GPS Error
Lanny,
Like I said in my message, the precision of your readings is very much
dependent on the satellite geometry. For example, if at a particular time all
of the satellites you are observing are relatively close together in the sky,
your position is very weakly determined. On the other hand, if the satellites
are "well" spaced, your position will be known to a greater degree of
certainty. If you can, you should record the PDOP value reported by the
reciever. This will give you an indication of the "goodness" of the satellite
geometry. I don't know how familiar you are with the GPS, so forgive me if am
telling you something you already know. The satellite geometry is constanly
chaging, so the precision of your readings is also constantly changing.
The differences in readings between recievers does not make alot of sense, but
it is probably a function of the method of computation or quality of the
reciever. The computations involved are rather lengtly, and there are several
corrections that must be made for atomospheric effects, and the methods used by
different recievers is very likely different. Some navigational recievers may
not even account for these sources of error. Also, the satellite ephemeris
being used by one reciever may be a newer or older one than that used by the
other (the ephemeris is the data sent by the satellites that allows the
reciever to calculate the satellites' position, and it is updated periodically)
With regards to making measurements from 15 min quad maps: Keep in mind that
the map standards dictate that positions plotted at that scale only have to be
correct to within about +- 100 feet, and elevations to within +- 1/2 of the
contour interval.
And with regards to errors in canyons: While multipath may be a problem there,
keep in mind that the canyon walls are probably blocking your view of several
satellites, thus reducing your precision.
It is very difficult to explain errors of several thousand feet. The figures
for DOD "guaranteed" precision of +- 100 metres is from their publications.
The 20 metres due to multipath is from the book referenced in the previous
message.
FYI: Using GPS for surveying work using geodetic recievers, we can routinely
get precisions in our measured baselines of +- a few centimeters (repeatable).
This is a somewhat more involved process requiring longer observation times and
post processing of the observations, but it does illustrate the precison
possible with the system.
Let me know if I can try to answer any questions–it really helps me to
reinforce what I have learned!
Happy Holidays!
Rob