Gps Accuracy?

[+PastorChip]

I tried to find a couple of caches today. On one it was obviously in a certain spot about 40 ft. off of where my GPS was pointing.

 

ON the other my GPS was pointing at a certain bush, but I was not finding it. THen somewhat suddenly it started pointing to a different bush, but I didn't find it there either.

 

I am using the simple Garmin Etrex. Are these symptoms common?

 

Are the locations of a cache generally as exact as I am expecting?

 

Other caches have been very precise.

[+Chuy!]

Your Etrex' compass is not electronic - meaning to get an accurate reading you must be moving in a relative straight line for a certain distance to obtain an accurate reading.

 

Generally, once you're within 40 feet or so, it's time to set the GPS aside and start using your noggin.

[+Smukke]

Your eTrex, on the satellite page, will tell you about how accurate it is. In very few circumstances, mine has professed to be within 16 ft of accuracy. Mostly it is within 30-40-ish. Trees, rain and power lines can cause great fluctuations in the actual accuracy of the device. It may tell you it is accurate to 16 feet in the trees, but it is laughing at you when it says that.

 

Of my not-quite-60 caches, only 2 have been located in spots where the GPS actually showed I was within 5 feet of the cache. The trick is to figure out how big that 'radius of uncertainty' is for your unit, at a certain battery level (I do think battery level affects mine, but it could just be a psychological issue for the person holding the GPSr), under certain kinds of trees, etc.

 

Generally, the best plan is to get to 30-ish feet away and then, to paraphrase Chevy Chase in caddy Shack: "Be the cache." After a few, you learn to think like a cache. When that happens, you will also see hiding places wherever you go, and always carry small containers with little logbooks in 'em. It's a disease.

[+Jeep_Dog]

PastorChip, the Getting Started Guide here has a lot of good advice for new cachers.

 

Specifically, the gc.com guide advises:

 

- When you get close to the Geocache (within 300 feet, which is the length of a football field), make sure to check your GPS unit signal. Sometimes the signal will have an error between 25-200 feet. Don’t concentrate as much on the arrow as the distance decreasing, as you get closer to the site.

 

- For the last 30 feet, use a compass or direct your buddy in the direction of the cache. In some cases we’ve had good luck circling the site with the GPS unit to get a good area to search.

 

- The final 30-100 feet is the hardest. It helps to think like the person who hid the cache. If there are stumps around, investigate around the base. Check for a pile of rocks. Some stashes, especially in people-trafficked areas, are pretty ingeniously hidden, so it helps to know the container they used.

 

This advise was pretty much covered by those posting above me. I just wanted to interject the "official" advice.

 

Additionally, if you are getting very frustrated, spending a couple of hours caching with a more veteran cacher can pay huge dividends in learning techniques and observing "geosenses" applied to caching learned from hard lessons in the field. I've had friends and family that I introduced to geocaching go along with me to learn general techniques, and by the 5th cache they were applying my advice and finding a really difficult cache using common sense of "where whould *I* hide the cache in this area, whilst I was fumbling around and cursing the GPSr. Er, do as I teach, not as I do...

[+StarBrand]

Generally you have an error around 20 - 25 feet and so did the hider's GPS so anything within 50 feet or so of your Ground Zero is where you will find the cache. Most of the time it is far better and you only need to search with 20 foot or so.

 

BTW - I have no scientific data, but fresh batteries, in my experience, give better readings than nearly dead ones.

[+PastorChip]

Normally my readings have been pretty accurate. I walk around the spot before I start looking and when in close go more by location than the directional arrow. In most cases I would get the exact reading of the cache when holding my gps directly over it. In all previous cases the display showing my location made sense and read consistently.

 

But this time it suddenly decided I was .003 minutes off of where it said I was a moment before.

 

I guess it must have switched satellites or something?

 

I want to check some other things too when I get a chance like the datum.

[+JohnnyVegas]

Generally you have an error around 20 - 25 feet and so did the hider's GPS so anything within 50 feet or so of your Ground Zero is where you will find the cache. Most of the time it is far better and you only need to search with 20 foot or so.

 

BTW - I have no scientific data, but fresh batteries, in my experience, give better readings than nearly dead ones.

I have seen a problem when my batteries are very week ( almost dead) I have seen very large erros, in one case the error was 500 feet. On another occasion the arrow in my sport trak color wpild not settle down, the batteries were almost dead so I changed them and them the arrow stop jumping all over and it go me to the cache.

[+Super_Nate]

That canopy of trees blocking out the sun overtop of you while you are scratching your head will aid in the problem of poor reception with full battery power

[+Night Stalker]

Accuracy will vary with the amount of tree cover, but even then my GPS is almost always less then 20' from the cache and most of the time within 10'. With that being said there have been times when the accuracy error of my GPS combined with the accuracy error of the cache hider have combined to make the distance a lot greater. In fact there was one cache that was more than 1/2 mile off. If it hadn't been for an intrepid cacher who had actually found it and posted corrected coordinates I never would have found the cache. I still wonder how that person ever found the cache.

[+Thot]

But this time it suddenly decided I was .003 minutes off of where it said I was a moment before.

The finest resolution your GPSr can get is .001 minute, which is about 6 feet. Therefore .003 is 18 feet. There are any number of reasons why your unit could fluctuate 18 feet. It won't do it all the time, but it's not terribly unusual either.

[+Greymane]

I can tell you from experience that the Magellan SporTrak tends to start wandering quite a bit when the batteries get below 1/4 full. Always snaps right back after new batteries are installed.

[ArmyCrayzee]

I did lots of Land Navigation courses when I was in the military and I was surprised how hard it was to find exact points. That was without a GPS. I have trouble finding points even with a GPS in my fingers. I get to the point and make a small circle until I find it. Sometimes I'd find two points or even three on the courses but caches are not located that close together.

[+omegabane]

We're lucky to have accuracy as good as we do. If you read the history of Geocaching, you'll find that it started when the satellite signal degradation was stopped. Prior to that, accuracy wasn't much better than 200-300 feet at its best. Now you can reasonably expect accuracy of about 30-40 feet. Remember, that is an average accuracy, so sometimes you'll be a lot closer and sometimes you might be farther away.

 

It is also important to remember that the cache owner has posted coordinates that are only accurate within the 30 foot range at best. When you go to find a cache, you might get different readings in the same places.

 

I got lucky on my very first cache and my GPSr lead me DIRECTLY to the cache. Perhaps that spoiled me, because I started to expect the same accuracy from all of my other finds. I think I've only been put right on top of the cache one other time since then. But really, it's that 30 foot search radius that makes it really fun.

 

Good luck!

[+TeamAO]

I've had next to dead batteries and accuracy to 10 feet. Age may make a difference because the older the GPS you use the less likely it has WAAS and other accuracy aiding abilities.

 

Usually bad accuracy has to do with tree coverage, terrain, and cloud cover.