Re: GPS measurement

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• Maybe in reply to: Jim Mitchell: "GPS measurement"

Jim:

Referencing my recent post concerning the use of GPS measurements where-in I
mentioned the use of corrected data.

I went into some of my files and took a recent trail measurement (albeit
done with motorcycle) for a competitive trail out in the Owyhee desert.

The differentially corrected arc length, i.e. map measurement with out
taking elevation changes was 131567 lf (about 24.9 miles) the slope length,
i.e. using elevation differences was 134064 lf (or about 25.4 miles) in this
case only a half mile difference since there is not much elevation change.

The uncorrected measurements were as follows: arc length 159830 lf and the
slope distance was 205614 lf. or 30.3 miles and 38.9 miles.

Since we are interested in actual on the ground measurement and therefore
use the slope distance in out measurements we find that uncorrected distance
of 38.9 miles less the actual corrected distance of 25.4 miles leaves us
with a discrepancy of 13.5 miles we could be off.

This type of discrepancy could get a ride manager very upset if confronted
with inaccurate data. That is why I caution users to be very careful with
what they do using relatively new technology with out knowing all the
ramifications. Many of the hand held units are capable of very accurate
measurements when used in the proper manner BUT the supplier never goes into
detail as to what this is.

The measurements were taken with equipment capable of submeter accuracy. If
you are interested in the procedure for differentially correcting the data
please contact me and we can go into that or I can refer you to some online
pages that have the technical information.

If you are out hunting and want to find camp, some place within 100 meters
is fine but if you have a boundary dispute and need a property corner then
10 meters is unacceptable and 1 meter most times is not close enough.

!!!!I put this out as a request for caution. PLEASE BE SURE OF WHAT DATA YOU
ARE USING BEFORE COMMENTING ON YOUR MEASUREMENTS.!!!

I put this out as a personal request. Please keep investigating and
experimenting as that is the only way we will advance in the use of this
equipment.

Bob Morris
Morris Endurance Enterprises
Boise, ID

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• Maybe in reply to: Jim Mitchell: "GPS measurement"

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