John,
according to [1] and [2] the names can be used inter-changeably
- longitude of the origin (central meridian)
- the scale factor at the origin (on the central meridian),
specifically:
"Scale factor at natural origin" (EPSG and GeoTiff)
"Longitude of natural origin" (EPSG, GeoTiff) vs. "central_meridian" (OGC WKT)
So I think the CF names should not change and example should be
updated to reflect the projection parameters.
I have used the correct parameters in the same example in ticket #80 -
"Add missing CF parameters to translate Coordinate Reference System
properties to/from OGC Well-Known Text format".
[1]
http://www.remotesensing.org/geotiff/proj_list/transverse_mercator.html
[2]
https://cf-pcmdi.llnl.gov/trac/wiki/Cf2CrsWkt
Regards,
Etienne
On Wed, Mar 14, 2012 at 8:57 PM, John Caron <caron at unidata.ucar.edu> wrote:
> Example 5.10. British National Grid
>
> ?int crs ;
> ? ?crs:grid_mapping_name = "transverse_mercator";
> ? ?crs:semi_major_axis = 6377563.396 ;
> ? ?crs:semi_minor_axis = 6356256.910 ;
> ? ?crs:inverse_flattening = 299.3249646 ;
> ? ?crs:latitude_of_projection_origin = 49.0 ;
> ? ?crs:longitude_of_projection_origin = -2.0 ;
> ? ?crs:false_easting = 400000.0 ;
> ? ?crs:false_northing = -100000.0 ;
> ? ?crs:scale_factor_at_projection_origin = 0.9996012717 ;
>
>
> but Appendix F has:
>
> Transverse Mercator
> grid_mapping_name = transverse_mercator
>
> Map parameters:
> ?scale_factor_at_central_meridian
> ?longitude_of_central_meridian
> ?latitude_of_projection_origin
> ?false_easting
> ?false_northing
>
> I think we can presume that
>
> ?scale_factor_at_central_meridian == scale_factor_at_projection_origin
> ?longitude_of_central_meridian == longitude_of_projection_origin
>
> central_meridian is probably clearer; so maybe just update the example ??
>
> or we can add this amendment to ticket #75= "fix documentation and
> definitions of 3 grid mapping definitions"
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Received on Wed Mar 14 2012 - 20:31:09 GMT
