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How Close Does The Gps Get You?


Thot
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I just read a post that said the closest one should expect to get using the GPS is within 50 feet. This was a revelation to me. Is there general agreement among experienced cachers that one shouldn’t expect the GPS to get them closer than 50 feet? Could some experienced others give their estimates of how close they typically get?

 

Fifty feet is a large area in which to find a microcache well hidden among in an zone of jumble or tall weeds. It seems like that could verge on the impossible.

Edited by Thot
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It's all dependent on the view of the sky that the unit has. Out in the middle of the lake, I've seen 3ft. accuracy.

The biggest problem is the differences in your GPS vs. the GPS the cache owner used, and more specifically, how long the unit had to sit there and settle before the waypoint was marked.

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It's all dependent . . .

Understanding there are a lot of variables, I urge you to try to take a shot at an overall estimate of your typical distance.

 

One way this is done in the estimating world is to add the closest you can reasonably expect + twice your best guess of typical/average + the greatest you can remember and divide by 4.

 

In my VERY limited experience this would be 2 + 2x4 + 40 = 50 divided by 4 = 12

Edited by Thot
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I find that about 42-45% of the time, I'm within 10 feet of the target and about 80% of the time I'm within 30' of the target. But that doesn't mean my expectation is to be within the 30' radius on my hunt. I expect to get within 50' 95% of the time. By the time I'm within 100-200 feet, I'm already using more than just the GPS to get me to my target. I'm looking at possibilities, triangulating on the move, and using my judgement on where I would hide a cache. It only so happens that when i let my GPS settle over the cache, sometimes unwittingly that I'll get great reads.

 

There are those few times where the GPS has led me straight to the cache and most of the time I found it right away. And then there are times I have yet to recognize it as a cache.

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95% of the time our unit puts us within 20 feet. 30% of the time it is less than 10 feet.

 

If you want to know how well your GPSr will typically do try setting a waypoint in your yard or driveway. Leave the area and do a goto to that waypoint. Go to that waypoint at several different times and on different days and compare how close the GPSr gets you to the original spot. It also helps you learn you unit.

 

John

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There have been times that I couldn't get a signal within 100' of the cache. :blink: Then there was the time with great signals that I walked up and settled the GPSr within 0.5' :) of the cache (a micro hidden...mmm, better not say so you can enjoy the hunt). Most times I (if I have the 3D turned on with WAAS) can get within 10' before having to start to search.

 

Please note that 0.001 minutes is about 6'. Many times I have set and averaged a 100 or so readings at a spot, then done it again, and again, and again. Often the numbers will vary by +/- 0.003 or more. That's a variation of 18' or more without moving the receiver.

 

If your GPSr can average a whole lot of points, then sit down with it in the open and try a bunch of repeated averages. Then walk away and come back an hour or two later (or next day at a different time of the day) to the very same spot and do it all over again.

 

Try it out in the open, then under a light leaf canopy, then under a heavy leaf canopy. If you can average within 10' of each other over a period of time, then you are doing excellent. (Most likely have 6+ sats in view and WAAS turned on.)

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I just read a post that said the closest one should expect to get using the GPS is within 50 feet. This was a revelation to me.

Over the course of the year, I make repeated visits to several locations hundreds of miles apart, so naturally, I have waypointed each and follow the gps to them on every trip. Sometimes, the gps indicates I am within a foot or two of the coordinates. Other times, 20 - 30 ft off. But I remember one instance where a waypoint that typically indicates within 3 - 8 ft indicated 161 ft!

 

As others have stated, the positions of the satellites at both the times the waypoint was taken and visited determine the accuracy of the coordinates to a large degree.

 

I have noticed from reading these forums that some brands of gps receivers appear to read more accurately if the waypoint had been taken (or was being sought by) units by the same manufacturer.

 

Many "Needle in the Haystack" micro caches exist where the coordinates are routinely reported to be 50ft or more "off." I have a hunch that in some cases, the inaccuracy was not accidental. :)

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Over the course of the year, I make repeated visits to several locations hundreds of miles apart, so naturally, I have waypointed each and follow the gps to them on every trip. Sometimes, the gps indicates I am within a foot or two of the coordinates. Other times, 20 - 30 ft off. But I remember one instance where a waypoint that typically indicates within 3 - 8 ft indicated 161 ft!

 

Hey, that's pretty interesting stuff... Thanks! :)

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If my gps says 12 feet from a set of coords that someone took with another gps that has a 12' accuracy, am I 24' off?  :)

It's been too long ago for me to be sure this is right but as I recall, cumulative error is the root mean square of the individual errors so in the case you suggest this would be:

 

Square Root of 12 squared + 12 squared or √144 + 144 = 17 feet.

 

I realize the error of the person placing the cache has to be taken into account. That’s one reason I wanted to get advanced cachers experiences. I can do tests to get a feel for my units performance but only field experience will show what one can expect from the combination of my error and the cache owner's error.

Edited by Thot
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Go find a benchmark with known coordinates, and stand on it. Try this with WAAS on and off. Do not confuse your "Accuracy Reading" with your actual accuracy. The "Reading" is pretty much just a programed in guess. A good exercise for anyone with a GPS.

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As was stated above, it depends on several things.

 

Are you reading satellites that are all over head as if you were in a valley or are you reading several sats on the horizon as if on a large body of water?

 

There are times when I can't even get a signal because I am inside of a cove and there are not enough overhead sats because the mountains are blocking them.

 

Also, atmospheric moisture, leaf cover, etc affects readings.

 

Here in the mountains of western NC, I am lucky to get 25ft as my most accurate readings.

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95% of the time our unit puts us within 20 feet. 30% of the time it is less than 10 feet.

 

If you want to know how well your GPSr will typically do try setting a waypoint in your yard or driveway. Leave the area and do a goto to that waypoint. Go to that waypoint at several different times and on different days and compare how close the GPSr gets you to the original spot. It also helps you learn you unit.

 

John

At one of our events here last year, we had an event where you were give a coord and a marker flag and had to go mark your spot. If my memory serves correctly, out of 60+ cachers tried and 95% were within a 10 square foot area. It's safe to assume there were many different brands and models used. My Garmin GPS12 usually averages 22-23 ft. just the other day I had it at 13 feet. IMO if your with in 30 feet you are good to go. I sometimes look at the satalite locks ( if I'm having trouble locating a cache)to see what the reading is and double that for errors.I do this because if the hidder had the same read and is off by say 20 ft and I'm off by 30ft at present reading, I'm still in the general accepted area. SF1

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I just read a post that said the closest one should expect to get using the GPS is within 50 feet. This was a revelation to me. Is there general agreement among experienced cachers that one shouldn’t expect the GPS to get them closer than 50 feet? Could some experienced others give their estimates of how close they typically get?

 

Fifty feet is a large area in which to find a microcache well hidden among in an zone of jumble or tall weeds. It seems like that could verge on the impossible.

The "accuracy" of todays units is +/- 3 to 15 meters. That's 10 to 50 feet. If you are looking to find an "A" order surveying point with coordinates to .001 feet you could conceivably be away from this point by 10 to 50 feet.

 

If I use my HH GPSr and give you a coordinate it can be expected to be 10-50' away from the highly correct .001 foot coordinate. If you then try to find it with a different configuration of satelites, I would expect you to be away by 10-50 feet. If I am away by 10 feet and you are away by 10 feet, we could still be apart by 20 feet.

 

Notice I an using "away". I am not right, you are not right. We are not using equipment with the ability to be right. Our equipment is very suceptible to multipath interference and signal blockage.

 

You are right 50 feet is a large distance but it is possible.

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I leave my Legend on my knee while I sign the logbook, then check it before I leave to see how close it 'settled' to the actual location. Twenty to thirty feet, usually. Eight feet was my closest. Last weekend, it was acting screwy all day, and I had one cache that was 297' out! I found that one by basically peeking under every daggum rock in the woods.

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So, from those who have used both older and newer units, how much accuracy does WAAS really add? I'm still using an old Eagle 100, which IMO was one of the best values of that generation. Having read the above and some other threads it seems that I'm already getting as close as most and closer than many.

 

BTW, I experienced some wild fluctuations last Saturday as well.

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In my vast experience of two weeks geocaching and five cache finds, plus about a dozen benchmarks :) , I have found my SporTrak Map can be pretty accurate, I have noticed as I approach the site it can be 50 feet out, a couple of times I have been past the cache site before it corrects itself. However, the longer I am in the general area, walking away a few feet then back towards the last indicated area the closer it seems to get, all 5 of my caches have been 3-5 feet from where the GPSr finally settled and indicated indicated. It does usually take 10 minutes or so of roaming to get the position narrowed down that closely.

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I second (and third...) the above. Even though I don't expect my old eMap (older generation of receiver, no WAAS, less precision in general) to get me much closer than 50 feet, it frequently does. Accuracy is largely dependent on the time of day, satellite pattern, canopy cover, care and accuracy of the listed coordinates, and all sorts of other factors, but in a "real world" situation, 25 feet is reasonable for my old 2001 unit. I sometimes get better, sometimes get worse.

 

I usually just get as close as it will get me within a 5- to 10-second waiting period, then put it away and "think like a Geocache" to see where it would be hidden nearby.

Edited by Seamus
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The original question is bogus. The same GPS will give different accuracy on different days, in different locations, under different conditions, and will report different estimated position error. It's simply not possible to estimate the accuracy you expect without more detailed information. Is it under tree cover, is the cover wet, what time of what day is it, is an external antenna connected, etc, etc, etc. In short, to quote Click and Clack, the Tappet Brothers, "Booooooooogguuuuuus!!!"

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I use an etrex venture and have so far been roughly 5-10 feet from the caches I've found. Maybe I've just been lucky... but I'll take luck when I can get it. The hardest part for me is finding the thing after I'm close enough. I seem to "build in" an error to my GPS and start looking too far away from where the GPS put me (over and over again), thinking that the GPS is wrong. Someday I WILL learn to trust the GPS - someday.

 

POLARBZ

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The vast majority of the time, we're anywhere from 15-30 feet "away" when we find the cache. It's unusual to be right on the nose, and unusual to be 40+ feet off unless reception is terrible. Typically what the GPS manual says is that your error will be within 49 feet 95% of the time. That doesn't mean it can't be more accurate than that, and usually it is. Of course, a cache isn't a good test of accuracy, but it's still unusual for the hider's reception AND ours to be so bad that we find a cache more than 50 feet away from our zero point. One virtual, in a truly awful spot (downtown, among high-rise buildings, with signals bouncing all over like crazy) was nearly 500 feet off. (Well, that's not quite true. It was 100 feet off, and then 50 feet off, and then 200 feet off in the other direction, and then 0.1 miles... it didn't stay at the same distance for more than a second or two!)

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I live in New Mexico, I got more sky than land out here and my etrex Legend has put me *right on top* of every cache I've found (if the cache is within arms reach, I think my GPSr has done it's job) When I go outside I usually get a lock on all but one of the birds available. Sitting inside my house I get a lock on five birds with a reported accuracy of 28 feet. Bottom line is I find caches, that's what it's all about.

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One local hider has hidden over 30 caches that I have hunted. He uses a Garmin V to hide, and I use a Sport Trak Pro to hunt. On 95% of his caches that I have hunted, I have been within an arms reach of the cache when I followed the GPSr to zero. I also re-check the cords on my hides when I run maintenance, and I am consistently under 10 ft.

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When hunting tuppers, I'm in most cases within 6' to 9'. But it has happened that I was 150' off, although I received 6 - 8 satelites, but all overhead and none around the horizon (I had a so called bad satelite geometry)

In this case, I usually leave, log a DNF, and return another day. Of course, distances in Switzerland are not so much a problem, as they might be in the US or any larger country, so it's easier for me to return to a place than it might be for others

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Regarding how close the GPS gets you...when vectoring in to a cache, and you are at that last 50-100 feet and low and behold, the cache appears to be in an area with heavy/dense tree cover, do most of you also use a compass to make sure your heading stays lined up with the bearing displayed on the GPS as you move into the heavy cover?

 

Let's face it...we all like to have that clear sky overhead because you can just "feel" the satellites leading you to the cache...but how many caches are sitting out in the middle of a peaceful field??...not many in Oklahoma I assure you... :D

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When I get within 100' or so, I take a bearing with my compass, and try to see about where the cache might be. If I don't find it right away, I may try coming in from a different direction, and shoot another azimuth with my compass. The intersection of these bearings should be about where the cache is.

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Okay, I made an attempt at averaging the replies. Occasionally this required doing some fancy windageing of the person’s complex reply, but I did the best I could. I was able to quantify 21 replies. This gave an average (arithmetic mean) distance of 16 feet from the cache. This would mean that half are closer and half further than 16'. For statistics fans the mode is 15' suggesting that the average is probably pretty good.

 

Given the spread in the replies, one should expect individual caches to fall in the range of 2' to 50'.

 

Cachers with less experience appeared to be more optimistic than experienced cachers. This makes me suspicious of estimates that most or all caches are closer than 10 feet. But, if I throw out all estimates less than 10' the average only increases to 18' and the mode to 20'.

 

All this would seem to say a newbie cacher should expect to be typically within 10-30 feet of the cache, with an average being in the range of 16-20 feet.

 

Update:

 

I found a few more estimates in other threads. This additional data did not change the overall average from 16’ but it did shift the mode up to 20’. Also, now, if I throw out the 10’ and unders this average also rises from 18’ to 20’. So maybe 20’ is a better estimate.

Edited by Thot
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Please note that 0.001 minutes is about 6'. Many times I have set and averaged a 100 or so readings at a spot, then done it again, and again, and again. Often the numbers will vary by +/- 0.003 or more. That's a variation of 18' or more without moving the receiver.

 

When stashing a cache I place my eMap in the clear and let it gather 100 or so readings. At this point the estimated accuracy should be down to about 15ft. If it isn't, I keep reading 'til it is. That nails it down pretty good. I've found some caches that were way out - and it wasn't just me. I think some people have the wrong map datums set on their GPS units when they take the reading.

 

C-A

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