[gdal-dev] question re netcdf grid_mapping

[Michael Sumner]

On Wed, Mar 2, 2022 at 2:31 PM Alan Snow <alansnow21 at gmail.com> wrote:

> Hi Michael,
>
> I don't claim to be an expert in the projection you are using or what the
> GDAL team would like to do in this scenario, but hopefully the information
> presented here will be helpful to you.
>
>

  excellent, thanks so much Alan this clarifies.

Best, Mike



> The grid mapping name "Oblique Mercator (LV95 - CH1903+)" in the netCDF
> file does not appear to be in the list of supported grid mappings:
>
> http://cfconventions.org/cf-conventions/cf-conventions.html#appendix-grid-mappings
>
> Using pyproj to dump EPSG;2056 to CF parameters (
> https://pyproj4.github.io/pyproj/latest/build_crs_cf.html), the grid
> mapping name is "oblique_mercator":
> >>> from pyproj import CRS
> >>> crs = CRS("EPSG:2056")
> >>> crs.to_cf()
> {'azimuth_of_central_line': 90.0,
>  'false_easting': 2600000.0,
>  'false_northing': 1200000.0,
>  'geographic_crs_name': 'CH1903+',
>  'grid_mapping_name': 'oblique_mercator',
>  'horizontal_datum_name': 'CH1903+',
>  'inverse_flattening': 299.1528128,
>  'latitude_of_projection_origin': 46.95240555555556,
>  'longitude_of_prime_meridian': 0.0,
>  'longitude_of_projection_origin': 7.439583333333333,
>  'prime_meridian_name': 'Greenwich',
>  'projected_crs_name': 'CH1903+ / LV95',
>  'reference_ellipsoid_name': 'Bessel 1841',
>  'scale_factor_at_projection_origin': 1.0,
>  'semi_major_axis': 6377397.155,
>  'semi_minor_axis': 6356078.962818189}
>
> If the pyproj version is correct, I am wondering if the appropriate action
> in this scenario is to contact the provider of the netCDF file to look into
> and potentially address the issue?
>
> If the swiss oblique mercator differs from the regular oblique mercator
> and is not correctly represented by the current grid mappings in the CF
> conventions, it may be worth opening an issue here to add it to the CF
> conventions: https://github.com/cf-convention/cf-conventions. Once the
> change is accepted there, I imagine a PR to GDAL will likely be welcome.
>
> Best,
> Alan
>
> On Tue, Mar 1, 2022 at 6:57 PM Michael Sumner <mdsumner at gmail.com> wrote:
>
>> Hello, I'm looking at the constants for CF_PT_* projection families in
>>
>> frmts/netcdf/netcdf_cf_constants.h
>>
>> I have a grid_mapping variable with attributes:
>>
>>    # float swiss_lv95_coordinates ;
>>        #          swiss_lv95_coordinates:_FillValue = -1.f ;
>>        #          swiss_lv95_coordinates:grid_mapping_name = "Oblique
>> Mercator (LV95 - CH1903+)" ;
>>        #          swiss_lv95_coordinates:longitude_of_projection_center =
>> 7.43958333 ;
>>        #          swiss_lv95_coordinates:latitude_of_projection_center =
>> 46.9524056 ;
>>        #          swiss_lv95_coordinates:false_easting = 2600000. ;
>>        #          swiss_lv95_coordinates:false_northing = 1200000. ;
>>        #          swiss_lv95_coordinates:inverse_flattening = 299.1528128
>> ;
>>        #          swiss_lv95_coordinates:semi_major_axis = 6377397.155 ;
>>
>> and by my understanding this would require something like
>>
>> #define    CF_PT_LV95     "Oblique Mercator (LV95 - CH1903+)"
>>
>> and then attendant logic in the driver for that case, this is
>> "EPSG:2056".
>>
>> It seems like this is a very long project of collating specific values
>> used for a crs name ... where the more general case would use those
>> parameters to build an Oblique Mercator - but this is Swiss oblique
>> mercator  with a simpler set of initialization params.
>>
>> Is it a case of needing a very specific else if to match this case, or is
>> there some more to this?
>>
>> Is it worth contributing cases as PRs?
>>
>> (I'm inclined to advise "use -a_srs 'EPSG:2056'". )
>>
>> Cheers, Mike
>>
>> Michael Sumner
>> Software and Database Engineer
>> Australian Antarctic Division
>> Hobart, Australia
>> e-mail: mdsumner at gmail.com
>> _______________________________________________
>> gdal-dev mailing list
>> gdal-dev at lists.osgeo.org
>> https://lists.osgeo.org/mailman/listinfo/gdal-dev
>>
>
>
> --
> Alan Snow
>


-- 
Michael Sumner
Software and Database Engineer
Australian Antarctic Division
Hobart, Australia
e-mail: mdsumner at gmail.com
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.osgeo.org/pipermail/gdal-dev/attachments/20220303/77ecdaec/attachment.html>