> Currently, I'm guessing that I and a student can have reasonable success
> operating a Garmin GPS II Plus with a little practice. I'm a little
> reluctant to get a unit with fewer features or less quality because we'd
> want our data on stream features and bird nest sites, etc., to be credible
> and verifiable, and we'd want to be able to transfer data to a classroom
> computer. But I haven't had any experience with this unit or any of the others.

Al Gore would be proud, hearing that you will be teaching GPS
hands-on to fourth graders.

I use GPS a lot, although I don't know that particular Garmin. I am
assuming you are talking about using a single GPS receiver without
applying differential corrections. In that case you really should
make your choice based on features you like, ease of reading in the
field, toughness, and battery life. I would not rely on a marine
receiver in the woods, because they can cut corners using "channel
switching" routines that work fine in the open ocean but can make it
very slow to achieve sattelite lock in the woods, but other than that
it really makes little difference. By the way, several receivers
transfer data to computers (mine was made in 1994 and it handles
all the chores the new ones do quite easily)

I don't imagine Trimble still makes the Scoutmaster with Topo, but if
you can find one the "over and up" programming (which tells you that
your location is so many inches over and up from a corner of a
Quadrangle map) can be very useful in the field. I have used it to
find section corners on survey jobs, when the heat and bugs addle my
brain way beyond wanting to deal with longitude and latitude or with
UTM.

The real reason I wrote is because your letter says you want the data
to be credible and verifiable. Please ask yourself just how credible
and verifiable you mean. Remember that, no matter what the salesmen
say, you will never get a mapper to accept a single GPS receiver
location as being any closer than 100 meters horizontally and 140
meters vertically. That's a radius, not a diameter. Draw a few 100
meter radius circles on a Topo map and you will see they take in
quite a bit of territory. And if you try to go back to an earlier
location, where you have a GPSed Long-Lat, and your receiver tells
you that you are within 200 meters, then you are right on within the
expected accuracy of the instruments. I don't think you could
relocate a bird's nest from that sort of information alone. Its true
that sometimes the receivers do better than that-- but the problem is
you will never know unless you have some independent means of
checking the work (for instance-- you know you were 200 feet east of
a road intersection and that falls within your circle). I have often
found that by the time I do the independent check I could have done
without the GPS receiver in the first place.

The only way to consistently improve on that accuracy is to apply a
differential correction. It makes no matter how many channels the
receiver can get, or how "good" it is, differential corretion is the
only way to correct for Selective Availability and for atmospheric
interference. One final point-- using a GPS receiver in the woods,
even with differential correction, requires a lot of training and
planning. The problem is that even though the receiver may be picking
up satellite signals in the trees, they are getting delayed and
reflected by the obstructions. The receiver doesn't know this, and
computes its position "thinking" it is picking up the signals with no
obstacle caused interference (this is called "multipath" error, and
is the same reason you shouldn't use one of these tools in a building
or in a car, without an outside antenna). I have found multipath can
cause errors of up to a quarter mile. Its also the reason these
things give you sattelite "maps", so you can know which sattelite is
has a clear line of sight and, through sophisticated software,you can
tell it to reject questionable signals.

This is not meant to discourage you, only to suggest that you
approach your mapping realistically, from the start. First of all,
only use the GPS receiver in the open, then use compass and pacing to
get to the individual mapping point from there. Believe me, it works
better than using the GPS receiver in the woods. Also-- this is most
important-- have one of your students keep notes in a notebook. The
keyboards on all GPS receivers (even the $10000-$20000 ones) are a
pain to use, and I would suspect that students will get very tired of
trying to type in waypoint names while everone else is looking at the
bird. Note taking is the most under rated skill in mapping, and it
is the most important. By taking written notes the student will
learn to observe the features that they will miss by punching buttons
on the little box, and it will relieve a lot of frustration back in
the classroom when you try to plot your readings and make sense out
of them.

Jerry Broadus
P.O. Box 249
Puyallup, WA. 98371