Tangential Topic - Hand held GPS units

[Rich Shepard]

On Fri, 22 Oct 1999, Agustin Lobo wrote:

> Look at: 
> 
> http://www.gps4fun.com/
> http://www.gps4fun.com/main_comp.html
> http://www.gps4fun.com/compare3.html

  There are several considerations in choosing a GPS for collecting data
which will be used in a GIS. I spent the first two days of this week on a
refresher course which came with the purchase of my new, high-precision GPS
receiver.

  All single-frequency (L1) GPS receivers, from the least expensive
Magellans or Garmins sold in the sporting goods or boating stores to the
most expensive sub-survey grade have the same inherent accuracy: 95% of the
time the position reading will be within 100 meters of its true position;
50% of the time the accuracy will be within 48 meters. This is because of
Selective Availability, the deliberate, pseudo-random degradation of the
satellite signals by the US Department of Defense.

  Achieving better accuracy costs money. What you get for that money are
several things: time, post-processing differential correction, attribute
collection and other useful things. For example, the $100-$500 units can be
tied to a radio receiver for real-time correction, but that's another big
expense. And, while some of these units have the ability to download (or,
more commonly, upload) positions (waypoints) in ASCII format, they cannot
download the raw data acquired from the satellites. This means that you
cannot process the data in a separate computer to correct the accuracy.
Also, you cannot collect information about your data while in the field and
have it in a digital format, associated with each position. I could go on,
but the main use of the inexpensive units (and we have one for the Palm
Pilot, too) is navigation. It will help you find your favorite duck hunting
position or your way out of the woods. And, for these uses, they provide the
best money.

  To get usable data for GIS input, especially for natural resource mapping,
you really need to spend more money. The units cost $3,000-$8,000 and they
allow you to collect feature attributes in the field as well as to
post-process the raw data to achieve 1-5 meter accuracy. Look to Corvallis
Microtechnology, Inc., Trimble, and Ashtech for these units. We used a CMT
March-IIe for about 3 years and it is plenty accurate for the work we do.
Except -- elevation readings.

  Calculation of position with a GPS receiver depends on a process called
trilateralization. It takes a minimum of three satellite signals (with good
geometry overhead) to determine a 2D position and a minimum of four
satellites to calculate a 3D position. The elevation/altitude reading
accuracy is generally 2-3 times less accurate than the horizontal position
reading because the former depends on the geoid model used in the
calculation. That is, while the 2D position is independent of the elevation,
the elevation is dependent on the mathematical model of the Earth's surface
at that point. This is the "gotcha'" with GPS data.

  To get usable altitude data requires a high-precision GPS system. That's
why I sold the March and bought a much more expensive unit. While we don't
need the 30 cm horizontal accuracy (95%, or 2RMS, of the time) for natural
resource features, we do need the 60-90 cm vertical accuracy to create DEMs
of our project sites. These units cost $8,000-$15,000 and come from the same
vendors as their slightly-less-accurate catalog mates. I don't know about
the rest of you, but I've not had sufficiently accurate data available to me
-- especially elevation data -- for any of my projects in the past 7 years.
Unless, that is, the client had the site flown and contours generated by
photogrammetry or they had surveyors map the site. Both of these options are
more costly than using GPS.

  Of course, you could go the route the surveyors do and get a pair of
survey-grade, dual frequency (L1 and L2) GPS systems. One is used as a local
base station on a know control point (a bench mark) within 10-20 km of the
project area, and it transmits a radio signal to the rover so that you
achieve (under the right conditions) a real-time, kinematic,
differentially-corrected position with a horizontal accuracy of 1-2 cm (and
vertical accuracy of < 6 cm). These systems cost $18,000 and up an are sold
by CMT, Trimble, Ashtech, Leica, Sokkia and others. This is great for legal
surveys, but overkill for natural resource work. Of course, you can achieve
the same degree of accuracy with one of the sub-survey grade systems, but it
means sitting on a point for up to 45 minutes (tracking the same 5
satellites continuously and with the base station tracking the same 5
satellites continuously) rather than for 30 seconds to 2 minutes. You pay a
big price for the time.

  Well, end of monograph. My point is to educate about the uses of different
GPS receivers. You can believe me when I write that if I could get the same
accuracy and functionality from a $500 GPS receiver, I would not have spent
almost $10,000 on one.

Rich

Dr. Richard B. Shepard, President

                       Applied Ecosystem Services, Inc. (TM)
              Making environmentally-responsible mining happen. (SM)         
                       --------------------------------
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 + 1 503-667-4517 (voice) | + 1 503-667-8863 (fax) | rshepard at appl-ecosys.com