Mike Mason said:GPS is Garmin GPSMAP 60CSx
Started the walk near Dockray at NY37990 21898. There is a spot
height very close to this of 412 metres.
The GPS was left on the car roof for 10 minutes whilst I put on boots
etc. The height it registered was 338 metres.
The return grid ref was NY37993 21896 but the height was 416 metres.
Looking at the track data the GPS started to give what appear to be
realistic readings somewhere between High Brow and Randerside and
somewhere between 37 minutes and 1 hour and 15 minutes into the walk.
There was no break in the track and no spurious track points. Can
anyone offer an explanation for this quite large difference in
heights please.
You didn't calibrate the altimeter at the start of the walk.
It was obviously set to auto-calibrate, which is why it gave sensible
readings later on, but auto-calibrate takes a while to settle down (at least
half an hour), which is why you should always calibrate it manually at the
start of each walk. It took longer in your case because you didn't calibrate
it.
The barometer mode should should also be set to variable elevation for use
when walking, which it probably was (I think that's the default).
The procedure is this:
1. Turn the GPS on and wait until you get a good signal (an EPE of at least
10m). It's important to do this first since the GPS elevation jumps around a
lot when you first switch it on.
2. Calibrate the altimeter. Use a known height if possible, but if it's hard
to determine accurately from the map, use the current GPS altitude instead
(but be sure you've got a good lock first).
3. It's a good idea to turn track recording on at this point, and to reset
your odometer etc. I always switch it on and off at the start and end of
each walk to avoid recording the car journey. It's best to do this *after*
getting a good lock and calibrating the altimeter, to avoid recording any
[censored] at the start of the track log.
That's it! I usually set mine at home in the car, before I start the drive,
since I know the exact elevation there. Also, it gives it plenty of time to
auto-calibrate by the time I get to the start of the walk. I can also use it
for navigation in the car too!
The more accurate the calibration the quicker auto-cal works. The reason it
works slowly is because it avoids big jumps in the GPS elevation which can
happen when the reception is temporarily bad. For this reason it's a good
idea not to start a walk where reception is bad, such as under trees or
surrounded by buildings. This is another reason to get a good lock and
calibrate before arriving at your start point.
Here's a tip if camping under trees or where reception is bad. Before you
turn your GPS off at the camp site, make a note of the elevation (saving a
waypoint is an easy way of doing this). Next morning, if you have difficulty
getting a lock, at least you'll be able to use the elevation from the
previous day to calibrate the altimeter (which in turn helps the GPS to get
a lock I think). Even if you can't get a lock, at least the altimeter will
give you some info.
It's surprising how much the elevation can change during the night due to
pressure changes. But that's actually a good thing since you can check the
barometer screen to get an idea about possible weather changes. Strictly
speaking, if you want to use the barometer mode properly you should turn
auto-cal off and change the mode to fixed elevation, but I tend not to do
this because I usually forget to change them back the next day. Seems to
work fine without doing that though.
Of course, pressure trend recording should be set to save always, so that it
records data when it's turned off. I just leave it on this setting.
Paul
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