[cf-satellite] The geos projection: proposal version 0.2

• To: "cf-satellite@xxxxxxxxxxxxxxxx" <cf-satellite@xxxxxxxxxxxxxxxx>
• Subject: [cf-satellite] The geos projection: proposal version 0.2
• From: Martin Raspaud <martin.raspaud@xxxxxxx>
• Date: Thu, 24 Nov 2011 11:29:48 +0100

Hi all,

After a first round of mail exchange, I think we can summarize the
proposal for the geos projection in the cf metadata as following:

There seems to be a need from the weather satellite people to add the
"geos" projection to the list of projections in the CF metadata.

"geos" stands for geostationary. The projection is described in detail
in the "CGMS 03, LRIT/HRIT Global specification" (Eumetsat) document.
(see here also: http://remotesensing.org/geotiff/proj_list/geos.html )

You'll find a possible text for this projection at the end of the email.

A practical example of use would be:

dimensions:
   y = 3712;
   x = 3712;
   reflective_bands = 3;
variables:
   short reflective_band_data(y, x, reflective_bands)
     :long_name = "Radiance of reflective bands of the SEVIRI instrument
(MSG)";
     :units = "milliwatts/m2/steradian/cm-1";
     :grid_mapping = "Projection";
     :coordinates = "y x";
   short x(x);
     :standard_name = "projection_x_coordinate";
     :add_offset = 1856
     :scale_factor = -208.16554260253906
     :units = "rad"
   short y(y);
     :standard_name = "projection_y_coordinate";
     :add_offset = 1856
     :scale_factor = -208.16554260253906
     :units = "rad"
   char Projection;
     :grid_mapping_name = "geos";
     :perspective_point_height = 35785831.;
     :semi_major_axis = 6378169.;
     :semi_minor_axis = 6356583.8;
     :latitude_of_projection_origin = 0.;
     :longitude_of_projection_origin = 0.;

As Tom rink pointed out, the negative scale_factor allows for different
scanning directions.


Geostationary
-------------

grid_mapping_name = geos


Map parameters:

        latitude_of_projection_origin

        longitude_of_projection_origin

        perspective_point_height

        false_easting

        false_northing

Map coordinates:

    The x (abscissa) and y (ordinate) rectangular coordinates are
identified by the standard_name attribute value projection_x_coordinate
and projection_y_coordinate respectively.
Notes:

    Notes on using the PROJ.4 software packages for computing the
mapping may be found at
http://www.remotesensing.org/geotiff/proj_list/geos.html . These notes
assume the point of observation is directly over the equator.
The projection coordinates in this projection are directly related to
the scanning angle of the satellite instrument.

==============================

Any comments ?

Best regards,
Martin Raspaud