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'[EE]:Barometer & Altimeter'
2002\09\04@171408 by Gordon Varney

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I have been asked to through together a barometer and altimeter.

A barometer is easy, using a Motorola MPX4115A. Just convert the voltage out to barometric pressure using a formuli....
Getting an altitude from this is also easy, Just run another one of them thar formuli.. and you have altitude. The
problem is, neither one of these measurements are a constant.

If I am driving up a mountain and a storm is approaching then what? The pressure drops from the storm and changes the
barometric pressure changing the altimeter, which is changing because I am driving up the mountain.

The MPX4115A is calibrated and compensated. Then pressure measured is correct, so how do I know when the change if from
the altitude or a storm? If I am not moving then no problem. The altitude is the constant, If there is no storm then the
relative barometric pressure is the constant.

Gordon Varney
Voice Active Remote Inc.
http://www.talk2it.com
>

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2002\09\04@173110 by Lee Jones

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> I have been asked to throw together a barometer and altimeter.
>
> A barometer is easy, using a Motorola MPX4115A. Just convert the
> voltage out to barometric pressure using a formuli....

Yep, getting local barometric pressure is straight forward.

> Getting an altitude from this is also easy, Just run another one
> of them thar formuli.. and you have altitude. The problem is,
> neither one of these measurements are a constant.

To convert local barometric pressure into altitude, you have to
assume some constants about the behavior of the atmosphere (which
are pretty stable) and know the local sea level equivalent pressure.

Local sea level equivalent pressure varies dramatically with the
current weather patterns in the area.  Before shooting an approach
under instrument flight rules, you are required to get sea level
equivalent pressure (i.e. altimeter setting) within a certain
distance of the airport.

> If I am driving up a mountain and a storm is approaching then what?
> The pressure drops from the storm and changes the barometric pressure
> changing the altimeter, which is changing because I am driving up the
> mountain.  The MPX4115A is calibrated and compensated. Then pressure
> measured is correct, so how do I know when the change if from the
> altitude or a storm? If I am not moving then no problem. The altitude
> is the constant, If there is no storm then the relative barometric
> pressure is the constant.

If you are moving, then there is no way to know if the change in
barometric pressure is due to your movement or atmospheric changes.

                                               Le

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2002\09\04@174945 by Peter L. Peres

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There is no way to solve that problem with a pressure gauge alone, but the
difference should be small enough not to matter seriously if you are
driving. Unless it's a hurricane or some other extreme weather system.

Peter

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2002\09\04@175412 by Dave King

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>If I am driving up a mountain and a storm is approaching then what? The
>pressure drops from the storm and changes the
>barometric pressure changing the altimeter, which is changing because I am
>driving up the mountain.
>
>The MPX4115A is calibrated and compensated. Then pressure measured is
>correct, so how do I know when the change if from
>the altitude or a storm? If I am not moving then no problem. The altitude
>is the constant, If there is no storm then the
>relative barometric pressure is the constant.
>
>Gordon Varney

You can't know what is affecting what. In aircraft altimeters the
barometric pressure is set to local pressures as you go
along if you are low level and to 29.92 inches of mercury if you are ifr.
The idea is that everyone in the same area/altitude
will at least be using the same base setting and be able to know what
altitude they are at.

With a fixed station position you can set the altimeter to a known altitude
and derive the current pressure. With your setup
unless you have access to a local/regional report or can set it to a known
altitude its as good as you can get.

Dave

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2002\09\04@180459 by Sid Weaver

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In a message dated 09/04/2002 17:32:09 Eastern Daylight Time,
spam_OUTleeTakeThisOuTspamFRUMBLE.CLAREMONT.EDU writes:


>
> Local sea level equivalent pressure varies dramatically with the
> current weather patterns in the area.  Before shooting an approach
> under instrument flight rules, you are required to get sea level
> equivalent pressure (i.e. altimeter setting) within a certain
> distance of the airport.
>
>

This also means that every day you will have to get the SLEP from your local
airport or Weather Bureau if you want extreme accuracy and factor in this
change in your formula.  For instance, if the SLEP is 29.50 at your location
when you set your formula, then the factor is 1 and formula/1 gives you your
altitude.  If the SLEP changes to 29.30 the next day, the factor becomes
29.50/29.30 = 1.007 so now your formula has to say formulas/1.007.

This could get pretty involved.

Sid

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2002\09\04@181756 by Olin Lathrop

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> The MPX4115A is calibrated and compensated. Then pressure measured is
correct, so how do I know when the change if from
> the altitude or a storm? If I am not moving then no problem. The altitude
is the constant, If there is no storm then the
> relative barometric pressure is the constant.

That's why airports have barometers so that pilots can compensate their
alitmeter to the pressure at a known altitude for the location they care
about.


*****************************************************************
Embed Inc, embedded system specialists in Littleton Massachusetts
(978) 742-9014, http://www.embedinc.com

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2002\09\04@182407 by Lee Jones

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>> Local sea level equivalent pressure varies dramatically with the
>> current weather patterns in the area.  Before shooting an approach
>> under instrument flight rules, you are required to get sea level
>> equivalent pressure (i.e. altimeter setting) within a certain
>> distance of the airport.

> This also means that every day you will have to get the SLEP from
> your local airport or Weather Bureau if you want extreme accuracy
> and factor in this change in your formula.

Yes.

This is exactly what you do when you enter the local sea level
equivalent barometric pressure in the Kollsman window of your
sensitive altimeter.  You are changing one variable of the
mechanical analog computer in your altimeter (or one variable
in the program running in your flight management system if you
are flying higher end, usualy turbine powered, aircraft).

Even "common" (non storm) weather changes can cause the local
barometric pressure to fluctuate enough to result in 200-300
feet of altitude variation.  Maybe this doesn't matter in a car,
but it sure matters in an airplane when you can't see the ground.


> For instance, if the SLEP is 29.50 at your location when you set
> your formula, then the factor is 1 and formula/1 gives you your
> altitude.  If the SLEP changes to 29.30 the next day, the factor
> becomes 29.50/29.30 = 1.007 so now your formula has to say
> formulas/1.007.  This could get pretty involved.

If you are building a sensitive altimeter, you have to allow for
entry of the local sea level equivalent pressure (aka barometric
pressure) during use.  It's not something you can build into the
formula as a compile time constant.

                                               Lee Jones

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2002\09\04@182419 by Nate Duehr

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This is way off-topic on this response, but in this day and age of
e-mail list archives ending up on Google...

Dave... you're going to end up hurting someone if you use 29.92 every
time you're IMC like that.

You're actually supposed to use the closest known correct altimeter
setting to your location.  And ATC has requirements to make sure that
you have that setting if you're in the IFR system unless you're in
uncontrolled airspace (rare even out here in the "West" anymore), then
it's the Pilot-in-Command's responsibility to get.

I'm too lazy to go to the basement to get the FAR, AIM, and Controller's
Handbook references right now, but please... don't just use 29.92
blindly.

You're friendly neighborhood aviation safety warning guy...

Nate

On Wed, 2002-09-04 at 15:46, Dave King wrote:
{Quote hidden}

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2002\09\04@182554 by Jason Wolfson

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You have 1 equation and 2 unknowns.

Barometric pressure drops 0.01 in Hg for every 10 feet of altitude gain.
Or 1" Hg every 1,000 ft is another way to remember it.

Aircraft set their altimeters to 29.92" above 18,000 ft. (you must be on
an IFR flight plan above 18,000 ft) Below that to the local barometric
pressure. If your on the ground you can set the altimeter to the field
elevation and read the barometric pressure.

Jason

>{Original Message removed}

2002\09\04@184047 by Lee Jones

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>> You can't know what is affecting what. In aircraft altimeters the
>> barometric pressure is set to local pressures as you go along if
>> you are low level and to 29.92 inches of mercury if you are ifr.
>> The idea is that everyone in the same area/altitude will at least
>> be using the same base setting and be able to know what altitude
>> they are at.

> Dave... you're going to end up hurting someone if you use 29.92
> every time you're IMC like that.

> You're actually supposed to use the closest known correct altimeter
> setting to your location.  And ATC has requirements

United States rules & procedures are not international.  I have
not flown IFR outside the US, but I believe that other countries
require setting altimeters to 29.92" Hg at much lower altitudes
than the US' requirement of 18,000 feet.

Dave's fully qualified domain name ended in .CA, which implies
he is in Canada.  He may very well have been quoting the correct
procedure for Canadian IFR.  I don't have Canadian equivalent of
the FARs handy (nor do I want to take the time to go searching).

                                               Lee Jones

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2002\09\04@184257 by Bob Japundza

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He's not entirely wrong.

Above 18,000 you use 29.92 for your altimeter setting.

Bob

{Quote hidden}

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2002\09\04@190224 by M. Adam Davis

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You are correct.  You cannot get both values out of a pressure sensor
unless you hold one of those values constant by not moving or by
stopping the weather changes (ie, getting a report from a local,
stationary pressure at a known altitude and compensating yours with it).

You can get a semi-useful altitude reading from a GPS unit, but they
have much worse altitude error than their location error (ie, +/- 100ft
as opposed to +/- 10ft).

What is the application/acceptable error rate (ie, +/- 1ft/hour is
allowed, etc)?  You can always integrate accelerometers and gyros to
find relative position (dead reckoning).  DGPS and WAAS can correct a
GPS signal so you have better resolution, etc.

-Adam

Gordon Varney wrote:

{Quote hidden}

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2002\09\04@193359 by Herbert Graf

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> > The MPX4115A is calibrated and compensated. Then pressure measured is
> correct, so how do I know when the change if from
> > the altitude or a storm? If I am not moving then no problem.
> The altitude
> is the constant, If there is no storm then the
> > relative barometric pressure is the constant.
>
> That's why airports have barometers so that pilots can compensate their
> alitmeter to the pressure at a known altitude for the location they care
> about.

       On can always go GPS, get a fix on at least four sats and the GPS spits out
current altitude, quite accurate AFAIK. TTYL

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2002\09\04@194627 by Dave King

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At 04:23 PM 04/09/02 -0600, you wrote:
>This is way off-topic on this response, but in this day and age of
>e-mail list archives ending up on Google...
>
>Dave... you're going to end up hurting someone if you use 29.92 every
>time you're IMC like that.

Oops I thought I typed over 18K imc ;-]

Oh well now adays I'm so old if I get over 500 feet I get nose bleeds and I
just
can't see whats going on at the local nude beach ;-]

Dave

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2002\09\04@195049 by Dave King

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>Relative barometric pressure is the constant.
> >
> > That's why airports have barometers so that pilots can compensate their
> > alitmeter to the pressure at a known altitude for the location they care
> > about.
>
>         On can always go GPS, get a fix on at least four sats and the GPS
> spits out
>current altitude, quite accurate AFAIK. TTYL

I just went flying with a friend who had a new gps with the map. He was
hitting on 5 satellites (I think)
and the altitude and airspeed were bang on what the instruments were
saying.  Very impressive.
And here I am with flying around my 20 year 50 cent esso road map......;-]

Dave

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2002\09\04@195926 by Nate Duehr

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I know.  He said "low-level".

On Wed, 2002-09-04 at 16:42, Bob Japundza wrote:
> He's not entirely wrong.
>
> Above 18,000 you use 29.92 for your altimeter setting.
>
> Bob
>
> > {Original Message removed}

2002\09\04@200101 by Nate Duehr

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GPS altitude is measured above a "datum" and sometimes is accurate,
sometimes not...

Nate

On Wed, 2002-09-04 at 17:05, Herbert Graf wrote:
{Quote hidden}

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2002\09\04@203927 by Dave Dilatush

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Gordon Varney wrote...

>I have been asked to through together a barometer and altimeter.
>
>A barometer is easy, using a Motorola MPX4115A. Just convert the voltage out to barometric pressure using a formuli....
>Getting an altitude from this is also easy, Just run another one of them thar formuli.. and you have altitude. The
>problem is, neither one of these measurements are a constant.
>
>If I am driving up a mountain and a storm is approaching then what? The pressure drops from the storm and changes the
>barometric pressure changing the altimeter, which is changing because I am driving up the mountain.
>
>The MPX4115A is calibrated and compensated. Then pressure measured is correct, so how do I know when the change if from
>the altitude or a storm? If I am not moving then no problem. The altitude is the constant, If there is no storm then the
>relative barometric pressure is the constant.

One more factor to consider is the effect of ambient temperature
on your pressure sensor.  Although Motorola's MPX series are
internally temperature compensated, that compensation is not
perfect.

There are residual errors in both offset and scale factor.  When
taken as a percentage of the MPX's entire span these are small;
but for barometer/altimeter use, where you are using only a part
of the sensor's full range, they could lead to significant
errors.

You might consider keeping track of the ambient temperature with
a suitable temperature sensor, and using that to correct any
offset/span changes in your MPX.

Dave D.

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2002\09\04@204751 by Herbert Graf

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> >Relative barometric pressure is the constant.
> > >
> > > That's why airports have barometers so that pilots can
> compensate their
> > > alitmeter to the pressure at a known altitude for the
> location they care
> > > about.
> >
> >         On can always go GPS, get a fix on at least four sats
> and the GPS
> > spits out
> >current altitude, quite accurate AFAIK. TTYL
>
> I just went flying with a friend who had a new gps with the map. He was
> hitting on 5 satellites (I think)
> and the altitude and airspeed were bang on what the instruments were
> saying.  Very impressive.
> And here I am with flying around my 20 year 50 cent esso road map......;-]

       Yes, I use it for long car trips and it's amazing (one of teh BGmicro
units), navigating is SO different with it since if you see something up
ahead you don't even think about NOT turning onto this dirt road in the
middle of nowhere since you know exactly where you are. Of course I always
have a paper map as backup. TTYL

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2002\09\04@205013 by Herbert Graf

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Excuse my ignorance but what is a "datum"? FWIW I have checked the altitude
reading I get from my GPS in several places and as far as I can tell it's
pretty darn accurate, I'd say to about 10 meters, more then enough for many
cases. TTYL

> GPS altitude is measured above a "datum" and sometimes is accurate,
> sometimes not...

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2002\09\04@214717 by Gordon Varney

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Yes, It will also have a temperature sensor and a humidity sensor as well.
The accuracy for this device is rough to say the least. It is for a portable
weather station, I am not sure why they want an altimeter, except that being
portable who knows where it will end up.

Gordon Varney
Voice Active Remote Inc.
http://www.talk2it.com



{Quote hidden}

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2002\09\04@215611 by Nate Duehr

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My understanding of it is that the GPS system gives an altitude above a
"fake" globe, which may or may not exactly match the "real" globe, which
isn't perfect and has bumps and bulges... (GRIN)...

Nate

On Wed, 2002-09-04 at 18:48, Herbert Graf wrote:
{Quote hidden}

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2002\09\04@222047 by Jim Korman

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Herbert Graf wrote:

>Excuse my ignorance but what is a "datum"? FWIW I have checked the altitude
>reading I get from my GPS in several places and as far as I can tell it's
>pretty darn accurate, I'd say to about 10 meters, more then enough for many
>cases. TTYL
>
>>GPS altitude is measured above a "datum" and sometimes is accurate,
>>sometimes not...
>>
Try the following page

   http://www.colorado.edu/geography/gcraft/notes/datum/datum.html


Jim

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2002\09\04@222442 by Jim

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From within: http://www.allgps.com/gps/chapter2n3.htm


Mathematical [GPS] models

This might be a good place to introduce a common engineering practice ... .
In the last section, we mentioned the use of mathematical models to correct
the effects of the atmosphere on GPS receivers. Many times, engineers use
mathematical models to solve problems.

One example of this is the use of empirical formulae.

<snip>

In GPS applications, yet another example of empirical formulae is the use of
the ephemeris.

This is essential, because the equations used by GPS receivers to correct
for the solar wind, the gravitation force of the moon, and other such
factors on the GPS satellites are too complicated to be predicted by
theoretical means. They are not like Kepler's equations, which can be
deduced from Newton's theory of universal gravitation. So the only practical
way to solve these real-world problems is, again, by using mathematical
models.

Your GPS receiver also has another way to utilize math models. It uses an
ellipsoidal model of the earth's shape to estimate the latitude, longitude,
and altitude of the receiver. The equatorial radius of the earth is
6,378.137 kilometers and its polar radius is 6,356.762 kilometers; the
difference is due to a slight flattening of the earth caused by its
spinning. Because it does not coincide precisely with the earth's surface,
this model causes discrepancies in the altitude calculation of the GPS
receiver; it is necessary, however, because there is no better way to
represent the real world. We will talk a little about this in Chapter 5, in
the GPS and GIS section.


RF Jim


{Original Message removed}

2002\09\04@222928 by Jim

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Addendum  (pay dirt) to the GPS altitude issue:

http://www.allgps.com/gps/chapter5.htm

- -  - -  - -  - -  - -  - -  - -  - -  - -  - -  - -  - -  - -  - -

We need to devote some additional attention to GPS elevation. This is not
actual height above the ground, and it is not elevation above sea level
either, because GPS receivers without GIS have no knowledge of the real
world.

It is the software in your GPS receiver, remember, that computes your
position, altitude, and speed.

The software uses an ellipsoid model, called the WGS-84 (World Geodetic
System 1984), to approximate the surface of the earth (see Fig. 5-3 and the
Mathematical Model section of Chapter 2).

Because of its rotation, the shape of the earth is that of a slightly
flattened sphere.

The calculated GPS height is the vertical distance above the imaginary,
mathematical surface of this sphere. This is a smooth ellipsoid surface that
carries no terrain information such as is provided by GIS. Unfortunately,
this surface does not coincide with sea level, either. This fact combines
with other GPS errors like selective availability and atmospheric delay to
make things even worse.

- -  - -  - -  - -  - -  - -  - -  - -  - -  - -  - -  - -

RF Jim

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2002\09\04@225428 by Herbert Graf

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       I still don't understand. While the earth has bumps they don't matter,
altitude is based on sea level, sea level doesn't have bumps, AFAIK. Perhaps
the problem has something to do with the partially "squished" globe that the
earth actually is? TTYL


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2002\09\04@225700 by Herbert Graf

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Thanks for the link, I HAVE heard of that problem, but, correct me if I'm
wrong, that error can be corrected manually since you know WHERE you are,
and unless you are travelling thousands of kilometers the error doesn't
change much. TTYL

{Quote hidden}

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2002\09\04@231405 by Dave Tweed

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Herbert Graf <.....mailinglistKILLspamspam.....FARCITE.NET> wrote:
> I still don't understand. While the earth has bumps they don't matter,
> altitude is based on sea level, sea level doesn't have bumps, AFAIK.

Actually, it does, even ignoring the effects of tides and weather.
The density of the earth is far from constant, and the "geoid" -- the
equipotential gravitational surface that corresponds to mean sea level --
is quite lumpy indeed.

-- Dave Tweed

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2002\09\05@061724 by Alan B. Pearce

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>On can always go GPS, get a fix on at least four sats and
>the GPS spits out current altitude, quite accurate AFAIK. TTYL

Not according to what I have read. It can be up to 50 feet out, and it is
because of the way it is calculated. This is borne out by personal
observation of having just got a GPS receiver and "finding where I live" and
comparing it to a survey map.

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2002\09\05@063012 by Alan B. Pearce

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>My understanding of it is that the GPS system gives an altitude
>above a "fake" globe, which may or may not exactly match the
>"real" globe, which isn't perfect and has bumps and bulges...
>(GRIN)...

My understanding of reading the info on the Motorola Oncore unit is that the
altitude is given from a perfect ball that is centred on the earths centre
of mass, and of nominal diameter the same as the average (or might be mean)
earth diameter.

However the earth is squashed at the poles because the rotational spin has a
centrifugal effect at the equator. The result is that the altitude given by
the GPS is not the height above Mean Sea Level, but the height above the
"ideal shape" (datum), unless your GPS is applying a fudge factor to the raw
data before displaying it.

It is probably possible to work out a suitable fudge factor according to how
far north or south of the equator you are, to remove most of the error
relative to MSL.

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2002\09\05@075012 by Olin Lathrop

face picon face
> sea level doesn't have bumps,

Yes, it does.  The general average "sea level" can be modeled with an
elipsoid.  There are local deviations because the planet's density is not
homogenious and the amount of material also differs (mountains, valleys).
These variations effect the path of the GPS satellites.


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(978) 742-9014, http://www.embedinc.com

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2002\09\05@135012 by Peter L. Peres

picon face
On Wed, 4 Sep 2002, Herbert Graf wrote:

>Thanks for the link, I HAVE heard of that problem, but, correct me if I'm
>wrong, that error can be corrected manually since you know WHERE you are,
>and unless you are travelling thousands of kilometers the error doesn't
>change much. TTYL

The error happens because the geoid is a model of reality at a certain
scale, using a mesh of some sort. Even if it models bumps and hollows it
does so at a limited resolution and if you are between two datum points on
the model you are guaranteed a lot of error even if the model mostly
follows the terrain where you are.

Peter

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2002\09\05@135014 by Peter L. Peres

picon face
On Wed, 4 Sep 2002, Herbert Graf wrote:

>        I still don't understand. While the earth has bumps they don't matter,
>altitude is based on sea level, sea level doesn't have bumps, AFAIK. Perhaps
>the problem has something to do with the partially "squished" globe that the
>earth actually is? TTYL

Sea level has several regular bumps that 'go around' (tides from sun +
moon mainly) plus it is like a liquid column barometer for the weather
above it plus dynamic current effects (Bernoulli & co) change the height
in straits and areas with low depth. Closed water (lakes etc) have
seasonal level variations sometimes into the tens of meters.

Peter

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2002\09\05@135017 by Peter L. Peres

picon face
On Wed, 4 Sep 2002, Nate Duehr wrote:

>My understanding of it is that the GPS system gives an altitude above a
>"fake" globe, which may or may not exactly match the "real" globe, which
>isn't perfect and has bumps and bulges... (GRIN)...

The fake globe is called a geoid and there are settings on GPS units to
switch that. The short answer is, that if the local altitude on your map
and the GPS current setting for reference geoid differ, you may misread in
altitude, potentially a lot (certainly enough to hit a tall point in a
plane in thick fog or to have tunnels dug into a hill at wrong heights on
the two sides, or to build a water pipe that will go uphill without your
knowing and other lovely things like that). The reference geoid (aka
survey date etc) is usually marked on the edge of the map near where the
scale is. If it isn't, worry.

Peter

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2002\09\05@141801 by Dave King

flavicon
face
At 11:15 AM 05/09/02 +0100, you wrote:
> >On can always go GPS, get a fix on at least four sats and
> >the GPS spits out current altitude, quite accurate AFAIK. TTYL
>
>Not according to what I have read. It can be up to 50 feet out, and it is
>because of the way it is calculated. This is borne out by personal
>observation of having just got a GPS receiver and "finding where I live" and
>comparing it to a survey map.

Thats not too bad, the average aircraft altimeter is +/- 75 feet. Even
after we ran
them through the shop they were never dead on accurate.

I've never worried about getting closer than that unless it was doing a survey
of some land (surveyor in a past life, before they told me about the bugs...)

Dave

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2002\09\05@163413 by Peter L. Peres

picon face
>I've never worried about getting closer than that unless it was doing a survey
>of some land (surveyor in a past life, before they told me about the bugs...)

What bugs ?

Peter

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2002\09\05@164924 by Dave King

flavicon
face
At 11:33 PM 05/09/02 +0300, you wrote:
> >I've never worried about getting closer than that unless it was doing a
> survey
> >of some land (surveyor in a past life, before they told me about the
> bugs...)
>
>What bugs ?
>
>Peter

Well if you go far enough north the mosquito's seem about the size of
buzzards...
and seem to swarm thick enough to cause eclipses...

Rumor is they carry you off to snack on later...

I was going through a bottle of deet every day, was dressed up like a mummy
and was still being perforated constantly.

Dave

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2002\09\06@045127 by Alan B. Pearce

face picon face
>Thats not too bad, the average aircraft altimeter is +/- 75 feet. Even
>after we ran them through the shop they were never dead on accurate.

Yeah, if you get a GPS receiver where you can get at the raw data, and leave
the aerial sitting at a stable point for an hour or so, and collect data, it
is quite revealing how the "position" wanders. And this is with SA turned
off :)

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