Quoted message said:GPSs have poor altitude resolution, here's a log of two
laps of a park using the forerunner:
Basic GPSr reporting of altitude is based on having decent
satellite config to get the 3rd dimension. If you're
marginal for 2-dimensional position (3 good satellites),
the 3rd dimension (elevation) (4 good satellites in a
decent config, not a small cluster or in a line) will be
similarly off - or worse. Some GPSr's (Etrex Vista,
probably not Forerunner) have built-in barometric
altimeter. This, in conjunction, with the 3-dimensional
location obtained from satellites tends to make a more
reliable elevation profile, although I'm not sure what the
response time is to changes in elevation. My usual trails
have many small hills, say 10 hills of 30-70 ft (vertical)
in a 1-hr run where I'm changing from up to down every 3
min or less, usually. (At least based on my understanding
of the way things work and from what I've gathered while
lurking in sci.geo.satellite-nav)
I'm not particularly convinced that elevation display even
on the Vista is more than a random number generator on my
usual routes as far as differentiating 600 ft from 800 ft
climb, for instance (it's probably in the 500-900 ft range
though). I'm sure the altimeters do better on larger hills
where you're going up for at least 15 min, sometimes hour or
more. The accuracy of the gps route on my trails when placed
over a map is insufficient to determine much in the way of
elevation change (gps horizontal error may put location at
bottom of hill instead of on top; also maps aren't that
accurate) so the altimeter approach is a better
approximation for me.
Quoted message said:
Quoted message said:Do either the Timex or the forerunner work in the woods
with alot of tree cover?
You'll probably lose signal a number of times depending on
the density and current satellite positions.
I'd just like to emphasize that the satellite config issue
is an issue regardless of the type of ground-based gps being
used. We were chatting about this the other night with the
tech that uses the survey-grade (1-cm resolution, although
it might be resource-grade - either way, better than any of
the hand-helds like we use). They have to pay attention to
satellite config when planning their work day to get the
most accurate data also.
That said, gps systems are more flexible (assuming you've
got a satellite lock) than accelerometers since you can use
them skiing, canoeing, snowshoeing, mt biking (no, I still
haven't gotten around to mounting computer on my bike), etc.
Dot
--
"Success is different things to different people" -Bernd
Heinrich in Racing the Antelope
