GPS receivers

[George Dasher]

I just want to bellyache...

I went cave hunting in Grant County, WV this weekend. We located five caves, and used my GPS receiver to obtain the coordinates.

First cave: we couldn't find.

Second cave: The coordinates are ok.

Third cave: The coordinates are waaaaaay off.

Fourth cave: The receiver wouldn't acquire.

Fifth cave: Way off.

Sixth cave: Way off.


Too too many steep mountainsides and probably too much leafy foliage. So it is probably not the receiver's fault. But what a pain!!

I'm back to pulling the locations on the topo map. Grrrrrr!!!

[Realms]

Just out of curiosity, what Datum did you have your GPS set on?

Realms

[lenslover2003]

For what it's worth,I have a Magellan Meridian Gold GPS,and it's been great. It might not be the newest high-tech model out there,but it's tough,and really seems to get a good signal and location. If you are in the market for a GPS,I highly recommend it.

[GoHighGoDeep]

I've got a eTrex... cheap, and dependable... I've had problems with coords i've gotten from people before... there was one time in particular, where 2 different people gave me ocoords that were both .25 mile off...

It really depends on how the GPS coords were come by... If they were marked by someone with a GPS or determined by a grid system on a map... There's always the introduction of human error when using a map... its accurate, but you have to check to datum the map refernces, and make sure the math was done right (i've seen people do the math with 100 divisions, instead of 60)

[George Dasher]

We're still using the 1927 North American Datum. But it is starting to cause some problems because the rest of the state is moving to NAD83. Fortunately, I can still convert everything I need to NAD27, so I'm having no trouble with my GIS system.

"We" is defined as the West Virginia Speleological Survey.

I think my problem was that all the caves I was trying to locate are down in deep valleys. I need to find some caves up on the ridge (but that is, like, work, climbing those mountains). The location that was "waaaaay" off is in a deep sinkhole in the middle of a big open field; I sure didn't anticipate a problem there.

Fortunately, most of West Virginia has good close-in landmarks, so pulling the locations off the topo was not a problem. Just a little time consuming, and that made me want to bellyache yesterday.



Grrrrr!! I wish we could open up this "post a reply" window. I get soooo tired of these little "post it note" size windows.

[cavemanjonny]

I had amazingly good luck finding a cave the other weekend using my garmin gps60. I punched the coordinates for the cave and took off into the woods. Had to hike half a mile or so, in the dark, through poison ivy, saw briers, and stinging nettle. Walked right to the trash can sized entrance though! I was amazed! I usually have a hard time getting a signal in thick Georgia jungle. Worked flawlessly this time though. Must be my karma is getting better or something.

I use NAD27 as my datum almost invariably. Most of the topos for the area surrounding Chattanooga (where I live) are based on NAD27 (I think), so I assume any pre-gps TCS, GSS, ACS coordinates were found using topo's and are therefore NAD27 specific.

Does anyone have any idea as to how much NAD83/WGS84 differs from NAD27? Would it really be that dramatic? I mean, if my gps has +- 10m accuracy, would using the wrong datum throw me off any more than my gps already does?

[cob]

I think my problem was that all the caves I was trying to locate are down in deep valleys. I need to find some caves up on the ridge (but that is, like, work, climbing those mountains). The location that was "waaaaay" off is in a deep sinkhole in the middle of a big open field; I sure didn't anticipate a problem there.

George,

down here in Ozarkistan, our hollows tend to be deep (not as deep as yours, God I love W Va) and jungle like. GPS locations tend to be problematic, to the extent that the MSS wants GPS and PLS locations on all caves. That way, if they don't agree, Scott House can know there is a problem right away and can contact the person.

I am a little bit older and still highly distrustful of all things "computerized", so I do PSLs anyway (I do have an e-trex because the MSS and AACS both want those locations, and I know how to get that datum out of it, but that is all I know about it). I still remember taking Bill Elliot to a Shannon Co cave that MDC had been looking for for years. Their location had it down in the bottom of a hollow, it was in fact about halfway up the ridgeline. When we got there, he got real excited, did the GPS and plotted it on the map right there in the woods... putting it right back in the bottom of the hollow. I looked at it and said, "Uh, Bill, that's not right, you were just there... It's actually up here..." He got a little embarressed, mumbling something about a "quick and dirty calculation" and that he would fix it back at the office.

On the flip side, we found a new cave this past winter, did GPS and PLS locations which agreed dead on the money. This past wkend we went back to survey (500' of virgin cave and more to go ) We came at it from a different direction and used a buddies GPS to guide us there... in a driving rain storm, thru vegetation as thick as anything in Venezuala, and very steep terrain. Took us right to it.

Sometimes they do work, and I guess they have their uses.

tom