Re: GPS to calculate mileage

[BMcCrary27@aol.com]

In a message dated 12/4/98 2:24:31 PM Pacific Standard Time,
rides2far@juno.com writes:

<< What Truman said about blocked satellite visability by foliage et al
 is true. But I work with GPS every day, in a very specialized research
 mode not with hand held units, but I know how the system works. And
 as a self-proclaimed GPS expert I would not trust a GPS derived mileage
 measurement for a 50 unless it was on a straight flat road!  A handheld
 GPS measurement is accurate to about 100 meters.  The up and down 
 accuracy is only about 300 meters. Thats pretty good for telling  
 where you are relative to a topo map, but if you are trying to get
 accumulated distance you need to put in a "way point" every time your
 trail bends and every way point will have 100 meters of error.  So if
 anybody ever tries to tell me a trail distance is wrong based on GPS
 I'll tell them to prove it with a wheel measurement.  >>

Since we are unable to access much of our trails by vehicle, we have
calculated mileage by three differnet methods: 1) Odometer on truck, 2) A map
measuring device with a little wheel. You set the device to correspond to the
scale of the map and wheel the device over the trail on the map (USGS topo
map), then add 10% to the reading. We've done comparisons over and over and
the 10% compensates for the zig zags and changes in elevation. The wheel
measurement plus 10% addition comes out to equal the odometer reading over the
same portion of trail. The 3rd method uses the cyclometer on a mountain bike.
We did all 50 miles of one of our rides. Some of the trail was so steep and
rocky, we tied a rope onto the handle bars and I ponied the bike from
horseback while my husband steered. I would love to have had a photo of this
procedure. But it worked!!

BMc