Le 26/10/2023 à 18:50, David Shean a écrit :
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.
This is actually already implemented (deducing the value of
GetUnitType() from the vertical component):
$ gdal_translate byte.tif out.tif -a_srs EPSG:4269+5703
$ gdalinfo out.tif | grep Unit
Unit Type: foot
$ gdal_translate byte.tif out.tif -a_srs EPSG:4269+8228
$ gdalinfo out.tif | grep Unit
Unit Type: foot
-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> 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)
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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