On 5/10/12 7:40 AM, Arthur Dent wrote:
I've found significant altitude errors using a GPS and the following quotes
found on the internet will explain why. From my experience of hiking
in the mountains of New Hampshire an aneroid altimeter will vary with
atmospheric pressure about 200 feet for a change of 0.2" of mercury
so you have to continually set it at known waypoints, just like setting a
frequency standard against a known reference, and then it will be
'accurate' for some length of time, and then you set it again. GPS altitude
will be off but it should be fairly consistent in spite of the changing
atmospheric pressure. The earth neither spins at a constant rate nor is
it a perfect sphere. Maybe we need to trade it in for a newer model. ;-)
"GPS altitude measures the users' distance from the center of the SVs
orbits.
Not precisely true. The SV orbits follow the usual Keplerian things so
the focus of the ellipse is close to the barycenter of Earth, but of
course, the moon and non uniform gravity of the Earth affect it too.
GPS fixes are relative to WGS84 coordinate system (x,y,z) 0,0,0 in
WGS84 is within a few cm of the center of mass of the Earth. WGS 84
also defines a datum for the surface (which is not, generally, the
geoid) as an ellipsoid of revolution. (compare to the Clarke 1866
ellipsoid)
These measurements are referenced to geodetic altitude or
ellipsoidal altitude in some GPS equipment.
This is a bit fuzzy... there are differences between geodetic and
geocentric altitude for instance. And then there's the reference
ellipsoid (e.g. Clarke 1866), or more generally the "geoid"
Garmin and most equipment
manufacturers utilize a mathematical model in the GPS software which
roughly approximates the geodetic model of the earth and reference
altitude to this model.
Mmm.. I don't know that it "roughly approximates".. WGS84 is precisely
defined (that's the coordinate system). The geoid (in terms of the "sea
surface" is defined in terms of spherical harmonics and varies some 100m
or thereabouts from the reference datum.
WHether your GPS uses the WGS84 datum (simple ellipsoid) or the fancier
geoid, is something you'd have to look up.
As with any model, there will be errors as the
earth is not a simple mathematical shape to represent. What this
means is that if you are walking on the seashore, and see your altitude
as -15 meters, you should not be concerned. First, the geodetic model
of the earth can have much more than this amount of error at any specific
point and second, you have the GPS error itself to add in. As a result of
this combined error, I am not surprised to be at the seashore and see -40
meter errors in some spots."
Actually, no.. the geodetic model (e.g. the EGM96) should be VERY close
to the actual sea surface (barring tides and local geographic effects..
the Gulf Stream sits several meters higher because it's warmer and less
dense)
The ellipsoid could easily be off by tens of meters.
"We have to make some assumptions about the shape of the earth. WGS84
has defined that shape to be an ellipsoid, with a major and minor axis. The
particular dimensions chosen are only an approximation to the real shape.
Ideally, such an ellipsoid would correspond precisely to "sealevel" everywhere
in the world. As it turns out, there are very few places where the WGS84
ellipsoid definition coincides with sealevel. On average, the discrepancy is
zero, but that doesn't help much when you're standing at the water's edge of
an ocean beach and your GPS is reading -100ft below sealevel. The deviation
can be as large as 300ft in some isolated locations. When the National Marine
Electronics Association came up with the NMEA standard, they decreed that
altitudes reported via NMEA protocol, shall be relative to mean (average) sea
level. This posed a problem for GPS manufacturers. How to report altitudes
relative to mean sea level, when they were only calculating altitude relative to
the WGS84 ellipsoid. Ignoring the discrepancy wasn't likely to make GPS users
very happy. As it happens, there is actually a model of the difference between
the WGS84 ellipsoid and mean sea level. This involves harmonic expansions
at the 360th order. It's a very good model, but rather unusable in a handheld
device. It was determined that this model could be made into a fairly simple
lookup table included in the GPS receiver. The table is usually fairly coarse
lat/lon wise, but the ellipsoid to mean sea level variation, known as geoidal
separation, varies slowly as you move in lat/lon."
And that is a more accurate description..
The question really is "what does YOUR receiver report".. if it's MSL in
NMEA strings then I would imagine all modern receivers use some form of
geoid model with error probably <1 meter. If it's WGS84, then it ignores
the geoid.
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