[gdal-dev] vector NODATA for Z values?

[Even Rouault]

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*
>
> *GIS developer*
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