Expected GPS Accuracy

[ckotchey]

I'm very new to Geocaching. I have a Magellan Explorist 210.

Thus far I've only looked for 3 or 4 geocaches, and have only found 1, but admittedly didn't look overly long for several of them.

This past weekend, I went looking for a geocache called Stroll Over the Troll at a local park. It was a dark, very very clear night with lots of stars, and walking towards the target, the GPS was giving me an 'accuracy' of around 20-30 feet with something like 6-8 satellites connected. For 5-10 minutes we circled around the location (according to the GPS) which seemed to only indicate 2 bushes nearby. Because of the name of the geocache, however, we suspected that it might not really be in those 2 bushes, but near/under a nearby bridge (which we only looked at briefly, being so dark out), but this bridge was a good 90-100 feet away from where the GPS was pointing us. Later looking at the website for this cache, I still suspect the bridge is the actual area.

 

So as a newbie, I'm wondering - is this common for people? I have some trust in the reliability of my GPS as I've mapped tracks onto Google Earth maps, etc., and I did easily find one other geocache, but I find it odd that occasionally I seem to be seeking the wrong locations - or at least that's what I suspect.

 

Any advice is appreciated.

[+Renegade Knight]

10' often.

20' normally.

30' sometimes.

40' rarely.

50' time to wonder if the cache coords are messed up.

 

The above is good for your GPS accuracy, and how far off your GPS will report when you find the cache.

 

If the clue says "under the bridge" and you are standing in a field. It's time to check the coords you keyed into the GPS.

Edited August 28, 2008 by Renegade Knight

[+briansnat]

Don't forget that there are two GPS units involved with a cache, yours and that of the hiders. Even if you had a $10,000 commercial grade unit with pinpoint accuracy, you can still be 30+ feet off thanks to the inaccuracy of the hider's GPS. When you add the two unit's inaccuracy together coords could be significantly off from what your GPS says. In those cases you have to look for other clues. I think in your case the cache name would lead me to concentrate my search around the bridge regardless of what the GPS said.

[+The Blorenges]

This can go to amazing technical depths if you want (2drms and HDOP ).

 

Basic GPS units should have an accuracy of better than 30 feet for 95% of the time which means for 5% of the time it could be worse (or better)... So if your GPS says it has an accuracy of 15 feet it is not necessarily true. This is because the signal can be degraded by ionospheric disturbance (upper atmosphere) and things like trees and buildings.

 

A simple guide is, if the accuracy figure is about 15 to 20 feet you are doing well. With correction from the WAAS satellites (EGNOS in Europe) the accuracy should be improved to better than 10 feet for 99% of the time.

 

But remember, the person who set the cache may have had an accuracy of 30 feet. You also have 30 feet when you come to find it so you could be within 30 feet or as much as 60 feet away!

 

This is where your cachers eye comes in and the hints on cache pages (if you want to read them). You know you are pretty close and that's when you start looking!

 

Chris (MrB).

[+Harry Dolphin]

And, the cache owner might not have had good coords. (Not everyone has the advantage of using my QC department...) I expect to find the cache within 30 feet, but fifty feet is not unusual. More than that, my QC department awards two paws down. The worst was 170 feet off (not counting the typographical error of .18 mile off.)

[+rambrush]

I have one cache that is driving me crazy as well. I go in and it advises its in a bush, at this point I start a grid search radiating out from that point. Usually that works. my GPS is usually at the 10' mark but will run out to 20-30 at times. So ckotchey you are not the only one not locating some it goes with the territory.

 

One suggestion I have for ckotchey is to go look at caching supplies and see what some of them small micros look like. If it is a larger cache those are usually easier to locate and suggested for starting out. Once you master those then start get challenged.

[+root1657]

I'm very new to Geocaching. I have a Magellan Explorist 210.

Thus far I've only looked for 3 or 4 geocaches, and have only found 1, but admittedly didn't look overly long for several of them.

This past weekend, I went looking for a geocache called Stroll Over the Troll at a local park. It was a dark, very very clear night with lots of stars, and walking towards the target, the GPS was giving me an 'accuracy' of around 20-30 feet with something like 6-8 satellites connected. For 5-10 minutes we circled around the location (according to the GPS) which seemed to only indicate 2 bushes nearby. Because of the name of the geocache, however, we suspected that it might not really be in those 2 bushes, but near/under a nearby bridge (which we only looked at briefly, being so dark out), but this bridge was a good 90-100 feet away from where the GPS was pointing us. Later looking at the website for this cache, I still suspect the bridge is the actual area.

 

So as a newbie, I'm wondering - is this common for people? I have some trust in the reliability of my GPS as I've mapped tracks onto Google Earth maps, etc., and I did easily find one other geocache, but I find it odd that occasionally I seem to be seeking the wrong locations - or at least that's what I suspect.

 

Any advice is appreciated.

 

I have a piece of golden advice that I think is going to change the whole sport for you. Yuo say you are 'brand new' to caching, so I'd suggest you check to make sure your GPSr is set to use the same coordinate system the site uses. Being on a different system might cause exactly what you say with the bridge issue, and if it's consistantly putting you way off like that, it's worth checking. you want it set to WGS84 and hddd mm.mmm to be correct. there are lots of other ays to measure the earth, but if you want to use the coords from the geocahing site, that's the one you want.

 

Further, the nem might be related to the bridge cause they wanted you to come see the bridge. Dont get target fixation on a name...

 

If you GPS gets you to within 20 feet of the cache, consider it dead on. This is minor variance that ould be the difference in your gps being a fet feet off one day and the hider off the other way the other day. Add in a little distortion or echo, and it's amazing the darn things work at all sometimes!

 

Check your settings, and go try a couple of 1/1 difficulty caches to check it out.

[+root1657]

oops, double posted somehow... darn interwebs

Edited August 29, 2008 by root1657

[+TropicalParadise]

This seems relevant to what I wanted to make a topic about, so I'll just post it here: I'm having trouble getting good accuracy for caches now because of overhead trees. Are there any tips or anything that can help me avoid the three DNFs I got today because of trees?

[+rambrush]

This seems relevant to what I wanted to make a topic about, so I'll just post it here: I'm having trouble getting good accuracy for caches now because of overhead trees. Are there any tips or anything that can help me avoid the three DNFs I got today because of trees?

1st question is which GPS are you using? the newer delorme and garmins usually work inside buildings they have great anttenas. I used my Delorme under trees in Montana with no problems. are you using WaSS or ? your unit maybe searching for them and not locating may have to turn that option off if not able to find them.

[+The Blorenges]

Trees.... couple of things to be aware of. The Etrex series have an antenna that is flat and horizontal in the unit which means you hold it flat in your hand (with the screen facing up). The GPS with a short 'stub' antenna (like the Map series and Colorado have a helix antenna so the unit should be held vertically. It can make a massive difference in signal strength if you get it wrong particularly if the signal is weak anyway under trees. Always a good idea to check the satelite screen to see how many satellites you have 'locked on' and what the accuracy is. You may not be able to do anything about it under trees but at least you know you are way off.

 

Chris (MrB)

[+briansnat]

This seems relevant to what I wanted to make a topic about, so I'll just post it here: I'm having trouble getting good accuracy for caches now because of overhead trees. Are there any tips or anything that can help me avoid the three DNFs I got today because of trees?

 

Some older units, particularly the older eTrex units ike the Legend and Vista, are not as good under trees as the newer units. They can still get reception but they tend to be sensitive to position. Units with a quad helix antenna get the best reception held vertically (e.g. Garmin Map 76 and 60 series, Magellan Sportrak and Meridian) while units with a patch antenna work best held flat, face up to the sky (e.g. all Garmin eTrex, Garmin Geko, Magellan eXplorist and Lowrance iFinder).

 

If you are having reception difficulty you need to hold your unit in the optimum position. I know the older eTrex units have to be held flat all the time under trees. The instant you clip it to your belt, hang it around your neck or just hold it in your hand dangling at your side you will lose reception under trees. With a Map 60, Map 60CS and similar units you will need to hold it vertically to retain reception. The problem with these units is that the compass only works when the unit is held horizontally, so you may have to alternate the position of the unit between vertical and horizontal every few steps.

 

A second tip is to turn your unit on well before entering the woods (perhaps while driving to the trailhead. Make sure it knows where the sats are, so if it does lose reception it can recover quickly.

 

A third but more expensive option is to buy a unit with better reception like the newer Garmins (with a CX, CSX or H in the model name), or a Lowrance iFinder. Older Magellans like the Meridian also do pretty well under trees.