[email protected] (Rammie) writes:
>I use a Garmin etrex with routes loaded from Memory map 2004.
>However, a few instances of batteries coming loose (mostly avoided by
>a piece of card wedged in the battery compartment) have demonstrated
>the sense in making sure you take a map with you too.
>I also tend to find that it does not work well in forests as it tends
>to lose contact with the satellites and so will not tell you where you
>are (does anyone have a solution this?).
First, make sure it's not in battery save mode, which hardly affects
wide open tracking, but kills tracking under trees.
Next, note that the problem is that it takes a much longer time of
uninterrupted signal to acquire a new satellite than to keep track of
one that's already been locked. So under trees the GPS can keep track
of the satellites it already has, as they flit behind passing boughs,
but can't see a new one for long enough to lock it. So gradually those
it has set below the horizon, and it never picks up any new ones.
The solution is easy. You simply have either to stop every so often
and let the unit acquire some new sats, choosing a spot with the
widest sky view. How often is "every so often"? Depends on how many
sats you amange to get in the clear spots. You can find out by
experiment or keeping an eye on the sat view page in your typical
woods.
Even in a very narrow steep sided and heavily wooded glen, about as
bad as you can get, I managed to get an almost perfect track on my
eTrex Summit simply by stopping as needed to let it keep up.
When trying to get a position lock in a very bad place, you can help
it a lot by watching the sat view page carefully and positioning it
just right so that you get most satellites. When boughs obscure the
sky moving a foot can make all the diffeence. And don't forget not to
obscure any with your head and body!
Note too that eTrexes have a 30 second period of grace in which they
allow position lock to be lost, then assuming you're moving in the
last direction and speed, and finally joining up the track if position
lock comes back within the 30 seconds. You can exploit this to mark a
waypoint in a place where you can't get position lock, by moving to it
at a constant velocity from somewhere within 30 seconds with position
lock. That way you can, for example, mark a bend or junction in a
track where you can't get lock and the tracklog will eaither be broken
or cut off the corner.
There are very few places where intelligent use of a GPS -- any GPS --
can't find out where you are. There are plenty of places where they
can't track you, but unless you're nailed to the spot they can still
be used to locate you, given a few to several minutes exploration
time.
--
Chris Malcolm
[email protected] +44 (0)131 651 3445 DoD #205
IPAB, Informatics, JCMB, King's Buildings, Edinburgh, EH9 3JZ, UK
[
http://www.dai.ed.ac.uk/homes/cam/]
