Re: [Paddlewise] Garmin eTrex GPS

|        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.

Hmmm.  I'll have to check.  I know I changed the settings the last time I
was playing around with the GPS and
topomaps.  Did I changed it correctly is a good question!  8-)

I went to the USGS website and eventually to the terra server and found the
satalitte images for the property.
They overlay a grid that is pretty close to what the GPS was measuring at
least on a North-South line.  The east
west line is the one that was really off.

The image is from 1993 and is out of date.  There are now clear cuts on
each side of my property and this would
make it very easy to determine boundries from the images.  The county also
has images take from airplanes that
are very detailed.  They were supposed to have been on line by March at the
latest...  I was talking with the people
in the county GIS office early in the year and the images they have were
amazing.

The topomap I have is from 1981ish and I know it is not quite accurate.
Maybe 90% correct.  Course that may be all one really needs.

I bring all of this up for a couple of reasons.  The original question was
getting a GPS with a map display.  My two cents is that the accuracy of the
GPS might exceed the accuracy of the map.  This may or maynot be important
to a particular user.  They have to decide.  Then there are the issues of
batteries and not having a chart because the GPS just went belly up.

The sat images on line is very very very nice.  How did we live without
this stuff?

Thanks for the Nad27 hint.  I'll check it tonight if I get home early
enough.....
Dan




***************************************************************************
PaddleWise Paddling Mailing List - All postings copyright the author and not
to be reproduced outside PaddleWise without author's permission
Submissions:     paddlewise_at_lists.intelenet.net
Subscriptions:   paddlewise-request_at_lists.intelenet.net
Website:         http://www.paddlewise.net/
***************************************************************************