40 feet, period. No "could be" and no "up to".

Edited by dfx
I think you're confusing the actual question asked with a question that you thought was asked, but wasn't.

If that's true, then it's easy to understand how this confusion occurred. Here's your original statement:

If both GPSrs had an error of plus or minus 20 ft, then when searching for a cache your GPS might tell you you're at GZ but you might be anywhere from GZ to 40 feet away.

If you were assuming both GPSr's had actual errors that were exactly 20 feet off, then you should have worded it that way. Instead, you described their errors as "plus or minus 20 ft." Not exactly 20 feet, but rather +/- 20 feet. Describing the error in this fashion generally implies that it is an estimated error, as in the actual error will be between 0 and 20 feet a certain percentage of the time (e.g., 90 percent).

The original statement you quoted was background of what I was told when I first started. The last paragraph in the OP contains the question that does NOT have anything to do with EPE.

The explanation you received when you first started was incorrect but for understandable reasons. The person oversimplified things to help you understand that the actual distance to the hidden cache might be farther than the estimated error displayed on your GPSr.

You tried to prove the explanation you had received when you started was true. You did this by misinterpreting his +/- 20 feet to mean exactly 20 feet. If you do this, then the 40-foot worst case scenario is indeed true. Why you would do this is a mystery to me.

The original statement you quoted was background of what I was told when I first started. The last paragraph in the OP contains the question that does NOT have anything to do with EPE.
OK, now I think you're just screwing around with us.

Is what I've explained above true for worst case error in a pair of GPS readings?

Long answer: All of the stuff posted above about EPE's, and statistics.

There's just no way to answer your question without getting in to all that stuff. I don't understand why that seems to irritate you so badly. Everyone is posting relevant, helpful answers to the question you actually asked, as opposed to the question you seem to think you have asked.

Fair enough. I've stated several times that I probably did a bad job of explaining myself and so I've restated the question in hopes of clearing up what was, and was not, asked.

But I'll dumb it down as much as possible so that you can understand it -

Here's the original post:

• 1st Paragraph: Once upon a time I was told some stuff about how GPS errors could possibly add.
• Picture: Showing some circles and points that are labeled.
• 2nd Paragraph: Summary of what I thought someone in the forums once said was not true at all.
• 3rd Parapragh: Asking, is the stuff outlined in the 2nd paragraph true or not?

Some people misunderstood, and assumed I was asking about EPE. I guess I can understand why.

I explain, no, ignore EPE, I'm asking strictly about readings being off by whatever amount, do these amounts add together? It seems obvious, but I just wanted to check.

And then lots of back and forth - you're asking about EPE, no I'm not, yes you clearly are, but I'm not, you must be, I'm not, if not your question is stupid, okay then it's stupid but I'm not, in that case I still think you're asking about EPE, I swear I'm not....

It seemed obvious to me too. It seemed like a no brainer. 20 plus 20 equals 40. So therefore the cache could be anywhere from 0 to 40 feet away.

But that's exactly what it doesn't mean. If the cache is 20 feet off the coordinates and some other set of coordinates is 20 feet off in the other direction, then the distance is 40 feet. But you say "could be between 0 and 40", so that's not what you're asking. Even if you don't want to admit it, at least one of the "20" in your equation is the EPE, and that's why your conclusion is not correct. If it wasn't the EPE, then the distance would be 40 feet, period. No "could be" and no "up to".

Whoops, my bad. The quote of mine above was not a correct interpretation on my part of my original question. I told you I was bad at communication in here.

You are correct, that 20 plus 20 is not "anywhere from 0 to 40". I said that it was on accident trying to reply too fast. Let me try again.

It seemed obvious to me too. It seemed like a no brainer. 20 plus 20 equals 40. So therefore the cache would be 40 feet away because in the worst case the two errors added.

The original statement you quoted was background of what I was told when I first started. The last paragraph in the OP contains the question that does NOT have anything to do with EPE.
OK, now I think you're just screwing around with us.

Is what I've explained above true for worst case error in a pair of GPS readings?

Long answer: All of the stuff posted above about EPE's, and statistics.

There's just no way to answer your question without getting in to all that stuff. I don't understand why that seems to irritate you so badly. Everyone is posting relevant, helpful answers to the question you actually asked, as opposed to the question you seem to think you have asked.

Fair enough. I've stated several times that I probably did a bad job of explaining myself and so I've restated the question in hopes of clearing up what was, and was not, asked.

But I'll dumb it down as much as possible so that you can understand it -

Here's the original post:

• 1st Paragraph: Once upon a time I was told some stuff about how GPS errors could possibly add.
• Picture: Showing some circles and points that are labeled.
• 2nd Paragraph: Summary of what I thought someone in the forums once said was not true at all.
• 3rd Parapragh: Asking, is the stuff outlined in the 2nd paragraph true or not?

Some people misunderstood, and assumed I was asking about EPE. I guess I can understand why.

I explain, no, ignore EPE, I'm asking strictly about readings being off by whatever amount, do these amounts add together? It seems obvious, but I just wanted to check.

And then lots of back and forth - you're asking about EPE, no I'm not, yes you clearly are, but I'm not, you must be, I'm not, if not your question is stupid, okay then it's stupid but I'm not, in that case I still think you're asking about EPE, I swear I'm not....

I'm just trying to follow along, and maybe this will help. WHAT readings (bolded above)??

The original statement you quoted was background of what I was told when I first started. The last paragraph in the OP contains the question that does NOT have anything to do with EPE.
OK, now I think you're just screwing around with us.

Is what I've explained above true for worst case error in a pair of GPS readings?

Long answer: All of the stuff posted above about EPE's, and statistics.

There's just no way to answer your question without getting in to all that stuff. I don't understand why that seems to irritate you so badly. Everyone is posting relevant, helpful answers to the question you actually asked, as opposed to the question you seem to think you have asked.

Fair enough. I've stated several times that I probably did a bad job of explaining myself and so I've restated the question in hopes of clearing up what was, and was not, asked.

But I'll dumb it down as much as possible so that you can understand it -

Here's the original post:

• 1st Paragraph: Once upon a time I was told some stuff about how GPS errors could possibly add.
• Picture: Showing some circles and points that are labeled.
• 2nd Paragraph: Summary of what I thought someone in the forums once said was not true at all.
• 3rd Parapragh: Asking, is the stuff outlined in the 2nd paragraph true or not?

Some people misunderstood, and assumed I was asking about EPE. I guess I can understand why.

I explain, no, ignore EPE, I'm asking strictly about readings being off by whatever amount, do these amounts add together? It seems obvious, but I just wanted to check.

And then lots of back and forth - you're asking about EPE, no I'm not, yes you clearly are, but I'm not, you must be, I'm not, if not your question is stupid, okay then it's stupid but I'm not, in that case I still think you're asking about EPE, I swear I'm not....

I'm just trying to follow along, and maybe this will help. WHAT readings (bolded above)??

I don't want to get into the fray, either... way over my head... but by "whatever amount" in the bolded text, I assume that "whatever" includes zero?

The original statement you quoted was background of what I was told when I first started. The last paragraph in the OP contains the question that does NOT have anything to do with EPE.
OK, now I think you're just screwing around with us.

Is what I've explained above true for worst case error in a pair of GPS readings?

Long answer: All of the stuff posted above about EPE's, and statistics.

There's just no way to answer your question without getting in to all that stuff. I don't understand why that seems to irritate you so badly. Everyone is posting relevant, helpful answers to the question you actually asked, as opposed to the question you seem to think you have asked.

Fair enough. I've stated several times that I probably did a bad job of explaining myself and so I've restated the question in hopes of clearing up what was, and was not, asked.

But I'll dumb it down as much as possible so that you can understand it -

Here's the original post:

• 1st Paragraph: Once upon a time I was told some stuff about how GPS errors could possibly add.
• Picture: Showing some circles and points that are labeled.
• 2nd Paragraph: Summary of what I thought someone in the forums once said was not true at all.
• 3rd Parapragh: Asking, is the stuff outlined in the 2nd paragraph true or not?

Some people misunderstood, and assumed I was asking about EPE. I guess I can understand why.

I explain, no, ignore EPE, I'm asking strictly about readings being off by whatever amount, do these amounts add together? It seems obvious, but I just wanted to check.

And then lots of back and forth - you're asking about EPE, no I'm not, yes you clearly are, but I'm not, you must be, I'm not, if not your question is stupid, okay then it's stupid but I'm not, in that case I still think you're asking about EPE, I swear I'm not....

I'm just trying to follow along, and maybe this will help. WHAT readings (bolded above)??

The readings that your GPS gives to tell you what your location is. If it tells you that you're standing at A, but you're really standing at B, then the GPSr is wrong. The error is the distance between A and B. This is not EPE, this is a solid number.

It also doesn't matter that the person holding the GPS doesn't know that the GPS is wrong. It's still wrong. And it's wrong by an exact amount at the time of the reading.

The original statement you quoted was background of what I was told when I first started. The last paragraph in the OP contains the question that does NOT have anything to do with EPE.
OK, now I think you're just screwing around with us.

Is what I've explained above true for worst case error in a pair of GPS readings?

Long answer: All of the stuff posted above about EPE's, and statistics.

There's just no way to answer your question without getting in to all that stuff. I don't understand why that seems to irritate you so badly. Everyone is posting relevant, helpful answers to the question you actually asked, as opposed to the question you seem to think you have asked.

Fair enough. I've stated several times that I probably did a bad job of explaining myself and so I've restated the question in hopes of clearing up what was, and was not, asked.

But I'll dumb it down as much as possible so that you can understand it -

Here's the original post:

• 1st Paragraph: Once upon a time I was told some stuff about how GPS errors could possibly add.
• Picture: Showing some circles and points that are labeled.
• 2nd Paragraph: Summary of what I thought someone in the forums once said was not true at all.
• 3rd Parapragh: Asking, is the stuff outlined in the 2nd paragraph true or not?

Some people misunderstood, and assumed I was asking about EPE. I guess I can understand why.

I explain, no, ignore EPE, I'm asking strictly about readings being off by whatever amount, do these amounts add together? It seems obvious, but I just wanted to check.

And then lots of back and forth - you're asking about EPE, no I'm not, yes you clearly are, but I'm not, you must be, I'm not, if not your question is stupid, okay then it's stupid but I'm not, in that case I still think you're asking about EPE, I swear I'm not....

I'm just trying to follow along, and maybe this will help. WHAT readings (bolded above)??
I don't want to get into the fray, either... way over my head... but by "whatever amount" in the bolded text, I assume that "whatever" includes zero?

Sure. It's got to be possible for a reading to be correct from time to time. If one GPSr error is zero and the other is 20, then the combined error would be 20 (as I understand how it works).

But I'll dumb it down as much as possible so that you can understand it -

Here's the original post:

• 1st Paragraph: Once upon a time I was told some stuff about how GPS errors could possibly add.
• Picture: Showing some circles and points that are labeled.
• 2nd Paragraph: Summary of what I thought someone in the forums once said was not true at all.
• 3rd Parapragh: Asking, is the stuff outlined in the 2nd paragraph true or not?

Some people misunderstood, and assumed I was asking about EPE. I guess I can understand why.

No one misunderstood. They were answering your question, in the only way that makes any possible sense.

1st paragraph: You were told a gross oversimplification so you would know to look beyond a few feet from the indicated GZ.

2nd Paragraph: Summary of what you thought someone in the forums once said was not true at all.

3rd paragraph: Question: is the stuff in the 2nd paragraph true or not?

The answer: No, it's not true. All the stuff about EPE is WHY it's not true.

I understand why the EPE reading was assumed by some to be the point of my question. And I've explained several times (in vain apparently) that it wasn't.

I take responsibility for my lack of clarity in my OP and following posts.

I've gotten the answer to my question, so I'm happy.

I've been told over and over that my actual question was something that it wasn't, so I'm also a little confused. But it's all good.

If anyone else wants to insist that I've actually asked something else, then I give up for the time being. Knock yourself out.

Even if you know that the two readings are off by exactly 20' (ignoring the statistical nature of EPE), the result will be a net error of 40' only if the two errors are directly in line with each other such that one adds to the other. The result will be a net error of 0' if the two errors are directly in line with each other such that one offsets the other. The result will be a net error of about 28' if the two errors are at right angles to each other. And so on.

Even if you know that the two readings are off by exactly 20' (ignoring the statistical nature of EPE), the result will be a net error of 40' only if the two errors are directly in line with each other such that one adds to the other.

So, in answer to the OP, YES, you are correct if one ignores the bolded part above.

The result will be a net error of 0' if the two errors are directly in line with each other such that one offsets the other. The result will be a net error of about 28' if the two errors are at right angles to each other. And so on.

Now getting out of this, remembering that the best way to a correct answer is to ask the correct question.

So IF both GPS units were off by 20 feet, in the worst case could the cumulative error be 40? And the answer seems to be yes.

I could have said if one was off by 15 and the other off by 34, in the worst case could the cumulative error be 49? And the answer would also be yes.

Right, but here's the problem: you don't know where the true coordinates are. If both GPSes are off by 20 feet, then it is equally likely that the two errors cancel and the actual error is zero.

This is not a stupid question, unlike what other posters have said. It strikes to the heart of measurement theory. Here is the basic problem:

Some physical quantity (in this case, lat/lon) has a true value. How can we know that true value? We can't ever know it exactly. All we can do is make measurements of it. Those measurements have uncertainties associated with them. So the information we have about the true value can only be expressed as a probability distribution.

From the perspective of measurement theory, statements such as "The GPS is 20 feet off" don't make sense. 20 feet off of what? We don't know the true coordinates so we can't ever say that. 20 feet off from a measurement made by another GPS? The the uncertainties in both measurements come into play.

Basically, measurement theory says that it does not make sense to frame the question that way. I know that the question seems straightforward and simple, but it's actually quite subtle and difficult. That's why people are telling you it's not a good question; but don't think for a second that it's a "stupid" one.

Worse yet, I recently read a published article that new measurements show that the North American continental plate is shifting to the west-south-west at just over 3 inches per year. Some larger fault lines around the US move even faster at around 8 inches per year. So even if you had super perfect coordinates today - it wouldn't take long for that spot to shift away. Just adding to the uncertainty and inexact measurements.

We live on an ever changing planet.

On caching's 50th anniversary - some of the oldest caches are going to be perceived as being quite a bit off. Somebody will blame our ancient and inaccurate GPSr's.

Even if you know that the two readings are off by exactly 20' (ignoring the statistical nature of EPE), the result will be a net error of 40' only if the two errors are directly in line with each other such that one adds to the other.
Correct. I was trying to indicate that by saying it was the worst case.

The result will be a net error of 0' if the two errors are directly in line with each other such that one offsets the other.
Which is exactly what the diagram I supplied showed. Again, worst case.

The result will be a net error of about 28' if the two errors are at right angles to each other. And so on.
I suppose that's true, but doesn't really address worst case, which is what this thread is about.
Now getting out of this, remembering that the best way to a correct answer is to ask the correct question.

I've admitted that I could have phrased the original post better. No problems there. I'm completely guilty of not being crystal clear my first attempt.

But if you've read the thread you could see that I've repeatedly explained that EPE wasn't part of my question. I didn't mean to allow for the possibility that the false locations were unknown to some degree (they may be unknown to the hider, but known to the reader of the question). I wanted to set up the question by saying that the false locations were exactly at A and C in the diagram.

I tried to make it clear that the EPE wasn't part of the question, that the locations were exact, that the false readings were located at a specific point, and I repeated myself here, and here, and here, and here, and here, and here, and here, and here, and here, etc, only to still have people try and tell me that I was asking about EPE and giving an answer that suggests the locations were somehow variable.

So you're right, there is a better way I could have asked my question, but when I tried to clear it up and ask it the right way nobody would listen. It's one of the strangest experiences I've had on these forums ever.

Well done for keeping calm and polite throughout!

Thanks to others for answering (slightly different) questions on the same theme.

I tried to make it clear that the EPE wasn't part of the question, that the locations were exact, that the false readings were located at a specific point, and I repeated myself here, and here, and here, and here, and here, and here, and here, and here, and here, etc, only to still have people try and tell me that I was asking about EPE and giving an answer that suggests the locations were somehow variable.

I still maintain that the reason you kept getting answers you didn't want is because your hypothetical situation can never exist in reality. And because it's not a very interesting question, since it boils down to "Is it true that 20+20=40?"

I tried to make it clear that the EPE wasn't part of the question, that the locations were exact, that the false readings were located at a specific point, and I repeated myself here, and here, and here, and here, and here, and here, and here, and here, and here, etc, only to still have people try and tell me that I was asking about EPE and giving an answer that suggests the locations were somehow variable.
I still maintain that the reason you kept getting answers you didn't want is because your hypothetical situation can never exist in reality.

It absolutely exists with every cache, every time you hunt it. The fact that GPSrs aren't 100% right should be enough to tell you that they're giving you inaccurate information. You're standing at A, the coordinates shown on the GPSr screen is B. True? That's the basis of my question.

The GPSr DOES say that you're probably within X feet of B, and that exact probability is up for debate in a different thread. But if you write down the coordinates of B when hiding a cache, you've written down the wrong coordinates. The amount that they're wrong is (B-A). There is no EPE on the piece of paper you've written down the coordinates on. Is there? When you look on a cache page online and see the coordinates, is an EPE given? No, the coordinates point to a specific place, which is almost always wrong by a little bit. Some have said a good rough estimate for the average error between published coordinates and actual cache location is 20 feet. Which leads us to my original post.

And because it's not a very interesting question, since it boils down to "Is it true that 20+20=40?"

Pretty much. I think it boils down to "Is it true that 20+20=40 in this circumstance, and is there nothing else that should be added or subtracted?"

My reply is that EPE is only a factor during the measurments of B and C. After you've taken the measurment and written down the wrong answer, you've now got a specific number. Is there anything else after that?

Your next reply is that EPE should either be considered, or the question isn't interesting.

It absolutely exists with every cache, every time you hunt it. The fact that GPSrs aren't 100% right should be enough to tell you that they're giving you inaccurate information. You're standing at A, the coordinates shown on the GPSr screen is B. True? That's the basis of my question.

Every time I hunt a cache, the owner has placed it exactly 20 feet away from the "true" coordinates, and I'm looking exactly 20 feet away in the opposite direction? I don't think so.

So now you've moved from "is 20+20=40?" to "How should I interpret the difference between the actual coordinates and what my GPS is telling me?" The answer to that question involves EPE and statistics, and is not "20+20=40."

It absolutely exists with every cache, every time you hunt it. The fact that GPSrs aren't 100% right should be enough to tell you that they're giving you inaccurate information. You're standing at A, the coordinates shown on the GPSr screen is B. True? That's the basis of my question.

...

Pretty much. I think it boils down to "Is it true that 20+20=40 in this circumstance, and is there nothing else that should be added or subtracted?"

My reply is that EPE is only a factor during the measurments of B and C. After you've taken the measurment and written down the wrong answer, you've now got a specific number. Is there anything else after that?

Your next reply is that EPE should either be considered, or the question isn't interesting.

The question isn't interesting because this is not how you hunt for caches, and we are, after all, in a geocaching forum. While I (now) understand your question and the assumption you're making, it ignores the real-life aspects of a cache hunt. The conclusion you've drawn is correct under the assumptions you've made, but those assumptions are arbitrary and so the conclusion/answer doesn't actually tell anyone anything useful. The danger is that the answer is so overly simple that you or other people may think it tells them something useful, while in reality it doesn't. This is why it makes more sense to give a broader explanation of how GPS works and what's actually happening out in the real world when they're hunting for a cache.

It absolutely exists with every cache, every time you hunt it. The fact that GPSrs aren't 100% right should be enough to tell you that they're giving you inaccurate information. You're standing at A, the coordinates shown on the GPSr screen is B. True? That's the basis of my question.
Every time I hunt a cache, the owner has placed it exactly 20 feet away from the "true" coordinates, and I'm looking exactly 20 feet away in the opposite direction? I don't think so.

No, what exists every time as cache is hidden is that the cache is hidden at A, and the coordinates published are at B. That's the basis of my OP. I continue to suggest that "in the worst case" the same happens in the opposite direction when you look for a cache. If you're standing at C because that's where your GPSr has led you, then your distance from the cache is (C-A). Period.

You're standing still. The cache isn't moving. The distance between the two of you is a single (unknown to the hider or finder) number. That's where my question started, and the question was, given this worst case is there something else that keeps us from adding the two numbers? Is there something goofy with the way all this works that makes this not a very obvious and boring question? I've heard that there was, but now I'm convinced I was remembering something from a similar but different discussion. So even though it looks super obvious that 20+20=40, let me know if in this situation there's more to it.

It's a boring question to you, but the situation exists with every cache hunt. It was a boring question with an obvious answer that I asked only to make sure about. How many times have you ever learned something new and said, "Wow, I would never have considered that. That's awesome to know."???

I don't know everything, and if I hear two conflicting things on a subject that interests me, I like to do more research and see what else I might be missing.

So now you've moved from "is 20+20=40?" to "How should I interpret the difference between the actual coordinates and what my GPS is telling me?" The answer to that question involves EPE and statistics, and is not "20+20=40."
Nope, I haven't changed my question at all. But amazingly enough, I have yet ANOTHER reply telling me that I'm asking about EPE.

I'm very curious how many times I have to say clearly that I'm NOT asking about EPE before you and other people stop telling me that I am.

It absolutely exists with every cache, every time you hunt it. The fact that GPSrs aren't 100% right should be enough to tell you that they're giving you inaccurate information. You're standing at A, the coordinates shown on the GPSr screen is B. True? That's the basis of my question.
Every time I hunt a cache, the owner has placed it exactly 20 feet away from the "true" coordinates, and I'm looking exactly 20 feet away in the opposite direction? I don't think so.

No, what exists every time as cache is hidden is that the cache is hidden at A, and the coordinates published are at B. That's the basis of my OP. I continue to suggest that "in the worst case" the same happens in the opposite direction when you look for a cache. If you're standing at C because that's where your GPSr has led you, then your distance from the cache is (C-A). Period.

You're standing still. The cache isn't moving. The distance between the two of you is a single (unknown to the hider or finder) number. That's where my question started, and the question was, given this worst case is there something else that keeps us from adding the two numbers?

The something that keeps us from adding the two numbers is simply that we have absolutely no idea what those numbers are. We do, however, have a reasonable estimate for one of those numbers... the EPE displayed on our GPSr.

I'm very curious how many times I have to say clearly that I'm NOT asking about EPE before you and other people stop telling me that I am.

Without getting into EPE, your question makes no sense. You aren't asking about EPE, because you don't want to hear about EPE, but the only useful way to think about GPS errors involves EPE.

It absolutely exists with every cache, every time you hunt it. The fact that GPSrs aren't 100% right should be enough to tell you that they're giving you inaccurate information. You're standing at A, the coordinates shown on the GPSr screen is B. True? That's the basis of my question.

...

Pretty much. I think it boils down to "Is it true that 20+20=40 in this circumstance, and is there nothing else that should be added or subtracted?"

My reply is that EPE is only a factor during the measurments of B and C. After you've taken the measurment and written down the wrong answer, you've now got a specific number. Is there anything else after that?

Your next reply is that EPE should either be considered, or the question isn't interesting.

The question isn't interesting because this is not how you hunt for caches, and we are, after all, in a geocaching forum. While I (now) understand your question and the assumption you're making, it ignores the real-life aspects of a cache hunt. The conclusion you've drawn is correct under the assumptions you've made, but those assumptions are arbitrary and so the conclusion/answer doesn't actually tell anyone anything useful. The danger is that the answer is so overly simple that you or other people may think it tells them something useful, while in reality it doesn't. This is why it makes more sense to give a broader explanation of how GPS works and what's actually happening out in the real world when they're hunting for a cache.

If I'm with a newbie cacher while showing them how to hunt for a cache and they ask me, "The GPS is showing me at the coordinates here, but you found the cache over there. Is my GPS wrong?" Tell me what is not correct with the following answer that I'm fully prepared to give:

"When the hider measured coordinates for this cache his GPS wasn't showing him exactly where he was because of errors in the calculations based on several things. The locations of the satellites as well as interference from trees, buildings, the atmosphere, etc can all make the readings slightly off. So he was standing where the cache was hidden, but wrote down coordinates somewhere around here. You're trying to put yourself at those coordinates, but because of very similar errors with your readings your GPS may tell you that you're at those coordinates but you'd be somewhere else too. These two errors will combine to put you anywhere from on the cache to the sum of the errors away. If the hiders error had him 8 feet off, and your error is 14 feet, you can be anywhere from 0 to 22 feet away."

You're standing still. The cache isn't moving. The distance between the two of you is a single (unknown to the hider or finder) number. That's where my question started, and the question was, given this worst case is there something else that keeps us from adding the two numbers?

Here's what your question is ignoring:

1. When hunting a cache, you're not standing still. You know the coordinates aren't perfect, so you don't expect to find the cache right where you stopped, so you start looking around.
2. Even if you're standing still, the coordinates on the GPS don't. They will keep changing as you just stand there.
3. More importantly, since you won't be standing still: every time you return to the same location, your GPS will show you different coordinates and a different distance to GZ, and a different direction to it.
4. Conclusion: in any particular moment, you may end up exactly 20 feet away from the posted coordinates (with the GPS showing 0 distance) and that may be exactly in line of where the cache is, in which case you will be 40 feet away. But that's such a specific case that it hardly ever happens. Even if you're exactly 20 feet off, it's much more likely that you're not exactly in line with the cache, which means you will be closer than 40 feet, but even more likely than that is that you're not exactly 20 feet off. In that last very likely case, it's more likely that you're closer than 20 feet from the coordinates and quite unlikely (but possible) that you're further away.
5. Conclusion to the conclusion: As you keep moving around and keep checking your GPS, you will be closer to the posted coordinates more often than further away. The most time you will spend within a certain radius of the posted coordinates. The EPE is what tries to give an estimate of how big that radius is.

"When the hider measured coordinates for this cache his GPS wasn't showing him exactly where he was because of errors in the calculations based on several things. The locations of the satellites as well as interference from trees, buildings, the atmosphere, etc can all make the readings slightly off. So he was standing where the cache was hidden, but wrote down coordinates somewhere around here. You're trying to put yourself at those coordinates, but because of very similar errors with your readings your GPS may tell you that you're at those coordinates but you'd be somewhere else too. These two errors will combine to put you anywhere from on the cache to the sum of the errors away. If the hiders error had him 8 feet off, and your error is 14 feet, you can be anywhere from 0 to 22 feet away."

Again, what you're ignoring is that the error you're seeing when hunting for the cache isn't static. It fluctuates, it could be 20 feet over this way one minute, the next minute it could be 15 feet over another way and the next minute it could be 0. And you don't know what it is at any given moment, or where. That's why you keep checking your GPS. The more often you do it and the longer you stick around, the better feel you'll get for where the "real" GZ is (that's the "it keeps bringing me back to this spot" moment). That's what you should be explaining to the newbie, and not give an oversimplified picture of how it works.

Your example can be nicely used to demonstrate this: if the hider's error was 8 feet and your error is 14 feet, then you can't possibly get a "0" reading at the cache site. You'd always miss it. I'm sure you know that this is not true.

Edited by dfx
It absolutely exists with every cache, every time you hunt it. The fact that GPSrs aren't 100% right should be enough to tell you that they're giving you inaccurate information. You're standing at A, the coordinates shown on the GPSr screen is B. True? That's the basis of my question.
Every time I hunt a cache, the owner has placed it exactly 20 feet away from the "true" coordinates, and I'm looking exactly 20 feet away in the opposite direction? I don't think so.
No, what exists every time as cache is hidden is that the cache is hidden at A, and the coordinates published are at B. That's the basis of my OP. I continue to suggest that "in the worst case" the same happens in the opposite direction when you look for a cache. If you're standing at C because that's where your GPSr has led you, then your distance from the cache is (C-A). Period.

You're standing still. The cache isn't moving. The distance between the two of you is a single (unknown to the hider or finder) number. That's where my question started, and the question was, given this worst case is there something else that keeps us from adding the two numbers?

The something that keeps us from adding the two numbers is simply that we have absolutely no idea what those numbers are. We do, however, have a reasonable estimate for one of those numbers... the EPE displayed on our GPSr.
No no no. You do NOT have to know the value of a distance for it to exist. If you're 20 feet from a tree but don't know how far away you are, you're still 20 feet from the tree. If you're 5.345 feet from an ant, you don't even have to know the ant is there and you're still 5.345 feet from it.

If the hider's error is 20 feet, and you don't know the value, and the finer's error is also 20 feet, and you don't know the value, it's still possible for them to be anywhere from 0 to 40 feet away.

I'm very curious how many times I have to say clearly that I'm NOT asking about EPE before you and other people stop telling me that I am.
Without getting into EPE, your question makes no sense. You aren't asking about EPE, because you don't want to hear about EPE, but the only useful way to think about GPS errors involves EPE.
It makes perfect sense. I'm not asking about EPE because it's irrelevant, not because I don't want to hear about it. Show me where on a cache page the EPE is listed. The cache is located at some unknown location. The coordinates shown are for a specific location. If those two aren't the same (and it's not important that we know if they are or not) there is a very specific distance between the two.
If the hider's error is 20 feet, and you don't know the value, and the finer's error is also 20 feet, and you don't know the value, it's still possible for them to be anywhere from 0 to 40 feet away.

But what use is that? You don't know the hider's error, and you don't know your own error. After you've found the cache, you can check your GPS and see what it says, at which point you'll know the combined error of the hider and your own at this particular moment. You still won't know either the hider's error nor your own.

You're standing still. The cache isn't moving. The distance between the two of you is a single (unknown to the hider or finder) number. That's where my question started, and the question was, given this worst case is there something else that keeps us from adding the two numbers?
Here's what your question is ignoring:

1. When hunting a cache, you're not standing still. You know the coordinates aren't perfect, so you don't expect to find the cache right where you stopped, so you start looking around.
2. Even if you're standing still, the coordinates on the GPS don't. They will keep changing as you just stand there.
3. More importantly, since you won't be standing still: every time you return to the same location, your GPS will show you different coordinates and a different distance to GZ, and a different direction to it.
4. Conclusion: in any particular moment, you may end up exactly 20 feet away from the posted coordinates (with the GPS showing 0 distance) and that may be exactly in line of where the cache is, in which case you will be 40 feet away. But that's such a specific case that it hardly ever happens. Even if you're exactly 20 feet off, it's much more likely that you're not exactly in line with the cache, which means you will be closer than 40 feet, but even more likely than that is that you're not exactly 20 feet off. In that last very likely case, it's more likely that you're closer than 20 feet from the coordinates and quite unlikely (but possible) that you're further away.
5. Conclusion to the conclusion: As you keep moving around and keep checking your GPS, you will be closer to the posted coordinates more often than further away. The most time you will spend within a certain radius of the posted coordinates. The EPE is what tries to give an estimate of how big that radius is.

Here's what your replies are ignoring:

1. When asking the question in the OP, I absolutely WAS standing still. I identified my location with the big letter C and a blue dot on the diagram.
2. This isn't a question about how to show someone how to find a cache, this is a question about how errors add together.
3. As stated earlier, a newbie is standing where they think the cache should be because that's where their GPSr put them and they ask , "Why am I not standing on the cache". They're not moving, they're a fixed distance from the cache and from the coordinates reported to them when they looked up and asked the question.
4. More importantly, you're ignoring me when I keep telling you that this thread isn't about how to figure out your distance from a cache and how EPE makes that distance hard to measure with accuracy. This is about how two errors add together.
5. Conclusion to the conclusion of your conclusion: You desperately want me to be asking a question about how to measure a distance and you'd like to make sure I know to consider EPE. But I'm not asking that question here and you can't accept that fact.

"When the hider measured coordinates for this cache his GPS wasn't showing him exactly where he was because of errors in the calculations based on several things. The locations of the satellites as well as interference from trees, buildings, the atmosphere, etc can all make the readings slightly off. So he was standing where the cache was hidden, but wrote down coordinates somewhere around here. You're trying to put yourself at those coordinates, but because of very similar errors with your readings your GPS may tell you that you're at those coordinates but you'd be somewhere else too. These two errors will combine to put you anywhere from on the cache to the sum of the errors away. If the hiders error had him 8 feet off, and your error is 14 feet, you can be anywhere from 0 to 22 feet away."
Again, what you're ignoring is that the error you're seeing when hunting for the cache isn't static. It fluctuates, it could be 20 feet over this way one minute, the next minute it could be 15 feet over another way and the next minute it could be 0. And you don't know what it is at any given moment, or where. That's why you keep checking your GPS. The more often you do it and the longer you stick around, the better feel you'll get for where the "real" GZ is (that's the "it keeps bringing me back to this spot" moment). That's what you should be explaining to the newbie, and not give an oversimplified picture of how it works.

Again, what you're ignoring is that I'm not trying to show a newbie how to find the cache, nor am I trying to show them how to figure out their distance to the cache from a specific point based on readings. I'm trying to explain that there are two errors, and they add together, and the cache location can be up to the sum of those errors away. That's it. The point of this thread was to determine if there was more to the story, but there's not.

Your example can be nicely used to demonstrate this: if the hider's error was 8 feet and your error is 14 feet, then you can't possibly get a "0" reading at the cache site. You'd always miss it. I'm sure you know that this is not true.
If the hider was at A, but thought he was at B. The finder is at C, but thinks she's at B, then in worst case she's AB+BC away. Best case she's AB-BC. You're right, if the two distances are not the same then she can't be zero feet away with that error. Keep in mind that she has NO IDEA what her error is, and doesn't need to know it, in order to be that far away from the cache. We're not measuring distances in this topic, we're adding the errors.
If the hider's error is 20 feet, and you don't know the value, and the finer's error is also 20 feet, and you don't know the value, it's still possible for them to be anywhere from 0 to 40 feet away.
But what use is that?
It explains to someone why they're not standing on the cache when they think they should be. That's about it.

You don't know the hider's error, and you don't know your own error.
Exactly!! Nor are we trying to figure out what the errors are.

After you've found the cache, you can check your GPS and see what it says, at which point you'll know the combined error of the hider and your own at this particular moment.
But we're not talking about figuring out the value of the errors, just that they're the reason the finder isn't standing on the cache when their GPS says they're at GZ.

You still won't know either the hider's error nor your own.
That's okay, that's not relevant. I don't CARE what those errors are. I'm not trying to figure them out. I'm just trying to use them to explain why the finder isn't standing on the cache when their GPS says they're at GZ.
That's okay, that's not relevant. I don't CARE what those errors are. I'm not trying to figure them out. I'm just trying to use them to explain why the finder isn't standing on the cache when their GPS says they're at GZ.

So you're just asking how trigonometry/geometry works, right?

That's okay, that's not relevant. I don't CARE what those errors are. I'm not trying to figure them out. I'm just trying to use them to explain why the finder isn't standing on the cache when their GPS says they're at GZ.
So you're just asking how trigonometry/geometry works, right?

Not at all. I'm well versed in both of them. There are some people here that could benefit from learning a bit more about how English works.

I was 100% sure that the two errors added together in the worst case to put you 40 feet away from the cache. Then at some point I read in here that this wasn't the case. Eventually I wanted to know why it wasn't the case bad enough to start a thread about it, just in case there was something I wasn't considering in such a simple explanation. It would have been really cool to find out that in this situation that blah blah blah blah and the numbers aren't additive, they have to be blended. Or whatever.

I've learned that I was correct.

I also learned that probably what I read someone say long ago which was NOT true, was that "you can expect to find the cache 40 feet away". There's a huge difference between "up to 40 feet away" (my version), and "will be 40 feet away" (the version that I probably read about in the past).

Not at all. I'm well versed in both of them.

Well, there you, because that's exactly what it is. The example in your OP can be described as a triangle, two sides are 20 feet long each (your two position errors) and the length of the third side is unknown, which is the distance to the cache. This length can be between 0 feet (the errors cancel each other out) and 40 feet (your "worst case"). Simple trigonometry. The actual answer then depends only on the angle in which the two errors run.

Alternatively you can express it as vector addition. Vector a is 20 feet long and vector b is also 20 feet long. The distance to the cache then is c = a + b. c will then be 40 feet long if a and b are in the same direction, and 0 feet long if they're in opposite direction, and anything in between in all other cases.

Edited by dfx