Hi Randy,
Funny that you bring this up, since the proposal I want to make originates from a discussion with Tom Rink, which apparently works also on GOES-R and netCDF/CF...
Anyhow, the difference between geos and vertical perspective is that the former is directly linked to the scanning angle, while the latter is related to the distance between pixels on a projection plane.
For example, in order to get the lon/lat with geos from the scanning angle alpha in radians from a satellite at height h, the projection coordinates to provide would simply be something like h*alpha. Instead, the vertical perspective would require something along the lines of h*tan(alpha) as projection coordinates.
Snyder is right of course when he says that vertical perspective is how the earth is viewed from space. The geos is actually distorted to that regard, since it's dependent on the viewing geometry of the satellite instrument. But that is the way we receive the data, and geos makes it easier to compute lon/lats from there, hence my proposal. For more technical details about geos, see the CGMS 03 document I cite in the original post.
Don't hesitate to ask more if I wasn't clear !
Best regards,
Martin
----- Reply message -----
Fr?n: "Randy Horne" <rhorne at excaliburlabs.com>
Till: "cf-metadata at cgd.ucar.edu" <cf-metadata at cgd.ucar.edu>, "Raspaud Martin" <martin.raspaud at smhi.se>
Kopia: "cf-satellite at unidata.ucar.edu" <cf-satellite at unidata.ucar.edu>
Rubrik: [CF-metadata] Making a proposal for the addition of the geosprojection
Datum: tis, nov 22, 2011 22:36
Martin:
I am working the GOES-R program (next generation geostationary weather satellite program).
It is our plan to generate CF convention compliant NetCDF product files that are projected from the idealized scanning imager on the satellite. We had thought the vertical perspective was the appropriate projection.
I went back and read through the "vertical perspective projection" in John Snyder's document (Map projections- A Working Manual), and it it unclear why this existing projection in the CF conventions is not applicable. It is true that the GOES-R satellite imager (i.e. camera) does not precisely faces the center of the Earth because it is scanning in approximately 500 km N-S swaths from west to east, but I am confused how this characteristic changes the projection geometry.
Could you augment the explanation you have already provided ?
very respectfully,
randy
---------- Original Message ----------------------------------
From: Martin Raspaud <martin.raspaud at smhi.se>
Date: Tue, 22 Nov 2011 13:28:32 +0100
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>Hi all,
>
>There seems to be a need from the weather satellite folk 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" document.
>(see here also: http://remotesensing.org/geotiff/proj_list/geos.html )
>
>My question is thus: how do we make a proposal for the addition of the
>projection ?
>
>On a side note, it looks like "Vertical perspective" and "geos" are
>mixed up in the current version of the conventions (the link to "geos"
>is provided under "Vertical perspective"). The difference is that the
>geos projection is related to scanning angles of the satellite, so that
>the projection is done on a piece of a sphere, while the vertical
>perspective is a kind of orthographic projection.
>
>Here comes a possible text for this projection:
>
>
>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.
>==============================
>
>
>Thanks a lot !
>Best regards,
>Martin
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