What would cause really inaccurate coordinates

[+Mr.Benchmark]

I've been looking for a cache for the past couple of months - I finally found it on sunday. On one of my attempts at this cache, I realized that the coordinates placed it more than 0.1 miles behind a fence that was posted "NO TRESPASSING". I conferred with the cache owner, showed him some aerial photos, and he agreed that his coords were BADLY off. He went back out, measured them again, and found that yes, indeed, his coords were off by nearly 700 feet! So he gets the approver to edit the coords.

 

I try the cache again on sunday, and get to ground zero, and it is obvious the coords are still WAY off. (It was obvious - the cache was near an abandoned structure, and at GZ, there was NO structure.) Given some ideas from the CO, and some research on my part, I found the structure about 200 feet away from the posted coords, and ultimately found the cache.

 

I suppose it's possible that my GPS is inaccurate somehow (I am using a garmin montana 650), although I've checked it on, I dunno, about 100 geocaches and dozens of other waypoints I'd marked on my prior GPS. For that matter, I've compared the montana with my old 60csx, and they generally agree very closely. Still, anything is possible - but I'm pretty sure my GPS isn't off by hundreds of feet. Near (within 150' or so) the cache in question is a tiny cemetery that's marked in the GNIS - the coordinates I marked for it agree to within about 40' of what's recorded in the GNIS, which seems in the ballpark, so I think my GPSr is working reasonably well. (The coordinates for the old cemetery are almost certainly scaled, and even if they aren't, there is NO TELLING whether or not I measured in the same spot as the surveyor who recorded the site.)

 

So my question is this - "what could this guy be doing wrong to cause such enormous errors?" or "what could cause such an enormous discrepancy between our GPSrs?" I can imagine that on the day the CO hid his cache, perhaps he just had the worst possible arrangement of GPS satellites, or some intermittent type of interference. But having such large errors twice suggests operator error to me. (Perhaps I'm wrong - or even if I'm not, I'd still like to help the guy out and suggest what he might be doing wrong.) Or, if somehow I am making a gross error of some sort, I'd like to figure out what that is and quit doing it! (I don't think I am making an error - but we are dealing with two imprecise measurements here - there is always the possibility that it is an error on my part or failure of my equipment.)

 

The area where the cache was located is wooded. I would call it moderately wooded - there are lots of trees (that's why I couldn't see the structure from 200' away until I was much closer), but it is by no means the densest woods I've seen. The CO's GPS is some type of Nuvi - I'm not sure which one. Mine is a Montana 650. I've seen cases where buildings and stuff caused multipath errors that had me walk 75' away from the cache before things settled down. But the differences here (700' and 200') seem really big, so I am wondering if there is some other source of error possible - operator or otherwise. Could he somehow have the wrong coordinate datum selected, would that do it?

 

Sorry for the long post - wanted to give as much background as I could.

 

TLDR version: What could cause GPS coordinate errors of hundreds of feet on more than one occasion?

[Grasscatcher]

I would guess that someone has (or had) a datum set incorrectly....either you or the cache owner...and.... it's either now or whenever the coordinates were input into the gps.

 

Do the GC coordinates agree favorably with the "actual" location on the aerial photos?

[+Mr.Benchmark]

I would guess that someone has (or had) a datum set incorrectly....either you or the cache owner...and.... it's either now or whenever the coordinates were input into the gps.

 

Do the GC coordinates agree favorably with the "actual" location on the aerial photos?

 

Yes, the GPS datum was my thought as well. I checked - my GPS is set for WGS84. I'll ask the CO about it.

 

The aerials don't show the structure, which is unfortunate. (It is lost in the trees - heck, I couldn't see it from the ground from a surprisingly close distance.) However, plotting the posted GC coords on a map, it is shown slightly NW of the little cemetery (either with my coords or the ones from the GNIS). However, my GPS places the cache 150' SE of the cemetery, and I can tell you from pacing it off with a compass, that is way closer to reality than 40' NW - it is definitely SE of the cemetery. So the coordintes I measured seem closer to reality.

[+Mr.Benchmark]

Well, I tried converting the coordinates for the cache from nad27 -> WGS83. The difference is only about 26 meters - not enough, and it's in exactly the wrong direction.

[+PokerLuck]

It seems a pretty large error for this to be the problem, but check that both of you have WAAS turned on. Raw GPS locations can be 100 meters off in certain circumstances. WAAS uses the locations of known installations around the US to send corrections to the GPS signal, which should in most cases bring the accuracy down to 10 or 20 feet.

 

By the way, whenever you perform an upgrace of a Garmin device, it turns WAAS off. I've been caught by that before, once when I put out a series of 25 caches. I didn't realize that it was turned off until I was done placing and measuring the caches. I had to do them all over again, and the corrections were around 20-40 feet.

[+GeoTrekker26]

 

By the way, whenever you perform an upgrace of a Garmin device, it turns WAAS off. I've been caught by that before, once when I put out a series of 25 caches. I didn't realize that it was turned off until I was done placing and measuring the caches. I had to do them all over again, and the corrections were around 20-40 feet.

 

This must be something Garmin has changed with the new eTrex line. I upgraded my eTrex 20 to firmware version 2.70 yesterday and just checked to confirm that WAAS is still on. It was on before and after the upgrade. That said, it is still a good idea to check all you system settings after doing an upgrade.

[Grasscatcher]

Go back to "square one"....

Now that you have found the cache, go back and completely start over.

Don't "Assume" ANYTHING.

Verify your GPS datum is set correctly, and go back to cache and get a good "averaged" and verified set of coordinates.

Also verify that any coordinates determined are interpreted correctly.(ie decimal coordinates vs DMS)

(that could also possibly be the original problem)

Verify,verify, verify with what is "actual" and work with only "your" data..

 

If you are working with any paper maps, verify it's datum also.

 

Very likely, something (datum)is getting "double converted" or a coordinate format is being misinterpreted, but by starting with the answer to the puzzle you should be able to work backwards and figure it out. Good Luck.....

[+coachstahly]

How many hides does the CO have? How new is s/he? Did the CO possibly use a phone to get the coords instead of a GPSr?