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Re: GPS Units...




Dwight K. Souder wrote:


Greetings everyone. I was wondering if some of you could give me
some information on gps (global positioning system) units. I've been
looking at them for over a year. I do a lot of traveling/hiking and I
would also like to take it into school for demos (Actually, I'd also like
to have it because I get lost a lot. :-)).
I'm looking for something that not only shows latitude and
longitude, but also elevation, accurate, not too pricey, direction and
distance to a location, speed and time of arrival to destination, sunrise
and sunset, versatile, fairly easy to use, etc.
Can anyone recommend a brand? Things to look for? Suggestions?
Any information would be greatly appreciated.

Dwight
dsouder@juno.com

I use two Trimble GeoExplorer units (one in the field while the other record
the differential correction data) very frequently for mapping forests to
produce orienteering maps. These are much too expensive if you just want to
know where you are, but I can give some information about accuracy and
precision.

The "SA" error imposed by the US Defence people definitely still applies, so
no civilian unit will give better than 100 to 150 metres accuracy operating
by itself. (That's why I have to use 2 units to get the <5m error I require
for mapping). My basefile positions, recorded in optimum conditions over
several hours, remain wrong, typically by more than 50 metres due to SA.
Single reading excursions can be much more.

The 100 metres is tolerable if you are out hiking with a 1:25000 scale map.
Here the error translates to 4 mm, so the gps reading is still very useful.
NOT SO, HOWEVER FOR ALTITUDE! Uncorrected altitude readings can be wrong by
300 metres a lot of the time. For routefinding this is too much. I use my
old Swiss "Thommen" aneroid altimeter when I'm hiking.

The other thing you might want to know concerns use under tree cover. The
cheaper receivers use one physical channel which is time-multiplexed to read
the four satellite signals needed. This means that they will not work in
less than ideal conditions. If (like me) you want the receiver to continue
working with noisy signals under tree cover, you need a more expensive one
with several channels working in parallel.

Mark.

*************************************************************
Mark Sylvester
United World College of the Adriatic, Duino, Trieste, Italy.
msylvest@spin.it
tel: +39 40 3739 255
*************************************************************