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Re: [Paparazzi-devel] Re: Paparazzi-devel Digest, Vol 61, Issue 16

From: Pascal Brisset
Subject: Re: [Paparazzi-devel] Re: Paparazzi-devel Digest, Vol 61, Issue 16
Date: Mon, 13 Apr 2009 19:16:05 +0200
User-agent: Thunderbird (X11/20090318)

Christophe De Wagter wrote:
I believe that within a UTM zone the NED coordinates and UTM coordinates are only translated (infinite UTM zone).

Unfortunately it is not the case. If you set a reference close to Toulouse and compare NED (or LTP for Local Tangent Plan) to UTM coordinates for a point 1000m away, there is a difference of 20m. Moreover, while the LTP is plan (!), the UTM projection is done on a cylinder wrapped around the earth, so the altitudes do not coincide in the two systems: 10km from the reference point, there is already a difference of about 10m. So, if the NED (LTP) coordinates are perfect for a "local" flight, they cannot replace UTM, at least in a simple way, for flights some kilometers around the reference point.

Altitude is really a problem when you want to use ECEF since there is no simple transformation from ECEF to LLH with the usual "H". While there exists an analytic formula to compute a height above the ellipsoid (the WGS84 model which tries to fit the earth), the computation of the height above the geoid (a model of a medium sea level) requires a database. I'm curious to know how the GPS receivers compute this height:


PS: Some test code is available in sw/lib/ocaml/test/ to be compiled with
    ocamlc -I .. unix.cma str.cma latlong.cmo

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