Re: [cf-satellite] Ang.: Making a proposal for the addition of the geos projection

• To: John Caron <caron@xxxxxxxxxxxxxxxx>
• Subject: Re: [cf-satellite] Ang.: Making a proposal for the addition of the geos projection
• From: Martin Raspaud <martin.raspaud@xxxxxxx>
• Date: Wed, 23 Nov 2011 10:26:06 +0100

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Hi John,

On 23/11/11 00:13, John Caron wrote:
>> This looks familiar indeed... The document cited there is the CGMS03,
>> which describes the geos projection. However, I am not sure what
>> scale_x and y are ?
> 
> they are from section 4.4.4 of:
> 
> http://www.eumetsat.int/idcplg?IdcService=GET_FILE&dDocName=PDF_CGMS_03&RevisionSelectionMethod=LatestReleased
> 
> something to do with the geometry of the specific instrument, needed to
> actually convert to km on the earth.
> 
> perhaps some expert can help us on this?

I understand now what they are, but I need some more information (units
for example) to know what they are scaling exactly. Is it integer pixel
positions to viewing angles in degrees ? or radians ? or the reverse ?
Also, I see the offsets are not included. Usually these are just the
pixel coordinates of the centre of the image but it would be nice to
include them anyhow.

>>
>> Should we merge the two descriptions somehow for the proposal ?
> 
> yes, if they are really the same. Its a port of this software:
> 
>  * Port Eumetsat MSG_navigation.c to java
>  * from
> http://www.eumetsat.int/idcplg?IdcService=GET_FILE&dDocName=zip_tools_msg_nav_c&RevisionSelectionMethod=LatestReleased
> 

This software you name is equivalent to the geos projection in the
proj.4 software library. The difference is how we provide input
coordinates to be transformed to lon/lats. (pixel coordinates or
scanning angles in Eumetsat's software, projection coordinates in metres
(=sat_height*scanning_angle) for proj.4)

So, in order to allow both to work, we need the scale parameters you
named, along with the offsets and the height of the satellite above the
earth (and of course I assume semi major/minor axis of the ellipsoid).

So it could look like this:

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

grid_mapping_name = geos


Map parameters:

        latitude_of_projection_origin

        longitude_of_projection_origin

        perspective_point_height
        
        scale_x
        
        scale_y

        offset_x

        offset_y

        false_easting

        false_northing



What do you think ?

Best regards,
Martin
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• Follow-Ups:
  • Re: [cf-satellite] Ang.: Making a proposal for the addition of the geos projection
    • From: Martin Raspaud

• References:
  • [cf-satellite] Making a proposal for the addition of the geos projection
    • From: Martin Raspaud
  • Re: [cf-satellite] Making a proposal for the addition of the geos projection
    • From: John Caron
  • [cf-satellite] Ang.: Making a proposal for the addition of the geos projection
    • From: Raspaud Martin
  • Re: [cf-satellite] Ang.: Making a proposal for the addition of the geos projection
    • From: John Caron