Altitude puzzle...

Nick Hopton said:

I went out for a walk yesterday around Pewsey, Milk Hill and Tan Hill, about 23 km, according to
the GPS. Also, according to my Geko 301, my total ascent was 2261 metres. Is this possible?

Not likely. I don't have an altimeter in my GPS but I gather they often produce excessive totals.
Have you got any software like Ozi or Anquet with which you can study the altitude profile of the
track? If you do, you'll probably find a number of spurious points which tend to skew the results.
It's not uncommon for a GPS to occasionally put a track point in completely the wrong place (usually
due to losing lock momentarily) which is obvious to see when you look at a track.

I've studied tracks of my walks where I've been plodding along at 3 or 4kph and suddenly I found
myself traveling at 150kph for a few brief moments, obviously due to one of these spurious points.
I imagine the same would happen with altitude, resulting in sudden jumps that would add a lot to
your total.

If you do have software like Ozi you can examine the track and profile to identify these spurious
points and edit them out. This should leave you with a pretty accurate result.

Paul
--
The October Project 2003 - Updated
wildwales.fsnet.co.ukoctober.htmlOpen ↗
wilderness-wales.co.ukwilderness-wales.co.ukOpen ↗
photosig.comuserphotosOpen ↗

The message <[email hidden]>
from Nick Hopton <[email hidden]> contains these words:

Quoted message said:

I went out for a walk yesterday around Pewsey, Milk Hill and Tan Hill, about 23 km, according to
the GPS. Also, according to my Geko 301, my total ascent was 2261 metres. Is this possible? I
started off walking along the canal at about 130 metres AOD and the highest point of the walk was
only 300 metres AOD. There was a fair amount of up-and-down, too.

Not in that area. That is much the same average gradient as the Welsh 3000s and getting on for 2
thirds of the peak to peak total. If you know how hard you walk you could make a reasonable stab at
the height by working back from the time and distance. At approaching 15 miles and 7500 feet of
ascent my old enemy Naismith would have taken almost 9 hours for such a trip.

My first trip out with my Summit was a low level meander from Ribblehead to Horton on a ng outing
that wasn't dignified with with its own numeral. On that occasion the Summit recorded approximately
twice the total height actually walked (gleaned from a careful study of the
1:25,000). I thought at the time that that was a inherent problem but once I started wearing the GPS
on my shoulder rather than my chest strap and visiting steeper areas the problem largely went away
and most recent trips have provided a believable total, occasionally even marginally less than I
had estimated from the map.

The one occasion in the recent past where I have had severe doubts was again a slow trip in company
where IIRC an expected 3000+ feet walk (Helvellyn range north from White Side to Clough Head) came
out at 5000+ but I have yet to get round to investigating that.

--
Roger Looking North over the Aire Valley (and Marley Gasworks) to Rombolds Moor Top Posters
killfiled on sight

The message <[email hidden]>
from Nick Hopton <[email hidden]> contains these words:

Quoted message said:

I went out for a walk yesterday around Pewsey, Milk Hill and Tan Hill, about 23 km, according to
the GPS. Also, according to my Geko 301, my total ascent was 2261 metres. Is this possible? I
started off walking along the canal at about 130 metres AOD and the highest point of the walk was
only 300 metres AOD. There was a fair amount of up-and-down, too.

2nd thoughts.

I don't want to cause offence but are you absolutely sure you have the height set to metres. The
units for height and distance are set independently on the Summit and I would have thought the Geko
was the same. 2261 feet seems a much more likely total for 15 miles in undulating country than 7500.

--
Roger Looking North over the Aire Valley (and Marley Gasworks) to Rombolds Moor Top Posters
killfiled on sight

Nick Hopton said:

I went out for a walk yesterday around Pewsey, Milk Hill and Tan Hill, about 23 km, according to
the GPS. Also, according to my Geko 301, my total ascent was 2261 metres. Is this possible? I
started off walking along the canal at about 130 metres AOD and the highest point of the walk was
only 300 metres AOD. There was a fair amount of up-and-down, too.

And how did you calibrate the altimeter when you started? I presume you had the unit set to auto-
calibrate the altimeter, and switched it on and cleared alt logging etc at the start of your walk.
In which case the quick shifting of apparent altitude as autocalibration brought the altimeter into
step with GPS altitude would be recorded as ascent (or descent) even if you were standing still.

Units with altimeters give more accurate altitude logs, but only *if* you take the extra care to use
them properly, the most important point being making sure you have a reasonable stable calibration
of altimeter *before* you start walk logging. You can either do this manually, or let the unit do it
by itself (autocalibration), given a good clear sky view and a couple of minutes *after* good sat
lock has been acquired. (I'm presuming here that the Geko 301's altimeter autocalibration works at
least as well as that on the earlier Summit and Vista.)

Note that on long walks in flat country the inevitable drifts and lurches in GPS altitude over time
(possibly worsened by keeping the 301 in a poor position, such as stuffed in a shirt pocket), and
failures to autocalibrate out any barometric pressure drift in the altimeter (again worsened by poor
reception conditions such as in shirt pocket) will add lots of little cumulative ups and downs which
didn't really exist.

If you still have the active log of the walk you can find out what happened by looking at the
altitude profile graph. There should be a menu function which allows you to run a cursor backwards
and forwards over the entire profile, giving you times of the altitudes. If I'm right you'll see a
very large sweep at the beginning as the autocalibration brought your altimeter into line.
--
Chris Malcolm [email hidden] +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/]

Paul Saunders said:

Nick Hopton wrote:

Quoted message said:

Quoted message said:

I went out for a walk yesterday around Pewsey, Milk Hill and Tan Hill, about 23 km, according to
the GPS. Also, according to my Geko 301, my total ascent was 2261 metres. Is this possible?

Quoted message said:

Not likely. I don't have an altimeter in my GPS but I gather they often produce excessive totals.
Have you got any software like Ozi or Anquet with which you can study the altitude profile of the
track? If you do, you'll probably find a number of spurious points which tend to skew the results.
It's not uncommon for a GPS to occasionally put a track point in completely the wrong place
(usually due to losing lock momentarily) which is obvious to see when you look at a track.

This doesn't happen with altitude on the altimeter models. They have other interesting ways of
getting the altitude wrong :-)

For example, suppose you switched it off during lunch break because you weren't moving. And the
barometric pressure changed equivalent to a drop of 300 metres (unusual but not impossible). Then
when you switched on again the unit would interpret that as having descended 300 metres, followed by
a climb of 300 metres as autocalibration pulled the altimeter back into line.

This week's homework exercise for the owners of GPSRs with altimeters is: how you could get the
battery savings of switching off during lunch, without risking the possible spurious large altitude
shifts in the altitude log due to barometric pressure change?
--
Chris Malcolm [email hidden] +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/]

In a recent message <[email hidden]>, Chris Malcolm <[email hidden]> wrote.

[...]

Quoted message said:

Quoted message said:

I went out for a walk yesterday around Pewsey, Milk Hill and Tan Hill, about 23 km, according to
the GPS. Also, according to my Geko 301, my total ascent was 2261 metres. Is this possible?

[...]

Quoted message said:

And how did you calibrate the altimeter when you started? I presume you had the unit set to auto-
calibrate the altimeter, and switched it on and cleared alt logging etc at the start of your walk.
In which case the quick shifting of apparent altitude as autocalibration brought the altimeter into
step with GPS altitude would be recorded as ascent (or descent) even if you were standing still.

Yes, I set the GPS to auto-calibrate the altimeter in the car park before I started walking and yes,
I gave the GPS a few minutes to stabilise before I reset the altitude data.

Quoted message said:

Units with altimeters give more accurate altitude logs, but only *if* you take the extra care to
use them properly, the most important point being making sure you have a reasonable stable
calibration of altimeter *before* you start walk logging. You can either do this manually, or let
the unit do it by itself (autocalibration), given a good clear sky view and a couple of minutes
*after* good sat lock has been acquired. (I'm presuming here that the Geko 301's altimeter
autocalibration works at least as well as that on the earlier Summit and Vista.)

I use GARtrip and the altitude plot looks about right. The highest point was the top of Milk Hill,
294 metres AOD, and the instrument recorded a maximum height of 292 metres.

[...]

Quoted message said:

If you still have the active log of the walk you can find out what happened by looking at the
altitude profile graph. There should be a menu function which allows you to run a cursor backwards
and forwards over the entire profile, giving you times of the altitudes. If I'm right you'll see a
very large sweep at the beginning as the autocalibration brought your altimeter into line.

[...]

No, the altitude profile for the starting, level section didn't have any significant ups and
downs in it.

One thing though, I keep the Geko in an Aquapack and this seals airtight. It doesn't seem to cause a
problem with the measurement of altitude, but I wonder what happens when I press the buttons,
perhaps this is compressing the air in the Aquapack? I'll have to check this. But on the other hand,
I don't see any obviously spurious points in the plot of altitude.

Regards, Nick.

--
Nick Hopton and Anne Hopton Caversham, Reading, England <[email hidden]

In article <[email hidden]>, Roger Chapman
<[email hidden]> writes

Quoted message said:

The message <[email hidden]> from [email hidden] (Chris Malcolm) contains
these words:

Quoted message said:

I wonder if every time you pressed a button it thought you went up and down a few feet? Or when
you took it from a pocket, belt, etc., and put it back? Or took a jolting footstep?

I am not sure what an aquapack is but I would have thought that any easily flexible envelope would
be almost transparent as far as air pressure is concerned.

It is a sealed bag for waterproofing electronics. See:

aquapac.co.ukaquapac.co.ukOpen ↗

Not sure why the op wants to waterproof a waterproof unit but each to their own.

Quoted message said:

Anyway I am retiring from this thread. As the op has ignored me I shall in the future pay him the
same complement.

Join the club.

<google.comgroupsOpen ↗
m=gVMk4KAo6%24c%24EwRS%40local.mail&scoring=d>

--

dscs.demon.co.ukdscs.demon.co.ukOpen ↗

In article <[email hidden]>, Roger Chapman
<[email hidden]> writes

Quoted message said:

I read off line and unfortunately cutting and pasting the link (in 2 sections) didn't work so I
remain in ignorance of what you meant, but perhaps that's for the best. :-)

I went out of my way to lookup information and post a helpful reply (giving all the requested
information) to the same op in a GPS group and there was no response.

Perhaps his newsfeed is patchy...

--

dscs.demon.co.ukdscs.demon.co.ukOpen ↗

The message <[email hidden]>
from [email hidden] (Chris Malcolm) contains these words:

Quoted message said:

This week's homework exercise for the owners of GPSRs with altimeters is: how you could get the
battery savings of switching off during lunch, without risking the possible spurious large
altitude shifts in the altitude log due to barometric pressure change?

Consult current height, switch off, <intermission> , switch on, calibrate height.

NB this will only work if you can remember what the height was at the start of the exercise so that
is me out for a start. :-)

Incidentally I have a log showing a long flat section while I was sitting eating my lunch.
--
Roger Looking North over the Aire Valley (and Marley Gasworks) to Rombolds Moor Top Posters
killfiled on sight

Nick Hopton said:

In a recent message <[email hidden]>, Chris Malcolm <[email hidden]> wrote.

Quoted message said:

[...]

Quoted message said:

Quoted message said:

I went out for a walk yesterday around Pewsey, Milk Hill and Tan Hill, about 23 km, according to
the GPS. Also, according to my Geko 301, my total ascent was 2261 metres. Is this possible?

[...]

Quoted message said:

Quoted message said:

And how did you calibrate the altimeter when you started? I presume you had the unit set to auto-
calibrate the altimeter, and switched it on and cleared alt logging etc at the start of your
walk. In which case the quick shifting of apparent altitude as autocalibration brought the
altimeter into step with GPS altitude would be recorded as ascent (or descent) even if you were
standing still.

Quoted message said:

Yes, I set the GPS to auto-calibrate the altimeter in the car park before I started walking and
yes, I gave the GPS a few minutes to stabilise before I reset the altitude data.

Quoted message said:

Quoted message said:

Units with altimeters give more accurate altitude logs, but only *if* you take the extra care to
use them properly, the most important point being making sure you have a reasonable stable
calibration of altimeter *before* you start walk logging. You can either do this manually, or let
the unit do it by itself (autocalibration), given a good clear sky view and a couple of minutes
*after* good sat lock has been acquired. (I'm presuming here that the Geko 301's altimeter
autocalibration works at least as well as that on the earlier Summit and Vista.)

Quoted message said:

I use GARtrip and the altitude plot looks about right. The highest point was the top of Milk Hill,
294 metres AOD, and the instrument recorded a maximum height of 292 metres.

Quoted message said:

[...]

Quoted message said:

If you still have the active log of the walk you can find out what happened by looking at the
altitude profile graph. There should be a menu function which allows you to run a cursor backwards
and forwards over the entire profile, giving you times of the altitudes. If I'm right you'll see a
very large sweep at the beginning as the autocalibration brought your altimeter into line.

[...]

Quoted message said:

No, the altitude profile for the starting, level section didn't have any significant ups and
downs in it.

Ok, so that wasn't it.

Quoted message said:

One thing though, I keep the Geko in an Aquapack and this seals airtight. It doesn't seem to cause
a problem with the measurement of altitude, but I wonder what happens when I press the buttons,
perhaps this is compressing the air in the Aquapack? I'll have to check this. But on the other
hand, I don't see any obviously spurious points in the plot of altitude.

Hm. It's very sensitive to small pressure changes...

I wonder if every time you pressed a button it thought you went up and down a few feet? Or when you
took it from a pocket, belt, etc., and put it back? Or took a jolting footstep?

--
Chris Malcolm [email hidden] +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/]

Roger Chapman said:

The message <[email hidden]> from [email hidden] (Chris Malcolm) contains
these words:

Quoted message said:

Quoted message said:

This week's homework exercise for the owners of GPSRs with altimeters is: how you could get the
battery savings of switching off during lunch, without risking the possible spurious large
altitude shifts in the altitude log due to barometric pressure change?

Quoted message said:

Consult current height, switch off, <intermission> , switch on, calibrate height.

Quoted message said:

NB this will only work if you can remember what the height was at the start of the exercise so that
is me out for a start. :-)

Or you could consult the altitude graph using VIEW POINTS or whatever. Probably easiest to set
a waypoint.

However, there's still another problem to address -- how do you stop the autocalibration and
tracklogging from seizing control and doing their thing after switch on and before you've done the
manual calibration?

--
Chris Malcolm [email hidden] +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/]

The message <[email hidden]>
from [email hidden] (Chris Malcolm) contains these words:

Quoted message said:

I wonder if every time you pressed a button it thought you went up and down a few feet? Or when
you took it from a pocket, belt, etc., and put it back? Or took a jolting footstep?

I am not sure what an aquapack is but I would have thought that any easily flexible envelope would
be almost transparent as far as air pressure is concerned.

Anyway I am retiring from this thread. As the op has ignored me I shall in the future pay him the
same complement.

--
Roger Looking North over the Aire Valley (and Marley Gasworks) to Rombolds Moor Top Posters
killfiled on sight

The message <[email hidden]>
from [email hidden] (Chris Malcolm) contains these words:

Quoted message said:

Or you could consult the altitude graph using VIEW POINTS or whatever. Probably easiest to set a
waypoint.

Quoted message said:

However, there's still another problem to address -- how do you stop the autocalibration and
tracklogging from seizing control and doing their thing after switch on and before you've done the
manual calibration?

Autocalibration is a switchable function.

I don't actually ever switch off except when I change batteries. I find the trip functions
too useful.

--
Roger Looking North over the Aire Valley (and Marley Gasworks) to Rombolds Moor Top Posters
killfiled on sight

In article <[email hidden]>, Chris Malcolm

url=''URL:/url said:

Roger Chapman said:

The message <[email hidden]> from [email hidden] (Chris Malcolm) contains
these words:

Quoted message said:

Quoted message said:

This week's homework exercise for the owners of GPSRs with altimeters is: how you could get the
battery savings of switching off during lunch, without risking the possible spurious large
altitude shifts in the altitude log due to barometric pressure change?

Quoted message said:

Consult current height, switch off, <intermission> , switch on, calibrate height.

Quoted message said:

NB this will only work if you can remember what the height was at the start of the exercise so
that is me out for a start. :-)

Or you could consult the altitude graph using VIEW POINTS or whatever. Probably easiest to set a
waypoint.

However, there's still another problem to address -- how do you stop the autocalibration and
tracklogging from seizing control and doing their thing after switch on and before you've done the
manual calibration?

What happens if you set the GPS mode to off during lunch, but leave the unit switched on. Does it
autocalibrate then? Certainly this reduces battery drain by a considerable amount, particularly if
you also have compass set to off at the sime time.

Mike
--
o/ \\ // |\ ,_ o Mike Clark <\__,\\ // __o | \ / /\, "A mountain climbing, cycling, skiing, "> ||
_`\<,_ |__\ \> | immunology lecturer, antibody engineer and ` || (_)/ (_) | \corn computer user"

The message <[email hidden]>

Quoted message said:

Join the club.

Quoted message said:

<google.comgroupsOpen ↗
m=gVMk4KAo6%24c%24EwRS%40local.mail&scoring=d>

I read off line and unfortunately cutting and pasting the link (in 2 sections) didn't work so I
remain in ignorance of what you meant, but perhaps that's for the best. :-)

--
Roger Looking North over the Aire Valley (and Marley Gasworks) to Rombolds Moor Top Posters
killfiled on sight