[gdal-dev] geolocation info in netCDF: "northernmost_northing" etc. not universal

[Frank Warmerdam]

On Wed, Jan 27, 2010 at 2:22 AM, Bruce Raup <bruceraup at gmail.com> wrote:

> We propose using terms such as
>
> grid_top_boundary_projected_y
> grid_bottom_boundary_projected_y
> grid_left_boundary_projected_x
> grid_right_boundary_projected_x
>
> Alternatively, grid corner coordinates could be used.
>
> What would the impacts of such a change of names in our files be?  If
> the GeoRef field is also in the file, would it break existing software
> for being able to navigate (geolocate) the data?  And, would the
> community consider the adoption of more universal terms more generally
> (i.e. in GDAL itself)?

Bruce,

I am not really an expert on the GDAL netcdf driver, but
I will attempt to respond.

First, I think it would be better to just document that the
existing tag names (eg. northernmost_northing) should
just be interpreted with the new definitions
(grid_top_boundary_projected_y) rather than trying
to change the actual tag names in order to gain some
possibly clarity in less common situations.

I have reviewed the GDAL netcdf driver and it seems to
look for two things.  First a GeoTransform value (which I
think is only a GDAL convention).  If that is not found it
looks for northernmost_northing, etc.  I presume these are
part of the CF conventions, though I don't know for sure.

I do no see any reference to a "GeoRef" tag.

So if you make the planned change, and if you do not
also provide the GeoTransform then older versions of
GDAL would not be able to read the georeferencing of
the files.  I presume this might also be the case of
almost all other software, even if you did support
GeoTransform.

However, if you do go ahead with this convention, then
please file  ticket with a small sample file so we can add
support for it.

Best regards,
-- 
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