[Gdal-dev] Understanding GEOS projected space

[Bas Retsios]

Enrico Zini wrote:

>Hello,
>
>I'm porting georeferentiation information from meteosatlib to GDAL, and
>I'm currently stuck dealing with the numbers I get from proj.
>
>What I cannot find is, in the GEOS projection, what do the projected
>values represent.
>
>Angles from the subsatellite point to the projected point with the
>vertex on the satellite, or something else?  In what measurement unit?
>
>I could not find this kind of information neither in
>http://remotesensing.org/geotiff/proj_list/geos.html nor in the section
>4.4.3.2 of the pdf document referred by the page, where the projection
>is described.  I'm afraid I lack the math background to infer it from
>the formulas of the projection.
>
>
>Ciao,
>
>Enrico
>
The projected coordinates are expressed in meters.
When using lon_0 = 0, x_0 = 0 and y_0 = 0 (defaults for the MSG-1 
satellite), point (0,0) in projected coordinates coincides with lat=0 
and lon=0.
Note that due to the large area covered by this projection (an entire 
earth's hemisphere), the "meters" are only correct near the nadir-point.
As an example, a box with bounds (xMin=-3500000, xMax=3500000, 
yMin=-3500000, yMax=3500000) would roughly include the Arfican continent 
and a great part of the atlantic ocean.

Best regards,

-- 
Ir. V. (Bas) Retsios
Software Developer
Geo-information Processing Department
International Institute for Geo-information Science and Earth Observation (ITC)
P.O. Box 6,  7500 AA Enschede, The Netherlands
Phone +31 (0)53 4874 573, telefax +31 (0)53 4874 335
E-mail retsios at itc.nl, Internet http://www.itc.nl