Jump to content

How Close Do You Get?


HotFix
Followers 1

Recommended Posts

I grabbed an eTrex Lengend off of eBay for $130. It is a fun little unit, but I have found nothing with it. I get right on top of it (GZ?) within 18ft accuracy. Does that mean the cache should be within 18 feet of me?

 

I'm going to hunt some ammo cans tomorrow - they should be easier than these tiny micros.

Link to comment

Eighteen feet is pretty good. It's usually more like twenty five. So...you're off by twenty five, the guy who hid it is off by twenty five...now you're up to fifty feet, as an average error. And fifty feet looking for an Altoids tin is no joke.

 

I'd stick to the full-sized caches until you get a bit of confidence...but at least you know the answer to the question, "if they give you the coordinates, where's the challenge?" :lol:

Link to comment

"I get right on top of it (GZ?) within 18ft accuracy. Does that mean the cache should be within 18 feet of me?"

 

No. It might be twice that due to a like error in the GPSr that recorder the location. If the person who placed the cache took several waypoints coming in from different angles, and/or used averaging then the location is probably more accurate. In that case you would liukely be more than 18 feet but less than 36.

 

The point of most micros is to be more difficult to find, but they still tend to be hidden in what looks like a good place to hide something.

Link to comment

You've got to develop the "cacher's eye." With experience, you will learn to spot the hiding places quickly--either because you recognize the camouflage, or because it's just where you would hide something.

 

Here's a tip--if you're having trouble finding a cache, walk away--perhaps 100 feet or so--and approach from a different direction than before. This is a good trick for locations where GPS reception is poor. I have often found that the second or third time I do this, I am right on top of the cache.

 

Here's another tip--for best reception, hold the GPSr up in front of you above your head level. This prevents you from blocking signals with your own body.

Link to comment

You will rarely find the cache exactly where your GPS tells you it is. Your GPS has an EPE (18 feet in your case) but so did the GPS of the hider. 20-50 feet off is common and 60+ feet is not unheard of.

 

Here's another tip--for best reception, hold the GPSr up in front of you above your head level.

 

How do you read it then? It may work with some units, but the Legend needs to be held horizontally with the face pointing to the sky for best reception. Anything you gain by holding it high will be lost by holding it vertically so you can see the screen at that angle.

Edited by briansnat
Link to comment

The very first cache I looked for with my GPS was at 0 when my GPS said 0. I think it was almost a year before this happened to me again. There are to many variables including the accuracy of the GPS used to the cache hider. I have found that some cache hiders are to impatient to use averaging, and in at least one case the cache hider inverted some of the coordinates so the cache was more than 1/2 mile from the 0 point. I would never have found that one, but a previous finder posted corrected waypoints. How he ever found the cache I will never know.

 

:blink:

Link to comment
Eighteen feet is pretty good. It's usually more like twenty five. So...you're off by twenty five, the guy who hid it is off by twenty five...now you're up to fifty feet, as an average error. And fifty feet looking for an Altoids tin is no joke.

No, you can't add error approximations like that - the total error will be smaller than the sum of the two errors (assuming that you are using the same level of confidence).

Edited by larsl
Link to comment
I grabbed an eTrex Lengend off of eBay for $130. It is a fun little unit, but I have found nothing with it. I get right on top of it (GZ?) within 18ft accuracy. Does that mean the cache should be within 18 feet of me?

It means that there is a X% chance that you are less than 18 feet from the waypoint, where X is the level of confidence used in the eTrex Legend, probably something like 95 or 99 (does anyone know?). The waypoint is almost certainly not where the actual cache is since the hider also had an uncertainty when he placed the cache, and unless you know this uncertainty you really don't know how far away the cache is likely to be.

Link to comment

Thanks for the replies. I should have read this before I went out caching today. I found a couple of ammos and learned the hard way about compounding my error with the placer's error. Once I found my first one I went "Ah - of course".

 

I also do not have a compass in my eTrex Lengend. I found that wlking in a straight line for about 20 feet gived me a good clue of where it is at.

 

I am still learning and will work my way up to the micros.

 

BTW: My girl went with me today and is now hooked. With her eye and my ability to carry her stuff (read: otherwise useless) - we make a good team.

Link to comment
BTW: My girl went with me today and is now hooked. With her eye and my ability to carry her stuff (read: otherwise useless) - we make a good team.

that's a good attitude to have, you'll do just fine. :P it's meant to be fun, and as long as you're having fun, not finding a cache is no big deal!

 

good luck, and welcome to geocaching.

Link to comment

You can answer some of your own questions regarding how accurate a GPSr is..

 

Go outside with your GPSr to an open area that has clear view of the sky. Once the unit has got a good signal lock and has located you fairly well, "save" the spot your standing at as a waypoint. Make some sort of mark on the ground to remind you exactly where the spot is, or perhaps use an existing landmark.

 

Now walk away from that spot, turn around and bring up that waypoint location on your GPS. Now you can walk all around the spot you saved and observe what the GPS tells you, regarding what direction "it" thinks your spot is and how far away it is. This should give you a decent understanding on the acuraccy of your GPS.

 

Now remember the test results will probably change quite a bit if you try it in a different environment, like a wooded area, or other features that affect visibility to the satellites. Also different brand GPSr's, and sometimes even different models from the same brand, can and will behave a bit differently. Also (just to add a final dose of frustration lol) the results could be different if you try the same test at the very same spot on a different day, as satellite aligment/orientation can be different & could cause more/less favorable triangulation.

Link to comment

Night Stalker makes a great point. If the difficulty is 1 and you have mulitple GPSr saying you are on top of it, you should be able to see it from there. If the difficulty is 4 or 5 and you have the world's best tracking system saying you are standing on it, you still may not see it. I had one cache I had to go to a couple of times. It was VERY obvious when you looked at it, but it took me a while to figure out WHERE to look.

Link to comment

There are a lot of variables that can change from day to day or minute by minute. Example: I went after a "MICRO" yesterday. My GPSr zeroed out in the open grassy area. I turned the unit off and back on (sometimes this helps) walked off about 100' and walked back...the unit zeroed out on the hill. I looked around there for a few minutes and repeated the GPS off/on/100' thing and the unit zeroed out in a different area. Bushes, trees, rocks, all with tons of places to hide a micro....so I head back to the van and log a DNF. Other days or caches I zero out on top of the cache container. Some caches I can spot the "hiding" place from 30' away others I walk around them, put my hand on them or even sit on them without finding them. It is all in a day of caching. :(

Link to comment

What they all said.

 

Stick to easy caches (I'd say 2/2 and below - and full sized) for now. Once you get more comfortable, you can go with other cache sizes, types, difficulty. Oh, and when your reciever starts to say you're within 30 feet, put it away,.

Link to comment

It means that there is a X% chance that you are less than 18 feet from the waypoint, where X is the level of confidence used in the eTrex Legend, probably something like 95 or 99 (does anyone know?).

GPS confidence level standard is 2-sigma which is roughly 95%. Except that I've heard that Garmin uses a confidence level less than this so their EPE looks good. Not sure if this is true.

 

From what I observed though, I believe that the errors we get for geocaching is much less than the EPE in many cases. The reason for this is that everyone's GPSr errors for a certain area (within a reasonable amount of time) are roughly the same. So just like differential GPS, the end result is that the errors got cancelled out and we get much closer to the cache than EPE would tell us. This has been my experience...More than half of the caches I found, the GPSr pointed me to less than 10 ft -- a third to within 5ft. I don't think I was lucky. There had to be a reason...

 

For this reason, I really believe that we shouldn't use WAAS or averaging to hide cache...while absolute position is more accurate, relative position for cachers may not be...

Link to comment

You can do this without the effort of walking. :(

 

Mark a waypoint. Without moving, select your GOTO function (whatever that is, navigation, etc). Immediately you will note your "destination" is at least 2-4 feet away. :D

 

I utilized this technique demonstrating how to use a GPSr to a happy new owner, and have done this several time when instructing on the use of a GPSr. The technique is ironic in that it is 100% accurate at hitting the target, unlike the accuracy of the GPSr. :D

Edited by Jeep_Dog
Link to comment
Oh, and when your reciever starts to say you're within 30 feet, put it away,.

I on the other hand stop when the GPSr states that I 30' away and let it catch up to me. I then start looking in the direction of the arrow and try to pick out where I think the cache would be. I then start walking real slow so I am more or less with the GPS signal. Once I get to zero I will turn it off and start looking (if I haven't already found it). If I don't spot it right away I will start looking in increasing circles for places that it could be...again keeping in mind that it could be 30' away. If I don't find it after a few minutes of searching I will walk off about 100' turn the unit back on, let it catch up and then follow the arrow again. After hours of searching I will stomp off and log a DNF. You will be surprised at how many caches you will find once you finally GIVE UP and start to head out. You will look at one more spot or a blue color will catch your eye, or you will finally see something that doesn't fit and there it is. Happy Hunting!!!

Link to comment
Also (just to add a final dose of frustration lol) the results could be different if you try the same test at the very same spot on a different day, as satellite aligment/orientation can be different & could cause more/less favorable triangulation.

Trimble has a free software program called Planning. It is used by surveyors who need to know when the best time is to use GPS surveying instruments. You can download it from Trimble.com. Then you will have the optimal satellite coverage time for your position.

Link to comment

During our last Mid Island mid winter get together we did a quick and dirty, and definately not scientific, comparison test of GPSr accuracy. We placed a table out on the beach then a bunch of us placed our GPS units on it and waited for a while. Then we wrote down the resultant positions. You can see the results by going to our website clicking on the Useful Stuff link. It is amazing how close they all were. All we know for sure of course. is where the table was!

Link to comment

We have a cache here in Spokane called GPS Accuracy where everyone leaves their GPS on a post out in the middle of a field and posts their readings. You are supposed to post the weather conditions, your GPS model (for those who care), lat/long, and how close you are with your GOTO feature. It is interesting to see how everyone did. My GPSr had me at 6' away...that is pretty good.

Edited by WARedBear
Link to comment
I grabbed an eTrex Lengend off of eBay for $130. It is a fun little unit, but I have found nothing with it. I get right on top of it (GZ?) within 18ft accuracy. Does that mean the cache should be within 18 feet of me?

 

I'm going to hunt some ammo cans tomorrow - they should be easier than these tiny micros.

What it means is you are within 18 feet of whwe the calculated waypoint is. There ae NO absolutes with a GPSr. Depending upon a lot of conditions you posigion is always a calculted value.

What I do is get close then stand there for a minute or so then start walking in one direction if the numbers go up then go in the other direction I keep doing this untill I zero out. Then I wait a few and do it again as need. Doing this I ahve gotten very close a few tiems other I have been lucky to be on the same planet.

Be patient and have fun.

cheers

Link to comment

Alot of good information here. Just remember, the EPE is ESTIMATED POSITION ERROR. In other words, 18' EPE doesn't mean "within exactly 18'", it means I'm pretty sure you're within 18'. And the hider's unit "thought" it was within X' as well. Different units use different criteria to come up with that number; some are more "optimistic" than others. Kinda like a Chevy telling you that Chevys are better than Fords. Can you really believe the Chevy, or is it going to be a bit biased?

 

When you get close, forget about trying to hit 0' and trust your "geo-sense". That "-sense" will improve the more you cache. :anibad:

Edited by 4x4van
Link to comment
Alot of good information here.  Just remember, the EPE is ESTIMATED POSITION ERROR.  In other words, 18' EPE doesn't mean "within exactly 18'", it means I'm pretty sure you're within 18'.  And the hider's unit "thought" it was within X' as well.  Different units use different criteria to come up with that number; some are more "optimistic" than others.  Kinda like a Chevy telling you that Chevys are better than Fords.  Can you really believe the Chevy, or is it going to be a bit biased?

 

When you get close, forget about trying to hit 0' and trust your "geo-sense".  That "-sense" will improve the more you cache. :anibad:

Great advice 4x4van!

 

There is a lot of good info here. (I'm still a bit puzzled about why we shouldn't use WAAS and averaging and that errors usually cancel each other out. :anibad: ).

 

The EPE is an estimate and shouldn't be given much more relevance than that. Also it seems each manufacturer treats its formula like a trade secret and they are different. I know my Lowrance always states an EPE double that of a friend's Garmin.

 

I pretty much use the 30' method. Once I get to 30' I continue my bearing and look around. Some recommend using a compass (magnetic or electronic) at this point, especially if there are a lot of obstacles. If I don't find it, I pace off to 100' elsewhere and try again.

 

The Trimble Planning freeware is a neat tool, but be aware of its intended use. Surveyor-class receivers are built to acquire the optimum PDOP (position dilution of precision) measurement, which occurs when using satellites that are scattered and very low on the horizon. The default elevation window for these calculations is between 0º and 15º. Consumer units with a patch antenna (Etrex & iFinder) may ignore signals from very low satellites (the iFinder requires a minimum of 9º) and this thereby reduces the effectiveness of this application. However I do recommend using it, especially to get a better understanding of PDOP and its effect on EPE.

 

If you really want to check your unit's accuracy do the following:

Go to your account page and click "View nearby benchmarks".

Select a "found" benchmark (the more "finds" the better).

Click "view original datasheet".

Record the coordinates, but only if they are "Adjusted". Do not use "Scaled"!

Note: These coor's are given in DD MM SS.SSSSS, NAD 83 datum.

Set your unit's Map Datum to NAD 83 and enter the coor's.*

Now find this benchmark. It's the most accurate reference point you'll ever find.

 

*It can be left in WGS 84, but you may be off by up to one meter.

Link to comment

Part of the enjoyment of geocaching, I think, is the fact that... once the GPS unit has gotten you close to where the cache is hidden, you still have "the thrill of the hunt."

 

When the GPS unit says that you're, say, within 50 feet of the cache ... start looking at the surroundings and using your wits. Read the description for clues. Take the time to actually enjoy where you are. No sattelite navigation technology available to civilians will ever allow you to "simply walk up to" the target coordinate, and if it could, obviously there would be no hobby.

Link to comment

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Loading...
Followers 1
×
×
  • Create New...