GPS and GIS Mapping Part II

[Jim Beidle <beidlej@yahoo.com>]

I've been reading with avid interest the continuing
thread on GPS. While reading my own, I realized I
hadn't really explained the use of Geographical
Information Systems (GIS) very well. That may have
inadvertantly helped contribute to some confusion.

GIS takes input from virtually every where and
crunches it into an accurate map. Raw GPS data is just
one of the inputs, and, taken by itself, is not all
that accurate as many people have pointed out. GIS
uses corrected GPS markers along with existing surveys
of a dozen different kinds, and the technician sculpts
it into an accurate-to-the-centimeter graphical
representation of the ground. Nobody who does this
professionally relies soley on the kind of raw data
you get from a "cheap" GPS transceiver.

Before the government turned off Selective
Availability a couple of years ago, civilain GPS were
limited to about 50 to 100 meters accuracy, and that
even meant standing still in one spot. Since SA is now
a thing of the past, accuracy has improved to about 5
to 20 meters or so (Some sources still say it can be
as much as 50 meters off; I'm quoting the "official"
numbers). Five meters is around 17 feet; 20 meters is
more like 66 feet. That's still a pretty big circle,
and you can see why those logs people talk about are
inaccurate.

In just about every county in the US there is a GPS
"Base Station" that logs the position of that antenna
second by second. The antenna is in a known, surveyed
position similar to the fence post mentioned in one
ridecamp posting, but more accurate. Using the log
from the base station, the GIS technician can correct
his GPS data and create a survey that's extremely
accurate and reliable. When he plots this with
surveyor's data, Cors of Engineers data, forestry
surveys, aerial photography and any number of sources,
the technician produces his highly refined plot. As an
example, the City of Everett mapping department is
present using over 80 layers, including some based on
surveys conducted in the 1920's to produce accurate
maps. And it's an ongoing process.

My dream project isn't quite so ambitious. Once I get
started, I will probably not use more than twenty or
so layers of data for a given ride. My main info will
come from the local base station, USGS mapping info,
GPS waypoints generated by volunteers, compass
surveys, and whatever info I can cadge from the RM
after the ride. 

I don't propose this to replace marking with
engineer's tap and chalk lines. That's simple and
works too well. My intention is to provide an accurate
graphical history of a ride and make the resulting map
available to RM and to interested riders. I'm still in
the stages of assembling equipment and developing a
plan; I hadn't really intended to present it before I
had those small but critical needs met. It'll be a
while yet before I get started.

If all goes as I hope, a rider could have reasonably
accurate way points to plug into they're GPS, if they
choose. Then if they want to know how far in
straight-line terms they are from the next stream,
they can look at they're GPS and find out within 30
meters or so. They could, if they wanted and the horse
allowed, figure out where on the trail they are, and
how far they've come using an accurate map that the RM
could provide.

I'm looking forward to what Truman and other old GPS
hands have to share about GPS. Have fun and See ya on
the trails,

=====
Jim Beidle


Please reply to beidlej@yahoo.com

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