<html><head><meta http-equiv="Content-Type" content="text/html; charset=utf-8"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;" class="">Hi Even and Javier,<div class="">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.</div><div class=""><br class=""></div><div class="">Example for EPSG:4979:</div><div class=""><br class=""></div><div class=""><div class=""> AXIS["ellipsoidal height (h)",up,</div><div class=""> ORDER[3],</div><div class=""> LENGTHUNIT["metre",1]],</div><div><br class=""></div><div>Example for EPSG:5703:</div><div><br class=""></div><div><div>VERTCRS["NAVD88 height",</div><div> VDATUM["North American Vertical Datum 1988"],</div><div> CS[vertical,1],</div><div> AXIS["gravity-related height (H)",up,</div><div> LENGTHUNIT["metre",1]],</div><div>...</div></div><div><br class=""></div><div>More complicated than specifying the generic <span style="caret-color: rgb(0, 0, 0); color: rgb(0, 0, 0);" class="">UNITTYPE in raster metadata, but seems like best practice.</span></div><div><span style="caret-color: rgb(0, 0, 0); color: rgb(0, 0, 0);" class="">-David</span></div><div><span style="caret-color: rgb(0, 0, 0); color: rgb(0, 0, 0);" class=""><br class=""></span></div><div><br class=""><blockquote type="cite" class=""><div class="">On Oct 26, 2023, at 4:45 AM, Javier Jimenez Shaw via gdal-dev <<a href="mailto:gdal-dev@lists.osgeo.org" class="">gdal-dev@lists.osgeo.org</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><div dir="ltr" class="">Thanks!<br class=""></div><br class=""><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Thu, 26 Oct 2023 at 12:27, Even Rouault via gdal-dev <<a href="mailto:gdal-dev@lists.osgeo.org" class="">gdal-dev@lists.osgeo.org</a>> wrote:<br class=""></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><u class=""></u>
<div class=""><p class="">Javier,<br class="">
</p>
<div class="">Le 26/10/2023 à 11:59, Javier Jimenez
Shaw via gdal-dev a écrit :<br class="">
</div>
<blockquote type="cite" class="">
<div dir="ltr" class="">
<div class="">Hi</div>
<div class=""><br class="">
</div>
<div class="">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.</div>
<div class=""><br class="">
</div>
<div class="">Is there any standard (or accepted) metadata to say the
units of those values?</div>
</div>
</blockquote><p class="">API wise in GDAL, you should use the
GDALRasterBand::SetUnitType() to set the unit</p><p class="">For TIFF, this will be encoded in the GDAL_METADATA TIFF tag like
the following:<br class="">
</p><p class=""><GDALMetadata><br class="">
<Item name="UNITTYPE" sample="0"
role="unittype">foo</Item><br class="">
</GDALMetadata></p><p class="">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
(<a href="http://cfconventions.org/Data/cf-conventions/cf-conventions-1.10/cf-conventions.html#units" target="_blank" class="">http://cfconventions.org/Data/cf-conventions/cf-conventions-1.10/cf-conventions.html#units</a>)</p><p class="">Even<br class="">
</p>
<blockquote type="cite" class="">
<div dir="ltr" class="">
<div class=""><br class="">
</div>
<div class="">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.</div>
<div class=""><br class="">
</div>
<div class="">Thanks<br class="">
</div>
<div class="">
<div class="">
<div dir="ltr" class="gmail_signature">
<div dir="ltr" class="">
<div class="">.___ ._ ..._ .. . ._. .___ .. __ . _. . __.. ...
.... ._ .__</div>
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</blockquote>
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