Hand held GPS or PDA Setup ????

Gordon said:

Paul Saunders <[email hidden]> wrote

Quoted message said:

Quoted message said:

But does it continue to record a track even if the GPS
loses lock? A normal GPS wouldn't, but what about a
barometric GPS, does it continue to drop altitude track
points even if it doesn't know its position?

Yes.

Quoted message said:

Lose a track point and you lost the altitude data with it.
I haven't studied what happens to the altitude on those
occasional false excursions, but altitude isn't critical on
the kind of walks I can manage nowadays.....

The GPS altitude error will tend to increase in proportion
to the position error, usually being between 1.5 and 2 times
as bad. But it won't affect the accuracy of the alitude
reported by an a barometric altimeter model using autocal.
Well, strictly speaking it will, but to a very small extent.

Quoted message said:

Quoted message said:

Another thing I'm wondering about, does a barometric GPS
use the barometric altitude to help compute a position
when it only has a 2D lock? This could be very useful
for more accurate grid refs in bad reception conditions
if it does.

Quoted message said:

Pass.

That would be a very useful feature. I've done some
approximate manual calculations to see how well this would
work, and it seems to work very well indeed. But AFAIK no
GPS currently does it.

Given Garmin's long lead in sophistication of barometric
altimeters, they're the obvious folk to make this next step.
However, there might be a conflict with things like NMEA
specs, i.e., some applications might insist that pure GPS
locations be reported. I believe some Magellans allow you to
input a presumed height for a 2D location, and allowing it
to be done as a single shot manual intervention like that
might be less controversial.

Also, given how hard many folk have found understanding
autocalibrated altimeters, such a facility might cause
more trouble in the general mass market than it was worth
-- to Garmin.

--
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/]

(RJ Webb) said:

On Fri, 2 Jul 2004 01:24:30 +0100, "Paul Saunders"
<[email hidden]> wrote:

Quoted message said:

Quoted message said:

Quoted message said:

I expect the Summit to give a slightly higher reading
that I can deduce from the map but if it adds on an
extra 1000 feet or more it gives me considerable doubts,
particularly when I take the trouble to check my initial
calculations and find them to be apparently correct.

Quoted message said:

Quoted message said:

I presume the key here is to ensure that the GPS retains
good reception throughout the walk. If auto-calibration is
on then sudden large altitude errors due to the GPS losing
and regaining lock would affect the autocal adversely.

No they don't. A good way of understanding how it works is
to suppose that readings are taken once a second, and that
what autocal does every second is to add .05% of the
difference between GPS altitude and altimeter altitude to
the altimeter height, keeping that as the new calibration.
So a spurious difference of 100 metres for one second would
add a spurious 5cm to the altimeter calibration. If the
spurious GPS error of 100m persisted for 10 seconds it would
add a spurious 50cm to the altimeter calibration. And so on.

Quoted message said:

Why I sometimes dont use it.... I like to try and record
how much climb I have "enjoyed".... In most of Europe spot
heights and reasonable contours allow resetting. In
wilderness areas autocalibration is a wonderful tool.

Quoted message said:

Likewise deviation from excepted heights can be a warning
of falling pressure.. nice to know.

You can find out exactly how much barometric pressure is
rising or falling whether or not you're using autocal simply
by manually calibrating the altimeter to the current height
(the default), and then looking at the sea level normalised
pressure, which is then an accurate calibrated reading. Some
of the newer Garmins allow you to set the unit to
recalibrate normalised pressure readings automatically,
without you having to do the prior manual calibration.
You're right, nobody can understand why they ever chose that
fiddly manual procedure in the first place, but they did.

Quoted message said:

The exageration of height gain (not absolute height) with
the sunmmit seems to be explained by sensitivity (the
sitting down argument) This also means small local pressure
changes too, gusts of wind etc.

So some folk maintain, but I doubt this. I've tried to get
my Summit to shift its alitude readings by turning it round
in a wind, without success. I'm sure it would work in very
fierce winds.

There are small local pressure changes, e.g., there are
small transient pressure changes several seconds to a minute
before it starts to rain, and several seconds to a minute
before it stops. You can't see these as changed in
barometric pressure, because the displayed reading is too
coarse, but you can see them as pseudo-altitude changes when
stationary without autocal.

Quoted message said:

The Suunto watches seem to give figures closely agreeing to
the map.. less spurious accuracy?

They incorporate thermal height correction factors which are
redundant in a system with GPS autocal and not provided in
Garmin altimeter models. So they work better than a Garmin
altimeter with autocal off.

Quoted message said:

Auto calibration is a good tool.. and we can chose to use
it when appropriate,

IMHO the only times autocal is inappropriate is when using a
stationary Garmin with altimeter as a barometer, and when
trying to derive your own accurate spot height calibrations
from nearby existing ones, which has to be done in periods
of stable atmospheric pressure.
--
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/]

Mark Cavendish said:

On Fri, 2 Jul 2004 01:24:30 +0100, "Paul Saunders"
<[email hidden]> wrote:

Quoted message said:

Quoted message said:

But does it continue to record a track even if the GPS
loses lock? A normal GPS wouldn't, but what about a
barometric GPS, does it continue to drop altitude track
points even if it doesn't know its position?

Quoted message said:

Mine can't be normal, then ;-)

Quoted message said:

If I drive through a tunnel the GPSr won't receive a signal
but the averaging continues and the track is smooth. Never
have worked out where the averaging ends and the wild
locations begin but suspect it varies (and has only
happened a few times)

Garmins continue to extrapolate a track for 30 secs after
loss of sat lock. If sat lock returns in that time, they
amend the dead reckoning to splice into the new track. If
more than 30 secs elapses they junk the hypothetical dead
reckoning segment and break the track.

Loss and regain of sat lock, with or without track break,
doesn't necessarily involve wild locations. That depends on
what sat cluster is recovered, whether it involves a
temporary reflection, and so on.

So if you know what you're doing you can make (& find)
accurate waypoints in the middle of an underground tunnel
where there is no sat reception :-)

--
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:

RJ Webb wrote:

Quoted message said:

Quoted message said:

Likewise deviation from excepted heights can be a warning
of falling pressure.. nice to know.

Quoted message said:

The latest GPS units record sea level pressure
independently of altitude as you move (using the autocal
feature to work it out).

All Garmins with altimeters can do that. But with the old
ones you have to do a simple manual recalibration to the
current altitude to correct the sea level pressure, whereas
the new ones can do that automatically.
--
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:

Mark Cavendish wrote:

Quoted message said:

Quoted message said:

Mine can't be normal, then ;-)

If I drive through a tunnel the GPSr won't receive a
signal but the averaging continues and the track is
smooth. Never have worked out where the averaging ends
and the wild locations begin but suspect it varies (and
has only happened a few times)

Quoted message said:

I believe when it loses lock there's a delay of 30 seconds
(with Garmins?) before it stops track recording where it
relies on dead reckoning. Fine if you continue moving in
the same direction and at the same speed.

If you don't and it recovers sat lock within 30 secs, it
corrects the dead reckoning to splice into the new track.
--
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:

Chris Malcolm wrote:

Quoted message said:

Quoted message said:

IMHO the only times autocal is inappropriate is when
using a stationary Garmin with altimeter as a barometer,
and when trying to derive your own accurate spot height
calibrations from nearby existing ones, which has to be
done in periods of stable atmospheric pressure.

Quoted message said:

But if the pressure is stable and the autocal has settled
down, it shouldn't affect the readings anyway, should it?

Unstable pressure won't affect your readings, but even with
the best sat view possible, the six hour semi-cyclic drift
of GPS altitude error will affect them. I'm talking about
situations where you want a precise altitude down to the
nearest foot.

Secondly, if you need to *know* that the pressure has been
stable while you were in transit, it's much easier to know
that if you switch autocal off. Returning back to the
starting point with a different altitude (autocal being off)
is a very sensitive test of pressure instability.

--
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/]

Quoted message said:

It occurs to me that this may be a fractal problem. Just as
the canonical 2.5D example of a coastline, which gets
longer and longer as your measuring ruler (unit of
approximation) gets smaller and smaller, so the amount of
upping and downing in a walk will increase as the
sensitivity of your measurements increases. In other words,
the Summit may be geting bigger ascent and descent figures
simply because it's sensitive to about a meter (I guess),
whereas it's being checked against a topographical map
abstraction which is probably sensitive to (say) 10 metres.

I thought that was the reason for the comparison with the
Suunto.. until reminded about the thermal correction. The
garmin is very "jumpy"/ greater sensitivity

Richard Webb

Roger said:

I expect the Summit to give a slightly higher reading that
I can deduce from the map but if it adds on an extra 1000
feet or more it gives me considerable doubts, particularly
when I take the trouble to check my initial calculations
and find them to be apparently correct.

I presume the key here is to ensure that the GPS retains
good reception throughout the walk. If auto-calibration is
on then sudden large altitude errors due to the GPS losing
and regaining lock would affect the autocal adversely.

So how do you know if you maintained good reception? When
checking the track afterwards on the PC it's usually easy to
spot any blatant errors, like the odd spurious track point,
or a gap in the track. If the track looks okay then probably
the ascent is realistic, yes?

But does it continue to record a track even if the GPS loses
lock? A normal GPS wouldn't, but what about a barometric
GPS, does it continue to drop altitude track points even if
it doesn't know its position?

Another thing I'm wondering about, does a barometric GPS use
the barometric altitude to help compute a position when it
only has a 2D lock? This could be very useful for more
accurate grid refs in bad reception conditions if it does.

Paul
--
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Chris Malcolm said:

Quoted message said:

Quoted message said:

If auto-calibration is on then sudden large altitude
errors due to the GPS losing and regaining lock would
affect the autocal adversely.

No they don't. A good way of understanding how it works is
to suppose that readings are taken once a second, and that
what autocal does every second is to add .05% of the
difference between GPS altitude and altimeter altitude to
the altimeter height, keeping that as the new calibration.
So a spurious difference of 100 metres for one second
would add a spurious 5cm to the altimeter calibration. If
the spurious GPS error of 100m persisted for 10 seconds it
would add a spurious 50cm to the altimeter calibration.
And so on.

Ah...

Quoted message said:

So some folk maintain, but I doubt this. I've tried to get
my Summit to shift its alitude readings by turning it
round in a wind, without success. I'm sure it would work
in very fierce winds.

Probably not the best situation in which to take critical
height readings then.

Quoted message said:

IMHO the only times autocal is inappropriate is when using
a stationary Garmin with altimeter as a barometer, and
when trying to derive your own accurate spot height
calibrations from nearby existing ones, which has to be
done in periods of stable atmospheric pressure.

But if the pressure is stable and the autocal has settled
down, it shouldn't affect the readings anyway, should it?

Paul
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Paul Saunders said:

Roger said:

I expect the Summit to give a slightly higher reading
that I can deduce from the map but if it adds on an extra
1000 feet or more it gives me considerable doubts,
particularly when I take the trouble to check my initial
calculations and find them to be apparently correct.

I presume the key here is to ensure that the GPS retains
good reception throughout the walk. If auto-calibration is
on then sudden large altitude errors due to the GPS losing
and regaining lock would affect the autocal adversely.

Why I sometimes dont use it.... I like to try and record how
much climb I have "enjoyed".... In most of Europe spot
heights and reasonable contours allow resetting. In
wilderness areas autocalibration is a wonderful tool.

Likewise deviation from excepted heights can be a warning of
falling pressure.. nice to know.

The exageration of height gain (not absolute height) with
the sunmmit seems to be explained by sensitivity (the
sitting down argument) This also means small local
pressure changes too, gusts of wind etc.The Suunto watches
seem to give figures closely agreeing to the map.. less
spurious accuracy?

Auto calibration is a good tool.. and we can chose to use it
when appropriate,

Richard Webb

Gordon said:

Walking a compass bearing (or rather walking to a
waypoint) with a GPS is easier

Far easier...

Quoted message said:

than walking a compass bearing in poor visibility, because
the usual trick of sighting a distant point and walking
towards it is drastically limited.

But more importantly because the GPS "compass" is really a
homing device and constantly points at your objective
irrespective of whether you walk in a straight line or not.
Which makes it particularly useful for *not* having to walk
in a straight line, so obstacle avoidance is easy.

Quoted message said:

I used to check my ability to walk a bearing by setting
off across the
1/4 mile of featureless grass in a local park with my head
down, staring at the compass.

Easy to do this with the GPS too. Just watch the bearing
number as you follow the arrow. If you walk in a dead
straight line the bearing won't change, but if you drift the
bearing will change.

Quoted message said:

I have a natural bias to the left, and was soon way
off course.

I've walked a few pretty straight lines with the GPS when
the terrain was virtually obstacle free. Easy to check with
the track afterwards. I once walked an almost straight line
for 1.5km in hill fog across Cefn y Cylchau with Roy Hill
who was doing the same thing with his compass. He drifted to
the left, perhaps a hundred metres or so. Pity he wasn't
recording his own GPS track to compare later.

Paul
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Gordon said:

However, when I am enjoying a walk in the country,
uncluttered by camera gear and walking poles (grin), I
sometimes forget that the purpose of the walk is to
maintain good satellite reception, finding myself
distracted by the scenery.

Much as I like GPS, it's not the most important thing on a
walk. I may try to maintain reception in difficult
conditions if I've got nothing better to do, but if I'm
scrambling or taking photos I couldn't care less. It's nice
to get a good track but I'm not going to spend the whole
walk worrying about it. I'm more interested in marking
accurate waypoints for key features than worrying about the
whole track.

Quoted message said:

My GPS is either in the strap on my shoulder or sometimes
in my hand when I actually use it to navigate instead of
peering at the map every few 100 yards. Usually I get a
good track.

I usually do too. It's rare to get a break on a hill walk.
It's the forested valleys that produce the garbage tracks
with me. I've read that the 60CS picks up a much stronger
signal than previous units. We shall see... I am slightly
jealous of Mark Cavendish's SporTrak's ability to maintain
lock in the Ystradfellte valleys.

Paul
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W. D. Grey said:

Quoted message said:

I have a natural bias to the left, and was soon way off
course.

Easily cured - put your wallet on you RH side!

Nah, put your camera on your RH side. Oh hang on, that would
cause a strong right bias instead...

Paul
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RJ Webb said:

Likewise deviation from excepted heights can be a warning
of falling pressure.. nice to know.

The latest GPS units record sea level pressure
independently of altitude as you move (using the autocal
feature to work it out).

Quoted message said:

The exageration of height gain (not absolute height) with
the sunmmit seems to be explained by sensitivity (the
sitting down argument)

High sensitivity should be good for determining which crag
or peat hag is really the highest point of a summit. Have
you tried using it for this?

Quoted message said:

This also means small local pressure changes too, gusts of
wind etc.

Really? Would it affect it that much? Such as when walking
across a windy col?

Paul
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Chris Malcolm said:

Unstable pressure won't affect your readings, but even
with the best sat view possible, the six hour semi-cyclic
drift of GPS altitude error will affect them. I'm talking
about situations where you want a precise altitude down to
the nearest foot.

Yes, but as I said, what I want is "relative" accuracy,
specifically the height difference between multiple tops on
a summit, or between a summit and its relative col. These
usually aren't far apart so it won't take long to get from
one to another. If the drift is a 12 hour cycle it's not
going to make much difference if it takes me say 20 minutes
to walk from a summit to a col or vice versa. The question
is, which would drift more in that time, the barometric
pressure or the semi-cyclic drift? You didn't say how large
this drift is, what sort of range are we talking about?

Quoted message said:

Secondly, if you need to *know* that the pressure has been
stable while you were in transit, it's much easier to know
that if you switch autocal off. Returning back to the
starting point with a different altitude (autocal being
off) is a very sensitive test of pressure instability.

True, but then I'd lose the advantages of autocal. I really
need to know how much the barometer can drift compared to
the semi-cyclic drift in order to determine which mode would
be the most useful for me. Even if I used the autocal,
couldn't I determine how much the pressure had changed by
other means when I got back, like from the barometer?

Paul
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