Gpsr =lousy Tape Measure

[cavecom]

After placing a multistage cache I was suprised to find veteran gps users having dificulty getting their gpsrs to acuratly show the correct distance. I had actually run the measurement off a 100' reel measure to 400 or so feet. While setting the cache up I checked the distance on my gpsr and it said 500 or so feet. I knew the tape was right so I didn't think too much about it. To my suprise 3 other cachers using 3 different units had projected the distance and were also off by 100'. Is this particular distance a problem for anyone else?

[+Renegade Knight]

They should only be the typical 20' off. However if they project the angle wrong, they could be 100' off.

[+embra]

Or add start position error (including your own possible error incurred in placement, destination error, and bearing error, and 100' seems possible.

 

Kudos for using the tape...that cuts down on even more error possibility. I haven't looked for many of this type offset, so I can't speak from experience as to what is typical in terms of how well they work. Theoretically, it would seem ok to get a searcher to a general area, but something well hidden or lacking a good hint could be elusive.

[cavecom]

There shouldn't have been a 100' foot error on four receivers at three different times from a point that has almost no obstruction to the sattelites. They all found the starting point with an acceptable 10' margin. Even when shooting backwards to the starting point there was a 100' discrepancy even though I retaped the distance to confirm. There just seems to be a distance near where the gpsr switchs from tenths of a mile to feet that they have this problem.

[+Night Stalker]

Did you advise the prospective finder if you were using magnetic or true north? this could be part of the problem.

[+EScout]

Night Stalker makes a good point. This is the first thing to check. Two of my caches use projections, one at 427 feet and one at 527 feet (yes this was on purpose.) I checked them many times, with different GPSrs, and projected back to the starting point, and they were always close, within the margin of error. Of course I have never taped them.

Do this: Create a route, and enter each waypoint. It will then tell you the distance and bearing from one to the other. The bearing will be True or magnetic as you have set your North Reference.

[cavecom]

The problem wasn't with the compass projection, all units lined up on the same heading, the difference was the distance measured. We all had different models of Garmins. I beleive it was a Legend, a base model E-trex, a GPS60C, and my own Vista. Even if there was a difference between true and magnetic North the declination here would not account for a 20 percent error. We have come to believe that a multi-billion dollar system has to be better than compass and pace but there may be a flaw in Garmin's distance measuring program. Check it out for yourself over a measured distance (not by the GPSr) between 4-500 feet and see if it's accurate. You might be suprised at the result.

[+as77]

Did you try to calculate the distance from the coordinates of the two points? There are many web pages that do this, e.g. http://jan.ucc.nau.edu/~cvm/latlongdist.html

[+Hard Oiler]

If it's 400 feet up or down hill then the GPSr will always read less than a tape. The extreme case would be 400 ft vertical in which case the GPSr will measure 0 ft since the coordinates at the top and the bottom are the same

[+IV_Warrior]

I had actually run the measurement off a 100' reel measure to 400 or so feet. While setting the cache up I checked the distance on my gpsr and it said 500 or so feet. I knew the tape was right so I didn't think too much about it.

Am I the only one who thinks perhaps you SHOULD have thought about it a bit more?

[cavecom]

I had actually run the measurement off a 100' reel measure to 400 or so feet. While setting the cache up I checked the distance on my gpsr and it said 500 or so feet. I knew the tape was right so I didn't think too much about it.

Am I the only one who thinks perhaps you SHOULD have thought about it a bit more?

Ok, maybe I should have thought about it more but the outcome is still the same- 450 feet on an almost deadlevel line of sight taped distance is still 450 feet no matter what. Forget about the true vertical distance, it dosn't matter here. My point is the assumption that the technology trumps the tape masure is (temptation to use all caps) completely wrong. While I thought it odd that the measurements didn't match do you suggest giving erronious info just because that is how a GPSr measures? For myself, I'm going to chalk it up to things you can and cannot expect a GPSr to do well. And my tape measure will continue to be my trusted rule of measure.