Re: [Paddlewise] Garmin eTrex GPS

>I wrote the previous paragraph last night but held off posting because I
>had taken about a dozen positions on some property this weekend.    So last
>night I tried to plot the positions on a 1:24000 topo map and the map is
>off anywhere from 200-300 meters both north to south and east to west.
>This is similar to what I seen on another topomap a few years ago.  I'm
>very familiar with the topography of the land, and I can very easily guess
>property boundries from ground features.  But the map features and the GPS
>positions don't line up.  Since I was walking in straight lines along a
>marked survey line I have confidence that the GPS numbers are accurate.
>They certainly are consistent.  Soooo, either the map is wrong or I can not
>plot points on a map........  8-)
>
        Here's a VERY important question:   Did you match your GPS reference
datum to that of the map?  Many GPS units default to WGS84, but most topos
are referenced to NAD27.  That alone can produce a 200-300 meter error.
        Here in NJ I routinely come within 100 feet of the correct position
using UTM coordinates.  If I take the time to get a few readings per
station, I get a much better rms error.  That will get me within a pencil
line drawn on the topo.  That's good enough.  In one instance where I had to
do better, I borrowed a DGPS setup that got me within one meter.
Joe P.

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