Re: GPS to calculate mileage

[Truman Prevatt <truman.prevatt@netsrq.com>]

As Angie pointed out the altitude measurement using hand held GPS is
very bad.  The GPS is a system developed by the Defense department to
distribute time world wide to within 50 nanoseconds.  Becuase of
relativistic effects associated with the rotating earth time tranfer
world wide is a very difficult technicial problem. This is the
overriding technical issue that drove the definition of the GPS program
in the early 70's.  The location is a by produce and is calculated by
measuring the time of transmission of the "same event" from multiple
space craft.  This calculation of location in the inexpensive hand held
units is to the reference World Geoditic System Geoid (WGS84 to be exact
and soon to become WGS 96 so most of the units will be obsolete <G>). 
In order to get accurate location (including altitude) the US Defense
Mapping Agency world altitude data base is required.  This takes up 100
CD's.  Sophiicated military systems use it but not hand held units.  

So an altitude quantization of almost 1/5 of a mile results.  So any
altitude variation less than 1/5 of a mile will not be counted in the
total milage.  

The other issue with the hand held models in the calculation of distance
is sampling time.  Most are on the order of minutes to sample the
location and then that data is used by a statistical algorithm - a
Kalman Filter in most units - to calculate the distance.  The sampling
interval is on the order of 4 minutes in the hand held units.  So every
4 minutes a location is calculate.  The unit then takes the last
location and "draws a line to the new location."  The distance of this
line is the distance that goes into the filter.  This is very sufficient
of hikers traveling at 2 mph.

Now a horse trotting at 7.5 mph.  It goes 1/2 mile in 4 minutes.  Now
assume it goes in a path of 1/2 of a circle with a circumference of one
mile.  You would go from a point to its antipodal point in 4 minutes. 
So, depending on the time of sampling, you hand held GPS would say you
went 1/Pi which is about 0.3183 miles when you really went 0.5 miles. 
So the error is about 15%.

Because of the way it estimates distance it will almost always measure
short.

So if somenone comes to me claiming their GPS measured a trail short,
I'd give them my mountain bike and tell them to "prove it"<G>.

Truman


> 
> 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.  >>
> 
>
-- 
Truman Prevatt
Mystic “The Horse from Hell” Storm
Rocket a.k.a. Mr. Misty
Jordy a.k.a. Bridger (when he is good)
Danson Flame - hey dad I'm well now and ready to go!

Brooksville, FL