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Re: Earth Radius for CF Grid Mappings

Dear Greg

You can include earth_radius with any of the mappings. At the start of App F
it says
"The attributes which describe the ellipsoid and prime meridian may be 
included, when applicable, with any grid mapping."
and earth_radius is one of those. If it's not specified but you need it, I
think you can assume any sensible value.

Cheers

Jonathan

On Mon, Dec 15, 2008 at 06:02:10PM -0500, Greg Rappa wrote:
> Date: Mon, 15 Dec 2008 18:02:10 -0500
> From: Greg Rappa <address@hidden>
> User-Agent: Thunderbird 2.0.0.16 (X11/20080707)
> To: address@hidden
> CC: "address@hidden" <address@hidden>,
>         "'John Caron'" <address@hidden>,
>         "Newell, Oliver" <address@hidden>
> Subject: Earth Radius for CF Grid Mappings
> 
> 
>    I recently discovered that in order to resolve the location of a grid
>    element (x/y-to-lat/long conversion), using software in the [1]General
>    Cartographic Transformation Package (GCTP), a value for the
>    Earth's radius is required.  For example, the GCTP functions
>    (lamazinvint and lamazinv) for converting x & y offsets to location
>    (latitude,longitude) of a grid mapped to a Lambert Azimuthal
>    Equal Area (LAEA) projection require: Earth's radius, center
>    latitude and longitude, and offset for east and north.
>    However, the CF Conventions document Appendix F (Grid Mappings),
>    only refers to an Earth radius in the Vertical Perspective grid
>    mapping.
>    For NetCDF file readers, how are the grid element locations to be
>    resolved for mappings such as LAEA if no Earth radius is provided
>    with the file?  Is a projection-specific radius to be implied by
>    NetCDF
>    readers?  Specifically, do all grid mappings use a prescribed set of
>    Earth radius or radii (major & minor) that NetCDF readers must
>    maintain?
>    The CF Grid Mappings appendix refers readers to a Web site that
>    discusses "[2]random issues" in which it is stated that GCTP always
>    uses a
>    radius of 6370997 meters (E019 - Normal Sphere).  I suppose this
>    answers my question, regarding the LAEA projection.  However, given
>    the multitude of grid mappings and Earth radius definitions that
>    exist, I'm
>    wondering if an earth_radius attribute ought to be added to the CF
>    grid
>    mapping parameter set for all projections (perhaps earth_radius_major
>    and earth_radius_minor, if applicable).
>    Greg.
>    ---------------------------------------------------------------
>    Greg Rappa
>    MIT Lincoln Laboratory               Group 43 - Weather Sensing
>    Phone: 781-981-3660, Fax: 781-981-0632, Email: [3]address@hidden
> 
> References
> 
>    1. http://gcmd.nasa.gov/records/USGS-GCTP.html
>    2. http://www.remotesensing.org/geotiff/proj_list/random_issues.html#laea
>    3. mailto:address@hidden

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Subject: Re: Earth Radius for CF Grid Mappings
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OK.  Thanks.  I didn't notice the definitions in Table F.1.  I'm all set 
now.

Greg.

Jonathan Gregory wrote:
> Dear Greg
>
> You can include earth_radius with any of the mappings. At the start of App F
> it says
> "The attributes which describe the ellipsoid and prime meridian may be 
> included, when applicable, with any grid mapping."
> and earth_radius is one of those. If it's not specified but you need it, I
> think you can assume any sensible value.
>
> Cheers
>
> Jonathan
>