Re: SEA LEVEL

In article <0Vipi.4883$Da.1660@trnddc07>, Rocky Top <rockytop@xxxxxxxxxxxx> writes

<bobbrooks220@xxxxxxxxx> wrote in message
news:1185240325.827473.55260@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
While sitting at the edge of the ocean yesterday my 76CSx fluctuated
between sixteen and thirty two feet but mostly hung around twenty two
feet elevation over a period of approximately 30 minuted. It was set
at NAD 27. Wasn't I sitting at sea level or closer to it than the
16-32 feet indicated? I did not have access to the barometric pressure
for the area. The technical answers to this question can't penetrate
this thick skull so is there a really simple answer? Thanks.


Try this rather detailed explanation:

http://www.esri.com/news/arcuser/0703/geoid1of3.html

Where you are, the ellipsoid for NAD 27 could very well be 20 feet off
from actual mean sea level.

And this (particularly the diagrams):

http://www.ngs.noaa.gov/GEOID/geoid_def.html

And this too for a simplified explanation:

http://www.unavco.org/edu_outreach/tutorial/geoidcorr.html

Quote: "We have learned that the GPS receiver uses a map of sea level called
a reference ellipsoid to calculate elevation. The GPS receiver uses the
reference ellipsoid Model of sea level, and so the number you see on the
screen is the elevation above the Model and not the real sea level. The
shape of the ellipsoid is a smooth squished sphere, but the shape of real
sea level is actually pretty bumpy."

Sadly they have learnt that wrongly. Even the lowly consumer receivers of ten years ago (e.g. the Garmin GPS38) had a model of the geoid in them to allow them to correct for the geoid / ellipsoid height difference. If they didn't the altitude displayed would be tens of metres out in many parts of the world.

Sure the geoid model may not be the most accurate / highest resolution available but it is there. The Garmins also output their value for the geoid / ellipsoid separation in the NMEA output so that any inaccuracies can be corrected for in logged data if a more accurate model is available.

Users would be pretty upset if their altitude readings were out by this many metres in many places:

http://www.colorado.edu/geography/gcraft/notes/datum/geoid84.html

The full 30 minute geoid height file is about 10 MB uncompressed so is hardly too big to fit in a modern receiver. Using a lower resolution grid and interpolation will still give acceptable accuracy for most consumer applications.

--

Dominic Sexton
.

Relevant Pages

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