Hi,
good question. To the best of my knowledge, in the official WKT / WKB
spec, it is a "everything or nothing" logic. Either all the vertices
have a well defined Z value and then you use LINESTRING Z, POLYGON Z,
etc.. Or none of them have, and you use the 2D versions: LINESTRING,
POLYGON, etc.
I believe QGIS has standardized using not-a-number NaN number for that
purpose, but officially, you can't export that to WKT... That said, for
the sake of interoperability with QGIS or other software using NaN, the
OGR WKT importer and exporter also supports its:
$ python -c "from osgeo import ogr; g = ogr.CreateGeometryFromWkt('POINT
Z (1 2 nan)'); print(g.ExportToIsoWkt())"
POINT Z (1 2 nan)
but I'm not sure if I'd recommend using that convention. I can imagine
exporting NaN to some formats could produce invalid content.
The good old shapefile uses a similar convention to the -1e300 trick you
propose.
https://www.esri.com/content/dam/esrisites/sitecore-archive/Files/Pdfs/library/whitepapers/pdfs/shapefile.pdf
, page 2 : "Any floating point number smaller than –10^38 is considered
by a shapefile reader to represent a "no data" value"
Even
Le 25/01/2024 à 18:07, Abel Pau via gdal-dev a écrit :
Hi,
there is any value in GDAL for VECTORS that indicates that a concrete
value of a Z is not known (z nodata value)?
I couldn’t find it anywhere.
In MiraMon format we use one concrete number documented in our format
pdf (-1.0E+300) an in the driver it’s planned to translate it to the
same number. I could translate it to the one I am asking. And the same
for detecting nodata Z and translate them to -1.0E+300 when reading
another format.
Thanks!
*Abel Pau Garcia*
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