Check it Out!
Re: RC: GPS and American River
Someone probably has an easier way of dealing with this problem than the
one I thought up, but I am glad to share my method (at the risk of
sounding dumb). Here goes:
For your "odometer" to work, your GPS must re-read its location every 2
seconds or so. It then calculates the distance travelled relative to the
previous reading and adjusts the "odometer" reading to reflect the
theoretical distance travelled. Most likely, at some point along the way
your GPS had a weak enough signal to misread your location by so many
miles. When it re-acquired a strong signal, it was able to read your
correct location again. It then added in the erroneous deviation (in
this case, about 4+ miles out and back) to your theoretical travelling
As trailmaster for the Norco Riverdance, I have had alot of practice in
mapping trails with a GPS. Out of necessity I have developed a (somewhat
cumbersome) system which accounts for this signal problem. I use the
"memorize location" utility on my GPS to lock into its memory what its
exact LAT/LON reading is about every 100 feet of travel down the trail.
I then use a mini-tape recorder to comment about that location.
(Example: "Landmark #128 is at the river crossing under the electric
wires, and with only 5 sattelites reading, the GPS estimates its margin
of error at 78 feet on this mark).
I then return to my desktop computer, and fire up my TOPO! mapping
program. After I key-in each memorized LAT/LON landmark (measured to
within one-one-thousandth of a degree of latitude and longditude), I
simply analyze the marks, ELIMINATE THE OBVIOUS LANDMARK ERRORS, and then
connect the remaining dots. The accurate readings fall in a logical
line; the wrong ones spike off into never-never land. I then ask the
computer to calculate the total distance marked, and print out a trail
map with mileages, my recorded comments, etc printed on the map.
Hope this helps you, should you decide to use your GPS to get serious in
mapping trails for you.
(SEE Y'ALL AT THE NORCO RIVERDANCE IN SEPTEMBER 2000!!!)
On Fri, 12 May 2000 13:13:31 -0700 "Dorothy Joiner"
> We rode the 30 mile ride at American River. Just for fun, we decided
> to take our GPS and set the odometer mode so that we would know
> exactly how far we'd traveled. The GPS and the mileage map were
> almost exactly the same...until the last 10 miles. At that point,
> according to the GPS, we had gone about 2 miles further than where
> they said that check was (Rattlesnake Bar). We chocked that up to
> going off the trail, down to the water, etc.
> However, when we got to the end of the ride, our GPS was reading 39
> miles--not 30. The folks we talked to there swore up and down that
> our GPS was wrong. So we checked it in the odometer mode travelling
> in the car. Its right on.
> Any ideas what would cause a GPS to ADD miles? I know they can
> sometimes cut them off if you don't have good sattleite
> reception--but add??
> Dorothy & Elly
> Butterfly Valley
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