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How Close Should I Expect Gps To Get Me To Cache?


Bozza
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Been out Geocaching for the first time this morning.

 

Using the clues that were provided and the local geography, pretty sure I was in the right place on both occasions.

 

However on one cache my GPS said about 20-25 metres away from the given co-ords, on the second it was more like 60m.

 

My GPS was saying accurate to about 9-11 metres at each of these which makes me think the first could be possible, given a possible small inaccuracy of the GPS of the person who left the cache, but the 2nd seems too far out.

 

Off out now on my third cache, but wondered what I should expect...

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I did a survey of experienced users here last year. The results showed that on average the GPSr gets you about 20 feet (7 meters) from the cache. Being an average, this means half the time it will be further and half closer. Occasionally it can be 50-60 feet (15-18 meters) or more.

 

This is discussed in my Beginner's Guide. See tagline.

 

Edited to add:

 

Make sure the "Datum" on your GPSr is set to WGS84

Edited by Thot
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Typically, I see around 6-8 meters EPE (Estimated Positional Error) when 'caching on the coast. Remember that your accuracy will vary with location and time of day as well. Ravines, canyons, heavy tree cover and tall buildings can cause significant positional errors. As well, don't forget that you have an error, and so did the hider. As well, if the 'cache has been found a few times, there's a chance that it may not have been put back in exactly the right place at some point. A 'cacher I know recently reported finding one of his 'caches replaced OVER 60m from where he hid it.

 

All that said, 60m does seem a little on the high side - if it seems to be doing that on a regular basis, try checking it against another GPSr (look up a fellow 'cacher in your regional forum, or look for a local event) or look for a nearby benchmark/datum and see what your error against that is.

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I would say about 20 feet with my Sport traks or my Meridian gold(3 Meters) I have seen a few that were over 40 feet, IMO these were cases of bad coordnates, most of these were posted by cachers that said they took several readings and the pick one they thought was the best of the bunch

Edited by JohnnyVegas
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With my yellow etrex and foretrex201 i usually get around 15 ft.. I hope that with my 60cs and glission amplified antenna (that are STILL in the mail) i'll be able to get just as good or better reception. BTW most of the caches that i've done (9) have been in pretty high up mountains in south korea..maybe that's why my reception has been good.

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Somewhat ambitiously, after this morning's adventure, we went out this afternoon on a 7 part cache. GPS had us hiking all over the place (urban Bristol, England) with not a lot of success.

 

Map Datum was not set to WGS84, but is now. Need to get out again to assess the difference it makes. I'm quite hopeful!

Edited by Bozza
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Map Datum was not set to WGS84, but is now. Need to get out again to assess the difference it makes. I'm quite hopeful!

Oh-ho! There's your deal. It'll make a huge difference -- like several hundred yards.

 

But beware! If you use mapping software like Fugawi, you have to set it to Ordnance Survey's OSGB36 or you won't be right on the map. But then you can't download from Fugawi to your GPSr.

 

I'm not trying to make this confusing on purpose. I screw this up all the time when I'm in the UK. People in the UK forum might be better at explaining it to you.

 

But if you're inputting the coords directly from GC, WGS84 is your ticket! That much I know.

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Your results will vary. There are many reasons... sattelite geometry, view of the sky, signals bouncing off of buildings, and many others.

 

Your GPS will tell you how "confident" it is of the numbers it's giving you with a number called DOP (Dilution of Precision). 1.0 is ideal; 2.0 not so good; 3.0 is basically worthless. Your GPS probably uses DOP, multiplied by the accuracy-figure, to draw a circle around your position. You could be anywhere in that circle and your GPS cannot tell you exactly where!

 

A Google search will lead you to literally thousands of sites that explain this, in as much detail as you could ever wish for.

 

Work your searches attentively as you approach the site. Observe features in the general area your GPS is indicating. Triangulate on the position from several angles. As you get close to the site ("inside the circle"), don't continue to rely upon the position figures the GPS is giving you. Once both you and the cache are "in the circle," the GPS can be of no further use to you. It's time to use your wits.

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Your results will vary. There are many reasons... sattelite geometry, view of the sky, signals bouncing off of buildings, and many others.

 

Your GPS will tell you how "confident" it is of the numbers it's giving you with a number called DOP (Dilution of Precision). 1.0 is ideal; 2.0 not so good; 3.0 is basically worthless. Your GPS probably uses DOP, multiplied by the accuracy-figure, to draw a circle around your position. You could be anywhere in that circle and your GPS cannot tell you exactly where!

 

A Google search will lead you to literally thousands of sites that explain this, in as much detail as you could ever wish for.

 

Work your searches attentively as you approach the site. Observe features in the general area your GPS is indicating. Triangulate on the position from several angles. As you get close to the site ("inside the circle"), don't continue to rely upon the position figures the GPS is giving you. Once both you and the cache are "in the circle," the GPS can be of no further use to you. It's time to use your wits.

What is "satellite geometry?"

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What is "satellite geometry?"

The current arrangement of sattelites in the sky above you. Your GPS uses triangulation to compute your position on the face of the Earth. Sometimes the sattelites are in a favorable position for giving accurate calculations at your particular spot on the globe, and sometimes not. (This isn't simply a matter of how clearly the signal from those sattelites can be received.)

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I use a Rino130 and (although I only have 10 finds so far) it has put me right ontop of the cache every time. One cache it lead me right to this tree stump.. and on one side of the stump it read zero feet to waypoint. The stump looked like a neat place for a hide, unfortunately I could not see anywhere that a hide could be placed. About ten minutes later I was still looking around and bumped into the stump knocking its side off exposing the cache. Very cool.

 

My point is sometimes these things can be EXTREMELY accurate.

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My point is sometimes these things can be EXTREMELY accurate.

A stopped clock has the exactly correct time twice a day. :ph34r:

 

Also, remember, it's not just how close your gadget gets it, the posted coordinates also contain the error of the person who placed. The best of these devices don't claim to be better than 9 feet. It's true they're sometimes closer, but you sure can't depend on it. The chance of the combined error of two independent readings(yours and the cache placer) being under than a foot is less than hitting #25 on a roulette wheel (probably less than hitting it twice in a row). :D

 

edited to add:

 

I failed to mention that the 3 decimal place resolution of coordinates listed on gc.com incorporates an average of 4+ feet of error even if the owners coordinates were perfect.

Edited by Thot
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I've found my explorist 100 to be extremely accurate so far, the only one that was off has been noted by others as being off & the only one that I had real snags finding was because I kept trying to be clever & looking where I thought it was & ignoring the GPS which was pointing me 3m to the right location!

 

I'm on <10 finds & as such probably aren't giving an "accurate reading" myself but so far its been pretty good.

 

Chalky

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Yeah 20 feet of accuracy on average is about right. Some caches were within a few feet of claimed "ground zero", others were 34-40 feet away. But 20 feet is a good average to assume.

 

And that's fine by me too. Hey, would you "really" enjoy GeoCaching as much if the GPRr was so good as to be able to get you within inches of every Cache? :ph34r:

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One other item that I have found to be invaluable while Geocaching is a good compass. I use a Ranger Ultra, but any compass that allows you to shoot a bearing and is of reasonable quality will do.

 

The way this works is as follows:

Assuming that you are getting bounced around an area of a cache. Take some distance back, and start to circle the area of the cache. Now use your GPSr ability to give you a bearing to the cache. Take the compass, set it for the bearing and take a look at what you see along that bearing.

 

Now walk around what would be an imaginary circle of where the approximate location of the cache is, say about 90 degrees along the circle. Now take another bearing from the GPSr, and shoot that bearing with the compass.

 

The points where those two bearings intersect, whatever objects are there, that is your focal point.

 

Of course you may have to take additional bearings, due to signal bounce, reception difficulties or whatever, but this has been a great aid for finding caches.

 

In addition, having the compass gives you a non-electronic backup for finding your way in the event your GPSr fails for whatever reason.

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Been out Geocaching for the first time this morning.

 

Using the clues that were provided and the local geography, pretty sure I was in the right place on both occasions.

 

However on one cache my GPS said about 20-25 metres away from the given co-ords, on the second it was more like 60m.

 

My GPS was saying accurate to about 9-11 metres at each of these which makes me think the first could be possible, given a possible small inaccuracy of the GPS of the person who left the cache, but the 2nd seems too far out.

 

Off out now on my third cache, but wondered what I should expect...

My experience has been that if the cache placer has given accurate coords and your gpsr is otherwise reasonably accurate then you should expect to get within 15-20 feet consistiently. Sometimes I have been fortunate enough to get to within 10 feet more or less. One thing that I have observed is that there is a lag time for my gpsr to display the coordinate changes as I hike and depending upon how fast I am moving, I will pass the coords before the gpsr has had time to react. When you get to within about 50 feet, adjust your pace to match your gpsr's react time and also to scope out the likely hiding places. And of course, unless you locate the cache quickly, re-check those coords 'one more time'.

 

Cache on Dude!! ;-)

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This brings up an interesting idea. Maybe a hider should put the GPSr brand and model they used to place the cache.

 

That way, we could all complain about the Etrex users. :lol:

 

This weekend I had two finds from the same hider. On one, my GPSr showed 1 ft. On the other it showed 20 ft.

 

On the other two finds of the day my Sportrak showed <10 ft. (under trees, in the rain :( )

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RK lies. He likes to give Magellan users a bad time. The fact is that his Garmin and my Magellan were normally within 6 feet of each other. Sometimes mine was closer and sometimes his was closer. I think now that I use a Garmin he is having problems adjusting to not being able to give me a bad time about the GPS I am using. Now for the actual question. I have been as close as 1 foot when it says 1 foot away, and the farthest was over 1/2 mile because the hider had inverted some of his numbers. You have to take into account that error is multiplied because 2 GPS's are involved.

 

:lol:

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Accuracy can also depend on whether you have WAAS enabled. I have noticed a slight difference in how close I can get with my Etrex Legend with WAAS turned on, but the decreased battery life isn't worth it.

At times I've been within a foot or two, but usually it gets me within 20 feet and I'm pretty happy. When you get within this distance, it's time to start looking around for an obvious hiding place or something that looks out of place and don't rely on the GPSr.

I keep WAAS turned off most of the time.

I agree that if you are off by more than 20 meters or so maybe you should ask the owner to post new coordinates.

I've only run into this once, and some previous posts also noted an accuracy problem, and the cache owner gladly revisited the cache and posted revised coordinates.

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Since this is in the "Getting Started" section, let us be fair to all the noobs out there. All this talk about accuracy is generally just men thumping chests over who has the best GPS out there. In general, figure your unit will get you to about 20 feet of the position IF coverage is good, weather is good, view of sky is good, batteries are good, antenna position is good, tree cover is not thick, the planets are aligned and Elvis is verifying the reading.

 

Yes, WAAS helps. Much of what is said here is true, BUT there are too many variables to say that on any given day at any given location, this unit is going to be accurate to X feet. Common sense and some orienteering skills will always help.

 

One day (if it hasn't happened already) someone is gonna sue one of these companies because their GPS said they were 5.1m from the edge of the cliff when their husband/son/uncle/boyfriend/dog walked off the edge.

 

JMHO

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I'm quite a newbie but my experience has been that most of my finds (except 5 out of 29) were within less than 10 feet. My guess is that everyone's GPSrs pick up the same errors therefore the errors cancel out.

 

Of course if someone does careful averaging or use WAAS results, while it represents more true position, I would end up having less accuracy due to GPSr errors. Hope I'm making sense :rolleyes:

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