Altimeters

Michael Thompson said:

I have a alimeter on my GPS, however I was thinking of getting a standlaone
one, because given enough attention I hear that they are more accurate.
True?

I would also like the functionality of knowing exactly how high I was during
a walk, though I can do this when I study the map of the route after.

Has any one got any reccomendations for a good altimeter?

I used a watch-type altimeter (Avocet Vertech) prior to getting my GPS,
but I've never worn it since. I've heard that GPS altitude can be a bit
sketchy, but I have the Geko 301 which is better in this regard. From
experience, barometric altimeters have problems of their own, and items
like the Vertech and especially the Suunto models suffer from poor
internal logic which doesn't always calculate altitude correctly.

If I could find my Vertech, you'd be welcome to try it out!

druidh

druidh bashed at the keyboard and came up with this gem of wisdom in
uk.rec.walking:

Quoted message said:

If I could find my Vertech, you'd be welcome to try it out!

LOL. If you ever move house and find it lurking in a dark corner, remember
me! 🙂

On Fri, 02 Dec 2005 17:48:20 +0000, Michael Thompson
<[email hidden]> wrote:

| I have a alimeter on my GPS, however I was thinking of getting a standlaone
| one, because given enough attention I hear that they are more accurate.
| True?
|
| I would also like the functionality of knowing exactly how high I was during
| a walk, though I can do this when I study the map of the route after.
|
| Has any one got any reccomendations for a good altimeter?

Altimeters measure Air pressure and so need setting at a known height
dally. Most of the old trig points and bench marks are no longer
survey regularly, only the Passive and Active stations are nowadays
surveyed regularly. http://www.gps.gov.uk/ Any changes in air pressure
throughout the day cause errors.

GPS systems have a different set of height errors.
--
Dave Fawthrop <dave hyphenologist co uk> Sick of Premium SMS scams,
SMS marketing, Direct marketing phone calls, Silent phone calls?
Register with http://www.tpsonline.org.uk/tps/
IME they work :-)

Dave Fawthrop bashed at the keyboard and came up with this gem of wisdom in
uk.rec.walking:

Quoted message said:

GPS systems have a different set of height errors.

I am aware that the more sats you are in contact with the better, but what
issues are there?

Michael Thompson said:

Dave Fawthrop bashed at the keyboard and came up with this gem of wisdom in
uk.rec.walking:

Quoted message said:

GPS systems have a different set of height errors.

I am aware that the more sats you are in contact with the better,
but what issues are there?

It's not just the number of satellites, they need to be spread over
the sky but also reasonably high above the horizon for optimum
accuracy. GPS height accuracy is usually around 3x worse than the
horizontal accuracy. This'd normally be somewhere around +/- 30m
vertically, +/- 10m horizontally for a single static measuremnent with
a reasonable satellite pattern.

Chris
--
Chris Eilbeck
MARS Flight Crew http://www.mars.org.uk/
UKRA #1108 Level 2 UYB
Tripoli UK Member #9527 LSMR

On Fri, 02 Dec 2005 23:18:08 +0000, Chris Eilbeck
<[email hidden]> wrote:

| Michael Thompson <[email hidden]> writes:
|
| > Dave Fawthrop bashed at the keyboard and came up with this gem of wisdom in
| > uk.rec.walking:
| >
| >> GPS systems have a different set of height errors.
| >
| > I am aware that the more sats you are in contact with the better,
| > but what issues are there?
|
| It's not just the number of satellites, they need to be spread over
| the sky but also reasonably high above the horizon for optimum
| accuracy. GPS height accuracy is usually around 3x worse than the
| horizontal accuracy. This'd normally be somewhere around +/- 30m
| vertically, +/- 10m horizontally for a single static measuremnent with
| a reasonable satellite pattern.

Also if you are on a 3,000ft (900m) mountain a 30m error is about 3%
of your height. 9% if you are on a 1,000 ft hill.

Not sure what will happen when EGNOS comes on line.
--
Dave Fawthrop <dave hyphenologist co uk> Sick of Premium SMS scams,
SMS marketing, Direct marketing phone calls, Silent phone calls?
Register with http://www.tpsonline.org.uk/tps/
IME they work :-)

In article <[email hidden]>, Chris Eilbeck
<[email hidden]> writes

Quoted message said:

Michael Thompson said:

Dave Fawthrop bashed at the keyboard and came up with this gem of wisdom in
uk.rec.walking:

Quoted message said:

GPS systems have a different set of height errors.

I am aware that the more sats you are in contact with the better,
but what issues are there?

It's not just the number of satellites, they need to be spread over
the sky but also reasonably high above the horizon for optimum
accuracy. GPS height accuracy is usually around 3x worse than the
horizontal accuracy.

It is usually quoted at 1 1/2 to 2 times the horizontal if there is a
good view of the sky.

Quoted message said:

This'd normally be somewhere around +/- 30m
vertically, +/- 10m horizontally for a single static measuremnent with
a reasonable satellite pattern.

The following pages show the measured accuracy of both horizontal and
vertical position in ideal conditions.

http://users.erols.com/dlwilson/gpsacc.htm
http://users.erols.com/dlwilson/gpsvert.htm

Comparing the figures for a Garmin eMap with an external antenna

Horizontal 95% 6.9m
Vertical 95% 13.7m

--

Dominic Sexton

Dominic Sexton said:

In article <[email hidden]>, Chris Eilbeck
<[email hidden]> writes

Quoted message said:

Michael Thompson said:

Dave Fawthrop bashed at the keyboard and came up with this gem of wisdom in
uk.rec.walking:

> GPS systems have a different set of height errors.

I am aware that the more sats you are in contact with the better,
but what issues are there?

It's not just the number of satellites, they need to be spread over
the sky but also reasonably high above the horizon for optimum
accuracy. GPS height accuracy is usually around 3x worse than the
horizontal accuracy.

It is usually quoted at 1 1/2 to 2 times the horizontal if there is a
good view of the sky.

Quoted message said:

This'd normally be somewhere around +/- 30m
vertically, +/- 10m horizontally for a single static measuremnent with
a reasonable satellite pattern.

The following pages show the measured accuracy of both horizontal and
vertical position in ideal conditions.

http://users.erols.com/dlwilson/gpsacc.htm
http://users.erols.com/dlwilson/gpsvert.htm

Thanks for the info.

Chris
--
Chris Eilbeck
MARS Flight Crew http://www.mars.org.uk/
UKRA #1108 Level 2 UYB
Tripoli UK Member #9527 LSMR

Michael Thompson said:

I have a alimeter on my GPS,

Do you mean that your GPS gives you an altitude reading, or do you have a
model with a built-in altimeter?

Quoted message said:

however I was thinking of getting a
standlaone one, because given enough attention I hear that they are
more accurate. True?

More accurate than normal GPS altitude, probably, but more accurate than a
GPS with altimeter, probably not. Standalone altimeters need to be
regularly recalibrated at known heights, whereas the GPS altimeter is
constantly auto-calibrated from the GPS altitude readings, so you never have
to worry about it drifting off.

Although GPS altitude tends to fluctuate a lot, it averages out quite well,
so when used to auto-calibrate the inbuilt altimeter, it gives suprisingly
accurate results.

Shortly after buying my 60CS I did a couple of "spot height drives". The
25k map of Swansea is liberally splattered with spot heights at many road
junctions, so I marked waypoints at these junctions and took a couple of
drives around Swansea, stopping at these junctions and marking GPS
waypoints, in order to later compare the readings.

I thought I wrote something about it at the time but I can't find anything,
so I've just realoaded the waypoints and checked them out again. Out of
perhaps 30 or so spot heights, almost every one was accurate to within 2m,
many were spot on. I think there were two that were 3m out, but in both
instances I don't think I marked the waypoint at the correct spot.

I think this accuracy is pretty impressive, so personally I wouldn't bother
with a standalone altimeter.

Quoted message said:

I would also like the functionality of knowing exactly how high I was
during a walk, though I can do this when I study the map of the route
after.

Any GPS will tell you that, there'll be an altitude readout somewhere on it.
On the altimeter models there's also a page which shows you a profile of
your route, and a barometer graph of the previous 48 hours.

Quoted message said:

Has any one got any reccomendations for a good altimeter?

Garmin Geko 301, 60CS, etc. :-)

Paul

Chris Eilbeck said:

GPS height accuracy is usually around 3x worse than the
horizontal accuracy. This'd normally be somewhere around +/- 30m
vertically,

In theory perhaps, but in practice I find that the GPS altitude is rarely
more than 10m out.

Paul

On Sat, 3 Dec 2005 19:49:48 -0000, "Paul Saunders"
<[email hidden]> wrote:

| More accurate than normal GPS altitude, probably, but more accurate than a
| GPS with altimeter, probably not. Standalone altimeters need to be
| regularly recalibrated at known heights,

Not regularly, *at the start of every trip*. Standalone altimeters
measure air pressure. Air pressure varies continuously as the weather
changes. The BBC weather persons have reintroduced isobars (lines
of equal air pressure) on the graphics. Watch them sweep across the
country, this is error on a stand alone altimeter.

--
Dave Fawthrop <dave hyphenologist co uk> Sick of Premium SMS scams,
SMS marketing, Direct marketing phone calls, Silent phone calls?
Register with http://www.tpsonline.org.uk/tps/
IME they work :-)

Dave Fawthrop said:

Paul Saunders said:

More accurate than normal GPS altitude, probably, but more accurate
than a GPS with altimeter, probably not. Standalone altimeters need
to be regularly recalibrated at known heights,

Not regularly, *at the start of every trip*.

I find they need calibration throughout a day trip at every reasonable
opportunity.
A good unit is a doddle to reset to a known spot height.
Knowing the direction in which the air pressure is changing when
recalibrating/checking can give clues as to the likely changes in the
weather.

GPS units with built in barometric altimeters do the re-calibration for you.
But can take away the knowledge of what's happening. They cost more and need
more batteries.
However, if taking a GPS anyway, then the additional cost is only the
purchase price of a model with the barometric functions.

Quoted message said:

Standalone
altimeters
measure air pressure. Air pressure varies continuously as the weather
changes. The BBC weather persons have reintroduced isobars (lines
of equal air pressure) on the graphics. Watch them sweep across the
country, this is error on a stand alone altimeter.

--
Nigel Cliffe,
Webmaster at http://www.2mm.org.uk/

Nigel Cliffe said:

Quoted message said:

Quoted message said:

Standalone altimeters need
to be regularly recalibrated at known heights,

Not regularly, *at the start of every trip*.

I meant regularly *throughout each trip*, especially at the start, which can
be inconvenient if you don't start at a spot height marker, so you have to
guesstimate based on contours.

Quoted message said:

I find they need calibration throughout a day trip at every reasonable
opportunity.
A good unit is a doddle to reset to a known spot height.

Do you find occasions when spot heights are few and far between?

Quoted message said:

Knowing the direction in which the air pressure is changing when
recalibrating/checking can give clues as to the likely changes in the
weather.

GPS units with built in barometric altimeters do the re-calibration
for you. But can take away the knowledge of what's happening.

If by "what's happening" you mean the overall pressure change, that's not a
problem since the GPS records pressure data for the previous 48 hours. You
can switch from path profile to a pressure graph, which shows how the
pressure has been changing for the past two days, which is helpful when
anticipating weather changes.

The pressure graph gets messed up during a walk of course, but unless you've
been walking non-stop for the previous 48 hours, the overall trend is easy
to see. When you switch the GPS off, it continues to log pressure changes
at regular intervals (using negligible battery power) and since the GPS is
switched off, there's no auto-calibration, so you see the actual pressure
change. This is very handy when you wake up your tent the next morning.

Tip: It's worth marking a waypoint at your campsite when you get there, so
as to record the altitude. This is useful for recalibrating the altimeter
before your next walk.

Paul

Paul Saunders said:

Nigel Cliffe said:

Quoted message said:

>Standalone altimeters need
>to be regularly recalibrated at known heights,

Not regularly, *at the start of every trip*.

I meant regularly *throughout each trip*, especially at the start, which can
be inconvenient if you don't start at a spot height marker, so you have to
guesstimate based on contours.

Quoted message said:

I find they need calibration throughout a day trip at every reasonable
opportunity.

Another issue with standalone altimeters (and I'm thinking Suunto /
Avocet here) is that the internal calculations which work out air
pressure vs. altitude can be thrown way off by variations in
temperature. If you're climbing during a non-typical altitide vs.
temperature day, they can get it way wrong (like in excess of 150M on a
950M hill.

druidh

Paul Saunders said:

Nigel Cliffe said:

Quoted message said:

>Standalone altimeters need
>to be regularly recalibrated at known heights,

Not regularly, *at the start of every trip*.

I meant regularly *throughout each trip*, especially at the start, which can
be inconvenient if you don't start at a spot height marker, so you have to
guesstimate based on contours.

Quoted message said:

I find they need calibration throughout a day trip at every reasonable
opportunity.

Another issue with standalone altimeters (and I'm thinking Suunto /
Avocet here) is that the internal calculations which work out air
pressure vs. altitude can be thrown way off by variations in
temperature. If you're climbing during a non-typical altitide vs.
temperature day, they can get it way wrong (like in excess of 150M on a
950M hill.

druidh

Paul Saunders said:

Nigel Cliffe said:

Quoted message said:

> Standalone altimeters need
> to be regularly recalibrated at known heights,

Not regularly, *at the start of every trip*.

I meant regularly *throughout each trip*, especially at the start,
which can be inconvenient if you don't start at a spot height marker,
so you have to guesstimate based on contours.

Whilst the attributions are correct, I wrote stuff which agreed with Paul,
recalibration being necessary throughout the day.

Quoted message said:

Quoted message said:

I find they need calibration throughout a day trip at every
reasonable opportunity.
A good unit is a doddle to reset to a known spot height.

Do you find occasions when spot heights are few and far between?

It happens. But, there are other ground features than spot heights which are
useful - if a contour line intercepts a particular feature, then that is
sufficient to set a reading.

The biggest problem is erroneous heights on footpath signs, some French
signs can be out by surprisingly large vertical distances, the height
referring to some object in the vacinity rather than the sign (and others
are just plain wrong!). Its always worth checking maps carefully before
re-calibrating against a footpath sign with a height marked on it !

Recently I have bought a Garmin Vista-C, its an awful lot easier just to use
that as a general data recorder throughout the day. If I need navigation
over the paper map & compass, it has it all there on the screen instantly.
As a result, the altimeter is reduced to very occaisional use, or on trips
where carrying batteries and charger for the Garmin is a problem.

Quoted message said:

Quoted message said:

Knowing the direction in which the air pressure is changing when
recalibrating/checking can give clues as to the likely changes in the
weather.

GPS units with built in barometric altimeters do the re-calibration
for you. But can take away the knowledge of what's happening.

If by "what's happening" you mean the overall pressure change, that's
not a problem since the GPS records pressure data for the previous 48
hours. You can switch from path profile to a pressure graph, which
shows how the pressure has been changing for the past two days, which
is helpful when anticipating weather changes.

True.
But assumes you've read the manual and have a clue how to do it.
If you just run the GPS on "automatic", then you don't find those tricks,
and because it works on "automatic", then the incentive to learn is reduced.
Its down to whether an owner of a device makes full use of what it can do,
and ideally learns those things in a non-threatening situation so they can
make use of it effectively if lost or in very bad weather.

Quoted message said:

The pressure graph gets messed up during a walk of course, but unless
you've been walking non-stop for the previous 48 hours, the overall
trend is easy to see. When you switch the GPS off, it continues to
log pressure changes at regular intervals (using negligible battery
power) and since the GPS is switched off, there's no
auto-calibration, so you see the actual pressure change. This is
very handy when you wake up your tent the next morning.

Agreed. And the same applies to any altimeter - stick it onto "barometric
weather pressure" rather than altitude when stationary.

- Nigel

--
Nigel Cliffe,
Webmaster at http://www.2mm.org.uk/

Paul Saunders said:

Standalone altimeters need to be
regularly recalibrated at known heights, whereas the GPS altimeter is
constantly auto-calibrated from the GPS altitude readings, so you never have
to worry about it drifting off.

I've looked at the manuals for various Garmin units with altimeter
function but I can't see any mention of auto calibration. It appears
that unless you manually calibrate then the accuracy is no better than
a stand-alone altimeter. Am I wrong?
--
remove the ox to reply

Chriss said:

I've looked at the manuals for various Garmin units with altimeter
function but I can't see any mention of auto calibration. It appears
that unless you manually calibrate then the accuracy is no better than
a stand-alone altimeter. Am I wrong?

There are only a few GPS models that have a built in altimeter, but I
believe they all have the capacity to auto-calibrate, you simply switch it
on or off in the options.

Which models have you looked at specifically?

Paul

The message <[email hidden]>
from "Paul Saunders" <[email hidden]> contains these words:

Quoted message said:

Quoted message said:

I've looked at the manuals for various Garmin units with altimeter
function but I can't see any mention of auto calibration. It appears
that unless you manually calibrate then the accuracy is no better than
a stand-alone altimeter. Am I wrong?

Quoted message said:

There are only a few GPS models that have a built in altimeter, but I
believe they all have the capacity to auto-calibrate, you simply switch it
on or off in the options.

The Summit certainly did and I am sure the Vista C does but there is of
course no reference to auto-calibration in the index and a quick sweep
through the various screens on the gps failed to provide a pointer. The
more complicated Garmin GPSs become the worse the instruction manual
(and the Summit one was [censored]).

--
Roger Chapman so far this year 62 summits
New - 28 (Marilyns 14, Nuttalls 5, Outlying Fells 10)
Repeats - 34 (Marilyns 16, Nuttalls 24, Wainwrights 12, Outlying Fells 0)