Dominic Sexton wrote:
> Baz writes
>> but something else
>> showed up which was not shown im MM, a 1.9 mile section of track at
>> the beginning
It's not usually as much as that, but it's not unusual, especially if you
switch your GPS on in a bad reception area, such as under trees or
surrounded by buildings. In the past, if I knew I'd be starting a walk in
such a location, I'd make a point of turning the GPS on in the car a mile or
two before I got there, to give it a chance to get a decent lock in advance.
And as I said, I never turn track recording on until I start the walk.
> If it has been switched off for over 2 hours (Garmin - other
> manufacturers may have a different time - up to 4 hours) the GPS makes
> some assumptions to get the initial fix and these can lead to large
> errors if there is a significant change from where it is last used but
> as you saw they generally correct themselves quite quickly.
>
> I recommend that people let their unit settle for at least a minute
> after turning it on when it has not been used for 2 hours or more.
> That way it will usually not give large errors. If you want to you
> can clear the track memory then and avoid such errors...
Nowadays I do things differently, due to having an altimeter in my GPS. I
always start by switching it on in the car at home, waiting for it to
settle, then manually calibrating the altitude to 71m. I find it better to
do that at home where I know the altitude with certainty, rather than
estimate it at the start of the walk (where it can often be difficult to
ascertain it precisely from the map, especially if it's a flat area), or
worse still, calibrate it from the GPS altitude.
Setting it in the car also has the advantage that it gives the
auto-calibration feature time to settle down, so it should be fairly
accurate by the time I reach my start point. Comparing it to the contours
in my GPS seems to validate that (although they can't always be trusted).
However, it's not uncommon for there to be a discrepancy between the start
and end altitude of my walks, and I suspect this may have something to do
with the car being a bit of a pressure bubble. I usually make a point of
leaving a couple of windows open if the weather permits, to help alleviate
this effect, but still, using an altimeter in a moving car can't be that
reliable, can it? Even with the windows open there'd still be a pressure
difference behind the windscreen, wouldn't there?
Anyway, just prior to starting my walk I reset the stats (to clear the data
from the drive) and turn the track recorder on, then turn it off again at
the end of the walk. If I choose to use the GPS for the return drive I'll
jot down the walk stats on a scrap of paper first.
When I get back home, the altimeter almost always reads 70-72m.
Paul
