Yokel wrote:

Unless the accuracy has improved markedly in the last few years, GPS is not
particularly good for altitude. The potential error is at least 15m
depending on the sattelite geometry [=where the sattelites being refererred
to are in relation to the receiver] and other factors

15m error would be enough to put me 7m below sea level!

--
Jonathan Stott
Canterbury Weather: http://www.canterburyweather.co.uk/

"
Unless the accuracy has improved markedly in the last few years, GPS is
not
particularly good for altitude. The potential error is at least 15m
depending on the sattelite geometry [=where the sattelites being
refererred
to are in relation to the receiver] and other factors (I believe the
civilian signal is deliberately "degraded" slightly), resulting in me
occasionally getting a "sea level" reading at my home! The error varies
rapidly in sign and value as the sattelite geometry changes - this can be
seen certainly on a timescale of hours if not tens of minutes.

For areas as "flat" as Cambridge, GPS is not sufficiently accurate.
Unless
you are in very hilly country, the best way of measuring your height with
GPS is to use it to get your position on a map, then read height off the
map
contours.
--
- Yokel -

That was my view until I used my Garrmin Etrex. I have used it in a number
of locations in Hertfordshire and Essex and have compared the altitude with
that read off 1:25000, OS maps and in every case the reading has agreed
within the limits of accuracy with the map contours. I was pleasnatly
surprised.

Alan

"Alan Gardiner" wrote in message
...

"
Unless the accuracy has improved markedly in the last few years, GPS is
not
particularly good for altitude. The potential error is at least 15m
depending on the sattelite geometry [=where the sattelites being
refererred
to are in relation to the receiver] and other factors (I believe the
civilian signal is deliberately "degraded" slightly), resulting in me
occasionally getting a "sea level" reading at my home! The error varies
rapidly in sign and value as the sattelite geometry changes - this can be
seen certainly on a timescale of hours if not tens of minutes.

For areas as "flat" as Cambridge, GPS is not sufficiently accurate.
Unless
you are in very hilly country, the best way of measuring your height with
GPS is to use it to get your position on a map, then read height off the
map
contours.
--
- Yokel -

That was my view until I used my Garrmin Etrex. I have used it in a number
of locations in Hertfordshire and Essex and have compared the altitude
with that read off 1:25000, OS maps and in every case the reading has
agreed within the limits of accuracy with the map contours. I was
pleasnatly surprised.

Alan

I have a Garmin MAP76S and this can be used in aircraft to evaluate TRUE
height. Can be quite interesting compred to what the pilot tells you !

Brian

"Brian Ludwig" wrote in message
k...

"Alan Gardiner" wrote in message
...

"
Unless the accuracy has improved markedly in the last few years, GPS is
not
particularly good for altitude. The potential error is at least 15m
depending on the sattelite geometry [=where the sattelites being
refererred
to are in relation to the receiver] and other factors (I believe the
civilian signal is deliberately "degraded" slightly), resulting in me
occasionally getting a "sea level" reading at my home! The error varies
rapidly in sign and value as the sattelite geometry changes - this can
be
seen certainly on a timescale of hours if not tens of minutes.

For areas as "flat" as Cambridge, GPS is not sufficiently accurate.
Unless
you are in very hilly country, the best way of measuring your height
with
GPS is to use it to get your position on a map, then read height off the
map
contours.
--
- Yokel -

That was my view until I used my Garrmin Etrex. I have used it in a
number of locations in Hertfordshire and Essex and have compared the
altitude with that read off 1:25000, OS maps and in every case the
reading has agreed within the limits of accuracy with the map contours. I
was pleasnatly surprised.

Alan

I have a Garmin MAP76S and this can be used in aircraft to evaluate TRUE
height. Can be quite interesting compred to what the pilot tells you !

Brian

Brian-what do you mean by true height?

All the best
--
George in Epping, West Essex (107m asl)
www.eppingweather.co.uk
www.winter1947.co.uk

On Sun, 09 Jan 2005 12:43:33 GMT, "Brian Ludwig"
wrote:

I have a Garmin MAP76S and this can be used in aircraft to evaluate TRUE
height. Can be quite interesting compred to what the pilot tells you !

Your GPS will display altitude above sea level. Above 'transition
altitude' aircraft fly at a 'pressure altitude' which may not coincide
with altitude above sea level.

--
Alan White
Twenty-eight miles NW of Glasgow.
Overlooking Loch Goil and Loch Long in Argyll, Scotland.
Web cam at http://www.ufcnet.net/~alanlesley1/kabcam.htm

On Fri, 07 Jan 2005 16:49:29 +0000, Mike Causer
wrote:

On Fri, 07 Jan 2005 16:30:34 +0000, Gavin Staples wrote:

Thanks very much Mike. I am interested to know the altitude (ASL) of my
home village of Horseheath, South Cambs.

There's a spot-height at the T-junction in the middle of the village (with
phone box) at 93m AMSL. The church is on the 95m contour, and the 100m
contour crosses the old Cambridge - Haverhill road at the junction with
the bypass to the east, and just at the end of the houses to the west.

You should be able to interpolate to about a metre with luck.

While I've got the map out, the spot height is pretty close to 56:06:00N
00:21:10E (do not trust Mapquest et al. for Lat & Long!)

Interpolation usually works, of course, but you do need to apply local
knowledge and allow for any discontinuities.

There are some low cliffs/steep slopes near me at Hillhead (Solent) of
around 10m height, with a large plateau at the top. Houses along the
clifftop road appear at first sight to be down at sea level and in
danger from unusually high tides but in reality, they are quite safe
as they are up around 9 to 10m amsl. The 10m contour is about 400m
inland and interpolation used in some digital OS based maps (such as
Memory Map) produces a significant error in the digital readout of
height above sea level. It's especially significant on the coast!

--
Dave

"Alan White" wrote in message
...

Your GPS will display altitude above sea level. Above 'transition
altitude' aircraft fly at a 'pressure altitude' which may not coincide
with altitude above sea level.

Even with an altimeter set to the correct sea level pressure (strictly the
"QNH"), the barometric altitude on the pilot's altimeter may not correspond
to the true altitude -- the altimeter makes standard assumptions about the
temperature of the air between its level and its reference level (usually
sea level) in order to turn a pressure difference into an altitude.

For air traffic control purposes, this is a Good Thing. It doesn't matter
if you fly around with 10000 ft on your altimeter when the true altitude is
9700 ft, as long as everyone flying at that level agrees to use the same
standard!

Julian Scarfe

On Sun, 09 Jan 2005 17:36:19 GMT, "Julian Scarfe"
wrote:

For air traffic control purposes, this is a Good Thing. It doesn't matter
if you fly around with 10000 ft on your altimeter when the true altitude is
9700 ft, as long as everyone flying at that level agrees to use the same
standard!

That's what I meant by 'flying at a pressure altitude'.

To enlarge, but you probably know this, above transition level all
aircraft set their altimeters to 29.92 or 1013. Thus, they are all
using the same standard and their true altitudes matter not. It's
their altitudes relative to one another that matters. Two aircraft,
one flying at an indicated 30,000 feet and the other flying at an
indicated 31,000 feet will maintain 1,000 feet separation even though
their true absolute altitudes could be more or less than 30,000 feet
and 31,000 feet, depending on the atmospheric conditions, i.e.
temperature and pressure.

The true altitudes of our two aircraft could vary considerably flying
through areas of widely different atmospheric pressure but relative to
one another their altitudes remain constant provide they both maintain
an indicated 30,000 feet and 31,000feet. Autopilots handle this quite
well.

When flying below transition level, the altimeters will have to be
adjusted at the behest of ATC otherwise things get tricky.

Jack can explain this better than me.

--
Alan White
Twenty-eight miles NW of Glasgow.
Overlooking Loch Goil and Loch Long in Argyll, Scotland.
Web cam at http://www.ufcnet.net/~alanlesley1/kabcam.htm

"Alan White" wrote in message
...

That's what I meant by 'flying at a pressure altitude'.

Yes, I guess I was splitting hairs, while agreeing with what you wrote. My
point was only that, even below the transition altitude, with the altimeter
set to the QNH, there can still be differences between barometric
(indicated) altitude and true altitude. While these are occasionally
significant for terrain clearance, it's OK for separation as long as
everyone has the same reference pressure set.

Julian

On Sun, 09 Jan 2005 19:15:22 GMT, "Julian Scarfe"
wrote:

Yes, I guess I was splitting hairs, while agreeing with what you wrote.

:-)

--
Alan White
Twenty-eight miles NW of Glasgow.
Overlooking Loch Goil and Loch Long in Argyll, Scotland.
Web cam at http://www.ufcnet.net/~alanlesley1/kabcam.htm