[gdal-dev] Tag for units in GeoTIFF

[David Shean]

Hi Even and Javier,
For the special case of a DEM, would it make more sense to specify the units with a vertical CRS?  This approach would define the LENGTHUNIT and the associated vertical datum.

Example for EPSG:4979:

        AXIS["ellipsoidal height (h)",up,
            ORDER[3],
            LENGTHUNIT["metre",1]],

Example for EPSG:5703:

VERTCRS["NAVD88 height",
    VDATUM["North American Vertical Datum 1988"],
    CS[vertical,1],
        AXIS["gravity-related height (H)",up,
            LENGTHUNIT["metre",1]],
...

More complicated than specifying the generic UNITTYPE in raster metadata, but seems like best practice.
-David


> On Oct 26, 2023, at 4:45 AM, Javier Jimenez Shaw via gdal-dev <gdal-dev at lists.osgeo.org> wrote:
> 
> Thanks!
> 
> On Thu, 26 Oct 2023 at 12:27, Even Rouault via gdal-dev <gdal-dev at lists.osgeo.org <mailto:gdal-dev at lists.osgeo.org>> wrote:
> Javier,
> 
> Le 26/10/2023 à 11:59, Javier Jimenez Shaw via gdal-dev a écrit :
>> Hi
>> 
>> Using a GeoTIFF with a single band and float values can be very useful to show any distribution over the terrain. One typical example is DSM (digital surface model), where the value is the elevation on every point.
>> 
>> Is there any standard (or accepted) metadata to say the units of those values?
> API wise in GDAL, you should use the GDALRasterBand::SetUnitType() to set the unit
> 
> For TIFF, this will be encoded in the GDAL_METADATA TIFF tag like the following:
> 
> <GDALMetadata>
>   <Item name="UNITTYPE" sample="0" role="unittype">foo</Item>
> </GDALMetadata>
> 
> Regarding the value itself, it is mostly unspecified by GDAL. It could be a good practice to suggest to use strings recognized by the "udunits" package (when possible), like done in the netCDF CF conventions (http://cfconventions.org/Data/cf-conventions/cf-conventions-1.10/cf-conventions.html#units <http://cfconventions.org/Data/cf-conventions/cf-conventions-1.10/cf-conventions.html#units>)
> 
> Even
> 
>> 
>> In the typical case of DSM it is usually "meter" or "foot". But it can be something else, like "people/km2" for population, or "mm" for rain precipitation, or "kg/ha" for agricultural yield.
>> 
>> Thanks
>> .___ ._ ..._ .. . ._.  .___ .. __ . _. . __..  ... .... ._ .__
>> 
>> 
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