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Posted by Thomas Magma on August 1, 2006, 1:28 pm


Does anyone know of a way to take the altitude from a GPS and calculate the
distance to the ground (if you are in a plane or helicopter). I assume you
would need a large lookup table describing the contours of the earth. Does
any company sell anything like this for the use with GPS?

Thomas



Posted by Ejo on August 1, 2006, 1:39 pm


Thomas Magma wrote:
> Does anyone know of a way to take the altitude from a GPS and calculate the
> distance to the ground (if you are in a plane or helicopter). I assume you
> would need a large lookup table describing the contours of the earth. Does
> any company sell anything like this for the use with GPS?
>
> Thomas
>
>

You take the WGS84 ellipsoidal height from the GPS, you subtract a geoid
model height and you subtract a digital terrain model height for your
location, and out comes the altitude of the helicopter.

Ejo

Posted by Sam Wormley on August 1, 2006, 1:51 pm


Thomas Magma wrote:
> Does anyone know of a way to take the altitude from a GPS and calculate the
> distance to the ground (if you are in a plane or helicopter). I assume you
> would need a large lookup table describing the contours of the earth. Does
> any company sell anything like this for the use with GPS?
>
> Thomas
>
>


The information can be extracted from the NMEA sentence GGA which
includes the distance between the surface and the Geoid.

Posted by Sam Wormley on August 1, 2006, 1:57 pm


Sam Wormley wrote:
> Thomas Magma wrote:
>
>> Does anyone know of a way to take the altitude from a GPS and
>> calculate the distance to the ground (if you are in a plane or
>> helicopter). I assume you would need a large lookup table describing
>> the contours of the earth. Does any company sell anything like this
>> for the use with GPS?
>> Thomas
>
>
> The information can be extracted from the NMEA sentence GGA which
> includes the distance between the surface and the Geoid.


Also see: http://edu-observatory.org/gps/height.html


Posted by Thomas Magma on August 1, 2006, 2:23 pm



> The information can be extracted from the NMEA sentence GGA which
> includes the distance between the surface and the Geoid.

Isn't the model of a geoid extremely rudimentary? Mountain ranges are not
part of a geoid model are they?

I guess I didn't really specify resolution to my original question. I guess
I'm looking for (what I now no as) a digital elevation model that can be
stored on a SD or Compactflash card and used in a embedded product.
Something like a GByte per continent.

Thomas



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