> I don't think that would work very well, at least for normal affordable G=
PS
> units. =A0The reported position can jump around quite a bit, especially w=
hen
> you're moving. =A0Most GPS units have something like a 10 meter 50% error
> radius.

That sounds larger than I have observed in practice. Especially so for
relatively time-adjacent readings.
If you change satellite set you will very probably get a step change.
Some years ago I took raw GPS data from driving around a network of
local streets and plotted it using Excel. This allowed subsequent
passes to & fro along the same road at similar and separated times to
be displayed and differences calculated.  Roads were typical NZ urban
residential roads (one lane each way plus kerbside parallel parking on
both sides.

A pass along a street, U turn at end and come back on opposite side,
total trip under say 1 minute, gave there and back tracks of close to
correct separation.

Wait a few hours and repeat and the mean centreline of the  new track
pair may be 20 to 40 FEET offset to one side.

ie the second by second differential positional accuracy is "very
good". The variability in side to side positional error compared to
the essentially straight line being driven is small. From somewhat
long ago memory it's perhaps a foot or so.

Interest only:    In the very very early days of GPS when there were
about zero commercial GPS receivers an academic published a paper in a
scientific journal describing the receiver that they had designed and
built. It was large and magnificently stationary but they published a
scatter plot of the position that it reported. From memory it showed a
locus about as wide as the building that they were in side to side
across the satellite track - maybe a few 10's of feet, and several
times larger along the track.  A footnote noted "now at ..." with the
name of what is now one of the major GPS receiver makers. (Which one I
don't recall). I will probably still have that paper here somewhere,
but my finding it for you would probably cost you several days labour
charges :-).


     Russell

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