GPS Error
Thanks Rob,
Actually, by now I have a pretty good handle on the GPS system (thanks to
people like yourself on various forums on CIS and a few publications). But I am
still learning.
I might change some of my methods of use though. When I finally decided to buy
a unit the thought was to use it–take readings and record them. Now, I am not
as impressed about the accurracy and will probably make note of the PDOP value
(or what similar data the Trimble actually gives me–don't recall at the
moment). The way I have been using it is to just record the lat/lon in the
field and put that in my notes.
From what I've seen of the readings though, they are nowhere near as accurate
as your stated +-100 feet of a topo map. Doing geology work, many readings are
in canyons or on steep mountainsides where at least one direction of the view
to the sky is obscured, but those readings taken in more moderate terrain
haven't been reproduceable to within 100-200 feet either. As an example, last
summer, a friend impressed me with his 3 year old Garmin as we drove to one
site. He read off the lat/lon from his notes and as we drove up the Garmin came
up with exactly that reading (to the thousandth of a minute I believe it was).
He turned it off, but I asked him to turn it back on and I got out of his truck
and took a reading with my Trimble, which gave a reading about 0.2 minute
different from his (which I believe is roughly 1,200 feet lat), he turned his
on and it now read about 0.12 minute greater (closer to my reading). And that
was within just a few minutes of the original reading. This was in a broad
canyon, but there still was enough rock walls and hillsides to mess things up I
suppose.
Lanny