Info on GPS

[Shueyi]

To add to the  GPS discussion.  We've been using a Trimble unit with post
processing capability for mark recapture studies of butterflies.  The results
have been excelent, and lat/long results are easily within 5m of the point of
data record (= initial point of capture).

Post processing adds to the work load, and we have to download the raw data
into a PC at the end of each work day.  Plus you have to have a source of post
processing data to use in conjunction with your field readings.  In my area,
Trimble data seems to be much more readily availible than other units
(magellan data is very hard to come by in the Midwest US the last time I
checked).  Later, we have to actually do the post processing in the office
(takes about 1hr per data dump).  So I don't get instant results in the field.
There are units with true-real time sub-meter acuracy - these usually cost
tens-of-thousands of dollars for the mobile unit (plus requires a base station
with a radio link to the mobile station) and are used mostly for land survey
work.  I don't know any biologists who can justify the expense for the added
accuracy (+ or - 2cm).

For getting lat/long/altitude, most manufacturers suggest that you take a
average reading of several hundred readings (my trimble takes about 4
reading/second).  The error around your "real location" is supposed to be
truely random, hence you should see jumps of up to 100m between individual
readings.  But because these points are randomly jumping around (within
certain limits) the true point, the average will increasingly resemble reality
as more individual samples are added to the database.  Most units have the
option of using a "sliding" average, which will calculate the last X readings
at any given moment.  So we just set the unit down while we work, and take a
look after five minutes or so.  These points always bring us back to within a
few meters of reality and always integrate easily with our other GIS
coverages.  Of course, working in the MIdwest, altitude is moot, and I have
never really checked that measurment.

John Shuey
Director of Conservation Science
Indiana Office of The Nature Conservancy