Affordable GPS systems

[M.P.Davey]

I know that civilian GPS systems can be inaccurate but I know that the
military do give licensed access to the proper system. Does anyone know
how much these cost etc?




On Wed, 28 Jan 1998, Margaret K. Thayer wrote:

> 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
> ***********************************************************
> Coleopterists Society: http://www.auburn.edu/beetles/
>