Affordable GPS systems

[Margaret K. Thayer]

Unless you're using differential GPS*, _don't_ expect to get accurate
elevation readings using a GPS unit.  As Mary Barkworth said, you can stand
in one place and watch your elevation rise and fall (I've seen changes of +
and - 100m, sometimes oscillating around the true elevation).  For ordinary
single-unit, civilian-available GPS use, the best accuracy you can count on
is +/- 100m for latitude and longitude, which is fine for many purposes,
such as general site location.  The accuracy for elevation is distinctly
_worse_ than that (I don't have a figure handy), and error of >100m in
elevation is probably acceptable for fewer purposes.  Like Mary Barkworth, I
carry an altimeter along, too (of course, you have to keep calibrating
those, as well).

*Doing differential GPS, for very accurate results, requires add-ons to a
stand-alone unit and/or a more expensive GPS unit _as well as_
simultaneously recorded (or transmitted) data from a second (fixed and
precisely known) location.

As someone pointed out earlier, it's especially important if you're using
GPS readings together with detailed maps to take into account the map datum
(model of the earth's curvature) that was used by each (preferably the
_same_ datum).  Different datums are better for (or in many cases designed
for) different parts of the world, though there are several general-purpose
ones in use for the whole world; I assume each of these has better and worse
parts of the globe.  The GPS's I've seen let you choose among numerous built
in map datums.  If you plan ahead of time to plot your GPS points on a
particular topographic map series, check to see what map datum it uses and
set your GPS for that one if you can.

The Garmin 75 I've been using for about 3 years (ca. US$600 then) has
performed well, though its receiver got out of tune (or something like that)
after about 2 years and needed to be recalibrated.  Since this problem
resulted in its not getting signals from _any_ satellites, I wound up with
no data (and quite aware of it!) rather than inaccurate data (and
potentially unaware of it).  After recalibration, it worked fine again.  The
service contract it came with (provided by the local distributor) included a
free recalibration within 18 months of purchase, which I had not taken
advantage of.  On the first trip I used it, the only place I had real
trouble getting a fix was in a rather dense grove of Coast Redwood trees in
California - even there, by moving around a bit I finally got one.

I've heard anecdotal reports that really inexpensive units (in the sub-$300
range) tend to have more trouble finding and/or keeping "hold" of
satellites, but no experience with them.

Margaret Thayer

P.S.  This is about all I know about map datums.  And, yes, the plural is
datums, not data.

Margaret K. Thayer      mthayer at fmnh.org  [note slight change of e-address]
Adjunct Curator  - Zoology, Insects
Field Museum of Natural History                http://www.fmnh.org
Chicago IL 60605-2496, USA
tel. 312-922-9410, ext. 838   fax 312-663-5397
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Coleopterists Society: http://www.auburn.edu/beetles/