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@lists.osgeo.org> wrote:
>
> Thanks!
>
> On Thu, 26 Oct 2023 at 12:27, Even Rouault via gdal-dev
> <gdal-dev@lists.osgeo.org <mailto:gdal-dev@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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