How accurate are the listed coordinates?

[TheMrCollin]

Decided to take a few friends caching yesterday and we had a few DNFs (found 5/8). We decided to stay as close to ground zero as possible when searching (+/- the 16 ft., of course), but still couldn't find 3 that were previously listed as found by others.

TL;DR:

I'm just wondering if it's a good idea to stick specifically to the coordinates (+/- 16 ft.) when searching, or can those be off by a bit sometimes?

Any other tips would be appreciated. So far we're getting used to the 2.0 D/T and lower, so just getting our feet wet at this point.

Thanks.

[+Viajero Perdido]

Anywhere within 5-10m (15-30') seems to be common.  Your unit reports some possible inaccuracy - eg the 16' you mentioned - but the hider's unit would also have been a bit inaccurate, possibly adding to your own inaccuracy.  Or cancelling it out, you never know.

Most newer caches seem to be better for accuracy; the hider would likely have used a newer and more accurate unit.  I'd start by looking close to your GZ, then gradually searching a wider area if no luck.

[+cerberus1]

Civilian GPS is still only accurate to 10' on a best day. 

That means the CO would have to make it a "best day" to place, and you have to have a "best day" to search.

Tall buildings, rock, heavy tree cover, and many other things affect signal, making you pretty distant from that best day. 

Since I've started, when walking in the direction of a cache, I put the GPS away around 20' or so, further if in a higher-terrain area.

[+cerberus1]

Was curious what you meant by the +/-16' "of course", but I think Viajero Perdido figured it out. 

[+justintim1999]

Sometimes the coordinates are off, sometimes your gps doesn't have a good signal.   It's always good to take a look at the previous logs.  Many times a cacher will post alternate cords if the posted ones are way off.

In many cases the best thing to do is get as close to ground zero as you can and put the gps away.  Look around for places you'd hide a cache.  You'd be surprised how well this can work.

You may have to accept that the cache could be missing or It just isn't your day.  I can't tell you how many times I've gone back to a cache I couldn't find and have it in hand within minutes.

[+BCandMsKitty]

We almost always read the previous logs. Very often we will find logs where folks say they were 20, or 30 feet or more from GZ when they found the cache.

Just gives us an idea how far off we need to be in our search.

I went out with a new cacher once (a retired engineer) who was complaining about why hiders didn't get accurate coordinates. He thought that with modern GPS units that it must just be sloppiness on the part of the hider to not lead him right to the cache.

He never did get it (that the coordinates would very seldom be right on, and that one needs to actually hunt for the cache) and has dropped out of the game.

 

[+boisestate]

Coords can easily be off, even more so around buildings, canyons or tree coverage where satellite signals can get bouncy. If I find a cache that is significantly off the posted coords (40ft +) i will indicatte it in my log. I will also indicate in my logs when the coords are spot on.

Its good to put down the gps device when your'e near GZ. Take a look around and ask yourself if i was going to hide a cache here, where would i likely hide it.  If there are a ton of hidey spots creating difficulty then the cache D-rating should reflect this. Correctly reading the difficulty and terraiin become factors and there is always the chance that the cache is missing.

You willl find that some cachers can be notorious for having hides with bad coords, its actually kind of funny. "Hey this is a so-and-so hide...probably gonna be off 30+feet"  Ive always taken time to make sure my hides have good coords and even verify them on the map to make sure, it takes minimal effort.

Side related story: Last year i went for a FTF very late at night, it was around 3am. Spent about 45minutes at the posted GZ but did not find the cache, i was all over this persons yard looking in hedges and such with a flashlight. Other cachers had the same bad luck earlier in the night. Turned out that the coords were WAY off, a few miles, and the cache was actually in a park. The CO made an error and used wrong coords when publishing the cache, he needed the reviewer repost the cache with the correct coords and i was lucky enough to pull off the ftf the next day.  Not sure whose yard I was in that night but I'm super glad the police were not called. I thought it might be the CO's yard.

[+Z_Statman]

Here in Florida we have interference from a variety of sources, not to include mountains, and I find that the average 'error' is 6-10' which is pretty good. When undercover, in thick swamps or narrow canals my focus tends to look at the difference between 'distance to destination" and 'GPS Error,' this gets me pretty close and pretty consistently within 3-6'

Edited October 18, 2017 by Z_Statman

[TheMrCollin]

22 hours ago, cerberus1 said:

Was curious what you meant by the +/-16' "of course", but I think Viajero Perdido figured it out. 

Haha, I should've clarified. It refers to the clarification found in the app which says "[+/- 16 ft]" underneath the distance away from it. That's found on the screen after tapping the compass icon in the top right. I've actually seen that number fluctuate many times, but it seems to steady out around 16'.

[TheMrCollin]

Thank you all for the input. Went 5/6 yesterday, so I'm improving a little. Appreciate the help from you all. Thought boisestate's story was especially interesting... a good reminder for me to be careful 

[+NYPaddleCacher]

4 hours ago, Z_Statman said:

Here in Florida we have interference from a variety of sources, not to include mountains, and I find that the average 'error' is 6-10' which is pretty good. When undercover, in thick swamps or narrow canals my focus tends to look at the difference between 'distance to destination" and 'GPS Error,' this gets me pretty close and pretty consistently within 3-6'

I'd think that as flat as Florida is, there would be very little interference except from buildings in larger cities.  

Many of suggested putting away the GPS when you get close to GZ and I agree with that.  However,  "when you get close" is very much situation dependent.   I've got a cache that is in a tree in a field about 100' from a building.  There are no other trees in the field so it's very obvious where the cache hidden from well over 100' away (you still need to find the container though).   Although some of suggested putting the GPS away when you're 20' from GZ, I typically start looking for potential hiding spots when I've 50-100'.  I might check my GPS when I get closer if I'm not seeing anything obvious but sometimes I don't have to.   I also notice that I'll tend to "follow the GPS" a little longer,  when there are a lot of potential hiding spots, for example, a cache on a foot bridge or on a long metal fence or in a densely wooded area.  

[+hzoi]

20 hours ago, NYPaddleCacher said:

I'd think that as flat as Florida is, there would be very little interference except from buildings in larger cities.  

I thought coastal Virginia would be pretty good for the same reason, but signal reliability here seems to be terrible.

[+tetley glc]

The accuracy of GPS units varies a lot. A lot of people are caching using only a smartphone. These can be accurate on a good day with no trees, buildings, mountains cloud etc. But when the going gets tough, they are hopeless and they freeze and jump about all over the place, with an accuracy of a few hundred feet. A dedicated GPS unit will get a far more accurate fix than a smartphone, and a high end GPS unit will be even better, as they have better aerials,and can pick up all the latest satellite systems- there are several other independent systems out there (eg GLONASS, SBAS and GALILEO), not just GPS which cheaper units and smartphones use.

I have just upgraded to a Satmap Active 20, and the accuracy is very good in extreme conditions, far better than my old Active 10, and way better than my phone. I have tested all 3 side by side, the Active 20 locks onto loads more satellites, and will get a signal indoors where the A10 never did. Under dense tree and cloud cover the A20 gets a rock solid fix with no freezing and jumping, so really easy to get to the published coordinates. The problem is how good are the published coordinates? I've recently found a cache that was 300 feet out! This was only found because the FTF contacted the CO for a clue, and published their own coordinates in the log, which were good to 10 feet accuracy.

[+cerberus1]

1 hour ago, tetley glc said:

The accuracy of GPS units varies a lot. A lot of people are caching using only a smartphone. These can be accurate on a good day with no trees, buildings, mountains cloud etc. But when the going gets tough, they are hopeless and they freeze and jump about all over the place, with an accuracy of a few hundred feet. A dedicated GPS unit will get a far more accurate fix than a smartphone, and a high end GPS unit will be even better, as they have better aerials,and can pick up all the latest satellite systems- there are several other independent systems out there (eg GLONASS, SBAS and GALILEO), not just GPS which cheaper units and smartphones use.

I have just upgraded to a Satmap Active 20, and the accuracy is very good in extreme conditions, far better than my old Active 10, and way better than my phone. I have tested all 3 side by side, the Active 20 locks onto loads more satellites, and will get a signal indoors where the A10 never did. Under dense tree and cloud cover the A20 gets a rock solid fix with no freezing and jumping, so really easy to get to the published coordinates. The problem is how good are the published coordinates? I've recently found a cache that was 300 feet out! This was only found because the FTF contacted the CO for a clue, and published their own coordinates in the log, which were good to 10 feet accuracy.

I think many of us has seen newbie mistakes (and even some errors from folks here a while) where they slipped on coordinates, and fixed after the FTF ("beta tester").  Fortunately they don't happen often.  With civilian GPS still only averaging 10' on a perfect day, any issues with coordinates a bit off are usually factored in by the searcher's common sense.

I'm often surprised that the other 2/3rd's iphone (7)  is just as accurate as my hand held GPSr, with just an issue of battery usage, and sensitive touch screens with old hands being the factor for me.  Trees and clouds haven't been an issue since a iphone 5, so far her only issue is river gorges and in rocky/boulder areas high in iron content.  I expect the 8s and 10 to fare even better.     On handhelds, my long-discontinued Garmin 60cxs got signal in my basement, which her iphone does as well.   That's nothing new. 

Curious how easily your gizmo accepts PQs, since it's not able to access caches using the new send to Garmin feature (with only Garmin products).   Thanks.  

[+Rebore]

Information about the estimated position errot:

EPE

Quote

But then:  What does EPE mean on my GPS?  If EPE reads 10 feet,  then I am within 10 feet of the actual  lon/lat position readout on the GPS- Right???

DEFINITELY NOT!!  EPE is generally an ESTIMATE OF POSITION ERROR and not a GUARANTEE of maximum position error.  In fact,  Garmin's EPE readout is generally accepted to indicate that there is an EQUAL probability that the error is GREATER or LESS THAN the indicated EPE.  This is the 50% CEP value given above.  As shown above,  to be 95% confident that your measurement is within a circle of a fixed radius,  you would have to multiply Garmin's EPE value by two.  To be 98.9% sure that your measurement is within a circle of a fixed radius,  you would have to multiply Garmin's EPE value by about 2.55.   Magellan's EPE numbers appear to be even more optimistic (maybe the 1 sigma value or even lower)  while Lowrance seems to be someplace between the RMS and 2 sigma values.

In short.  Your GPS's EPE readout is just a "figure of merit".  It is NOT an indication that the given position readout is within "EPE" feet of absolute perfection.

Joe Mehaffey