GPS accuracy question....

[FastEddy454]

Hey everybody. I'm new to GPS and geocaching and have a question about the accuracy (ie. how close you are to the given coordinates) I got an eTrex Legend for Father's Day and finally got a chance to use it yesterday. Before I set out I loaded three relatively close together caches for me to try and find. It seems that when I would get close to the coordinates, say the GPS was telling me it was 3 ft. in front of me, the arrow would start pointing in all directions, and giving me all kinds of remaining distances and directions. My first two caches were busts, I couldn't find 'em. I actually managed to find the third (under some very heavy tree cover I might add!) The GPS seemed to lead me right to this one, I stood right in front of it and the unit said I was 3 ft away. While I was on the trail I marked some waypoints to go back to later to check the accuracy against some known landmarks. The landmark waypoints, with me standing right on top of them, my GPS was telling me I had between 6 and 10 ft. to go to be right on the spot. So, my question is, is this normal accuracy, or should the given cache coordinates put me right on the spot marked? Or does the GPS just point me in the right direction, get me within 25 ft. and the actual cache site is up to me to find from there? Any insight or advice is appreciated! Ed.

[Stephen2]

Sounds normal to me. The GPS will get you there. The rest is up to you. In most all cases it will not lead you directly to the cache. It may be close, but usually you will have to search for it.

 

Expect to search as far away as 30' from ground zero. Sometimes as far as 50' although that is very uncommon.

[+Miragee]

Welcome to the Forums!

 

Your GPSr doesn't have an electronic compass, so the behavior of the arrow is normal. The arrow will only point towards the cache when you are moving at a normal walking speed, or faster. Thirty feet is about the best you can expect, although there is one cacher here who must have the same GPSr I have because I always find his caches at the '0' distance. For other cache placers . . . well . . .

 

If you are having trouble finding the cache, and you can approach the area from different directions, that might help you pinpoint the location. Also, sometimes I'll look at the cache waypoint, remember the last three digits of the latitude and longitude, and then go to the Satellite Page and try to match up those numbers. That little trick has helped me find a couple of caches when the distance kept changing, and the arrow kept turning around . . .

 

Happy caching!

[+Markwell]

Right on the front page of my geo-page:

Aren't GPS Receivers extremely accurate since the release of Selective Availability? How is this challenging?

To answer the first question, in the grand scheme of things, yes they are extremely accurate. Usually down to about 30 feet. That means that my GPS could get me any where within a radius of 30 feet or rougly 2827½ ft². That's a pretty big area. Couple that with the fact that the hider's GPS could be off by that much and that's even more area to search (but not double). Then, people use devious methods to hide their caches. The GPS gets you to the grove in the forest. You have to find the exact hiding spot.

[FastEddy454]

Thanks alot folks! I really enjoyed looking for the caches even though I didn't find them. I'll definitely try the first two again.

[+StarBrand]

When the GPSr says about 20 feet or less to go - put it away and start looking for likely hiding spots.

 

[FastEddy454]

When the GPSr says about 20 feet or less to go - put it away and start looking for likely hiding spots.

 

 

Thanks!

 

Another question.... Are the more expensive models with external antenna, better chipset etc. more accurate or about the same?

[+briansnat]

Thanks alot folks! I really enjoyed looking for the caches even though I didn't find them. I'll definitely try the first two again.

 

The chief mistake novices make is expecting the cache right where the GPS says it is. In reality it could be 20, 40, 50 feet or more away.

[+Miragee]

When the GPSr says about 20 feet or less to go - put it away and start looking for likely hiding spots.

 

Thanks!

 

Another question.... Are the more expensive models with external antenna, better chipset etc. more accurate or about the same?

They are basically the same. The only reason I upgraded from my monochrome Vista to the Vista C was because I wanted the auto-routing it offered when I purchased and installed the City Navigator maps.

 

Edit to add appropriate quote . . .

Edited June 25, 2007 by Miragee

[FastEddy454]

They are basically the same. The only reason I upgraded from my monochrome Vista to the Vista C was because I wanted the auto-routing it offered when I purchased and installed the City Navigator maps.

 

Cool. How are the map programs? My Legend came in a package deal with Canadian Topo map software and a carrying case. (and it only cost me an extra $40!) I wanted to try just the GPS first before I dove into using map software. Do you recommend I put them on my GPS?

[+Miragee]

Your blue Legend only holds 8 MB of maps, but that should be adequate for your area, and for the Topo maps. I've read in these Forums that the Canadian Topo maps have auto-routing information. I'm not sure about that for the maps you got. At any rate, your GPSr cannot use that functionality. You would have to upgrade to a Legend C, Vista C (now discontinued) or any of the 'x' versions.

[+QuesterMark]

Thanks alot folks! I really enjoyed looking for the caches even though I didn't find them. I'll definitely try the first two again.

 

The chief mistake novices make is expecting the cache right where the GPS says it is. In reality it could be 20, 40, 50 feet or more away.

 

Look at it this way: You could have an error of, say, 15'.

The 'cacher that placed the cache could have an error of, say, the same: 15'.

 

That means your combined error could be anywhere within a radius of 15' from any point within a radius of 15'. from the cache. You might get it spot on. But probably not.

[+wayfarer222]

I've often found that the GPS will get me close, but inevitably the cache is located in thick tree cover. At that point, I'm relying on the look of likely places---and trambled down ground. I've been finding several caches that are extremely popular now that it's tourist season, some visited 2+ times a day. Look for the trambled down weeds and you're at least on the track of the last hunter.

 

Chris

[+rlridgeway]

The rule of thumb for recreational grade GPS accuracy as I was taught:

 

95% of the time GPS position will be within 3-12 meters of a "known" location (say, a survey monument). The other 5% of the time, the GPS positional error could be greater than that, sometimes by a whole lot. To make matters a little more confusing, the 5% will occasionally bunch together, giving a false impression of location. (There are industrial grade GPS setups that have sub-meter accuracy, but these cost thousands of dollars.)

 

Here is an example of normal readings while standing still:

The GPS in one instant may say a point is 5 feet in front, the next reading 15 feet behind, the next 27 feet to the left, the next 4 feet to the right. These readings would be with the 95 percentile. If I waited long enough I would get readings that could be 200 feet or even 1000 feet away (a part of the 5%).

 

As others have mentioned, most geocaches are not really at a "known" location. Their coordinates are usually obtained by using recreational grade GPS methods, so their coordinates will be off a little. Most geocache owners try to eliminate the wacky errors by taking several readings. Even so, in the best of circumstances, the owner's coordinates will be off a little and your coordinates are also off a little.

 

All that being said, most of the time GPS works great at getting me within about 50 feet of a geocache. If I am having trouble I try to determine if I am getting the 5% error or if the geocache is just well hid. Its usually just well hid. My last three caches read 8, 6 and 24 feet from actual locations.

 

Edited for clarity

Edited June 26, 2007 by rlridgeway

[+scavok]

I usually go by two basic rules...

 

When the "distance to destination" is less than the current accuracy reading it is a good point to start searching. I.e. if accuracy is 25ft on my GPSr and the cache is bouncing around at 20ft to go, then it is close enough.

 

Then I take into consideration what best accuracy I can get at the location. In the open mine usually gets 15ft, so I figure if that is my max possible error, and is also the max possible error of the hider, then I search in a 30 ft. radius from my zero out point. Poorer signal means more searching (like under trees n such).

 

Of course if the GPSr leads you to the middle of a parking lot and there is a lamp post 15 ft away, then you should probably check the lampost (: