[gdal-dev] Shapefiles and z dimensions

[A Huarte]

Hi, from ESRI shapefile specification: 
https://www.esri.com/library/whitepapers/pdfs/shapefile.pdf

the header file always contains the Mmin and Mmax values (Page 4):

    "but these limits can contain "no data" values (see Numeric Types on page 2) for shapefiles      of measured shape types that contain no measures."
for geometries (Page 15):     "Shape Types inX,Y,Z SpaceShapes of this type have an optional coordinate M. Note that "no data"      value can bespecified as a value for M (see Numeric Types on page 2)."
    "PointZ - A PointZ consists of a triplet of double-precision coordinates in the order X, Y, Z plus ameasure."    ...
the coordinate M is always present, but it can contains a "no data" value to indicate if the geometry is a pure Z-type geometry or a ZM-type geometry.
I think you only can distinguish between both types reading the Mmin and Mmax values of the header file.


      De: Nyall Dawson <nyall.dawson at gmail.com>
 Para: gdal dev <gdal-dev at lists.osgeo.org> 
 Enviado: Sábado 9 de diciembre de 2017 8:05
 Asunto: [gdal-dev] Shapefiles and z dimensions
   
Hi all,

I'm trying to track down some odd behaviour in QGIS, and determine
whether this is a qgis bug, gdal bug, or shapefile limitation.

If I create a new shapefile layer using GDALDatasetCreateLayer, with a
geometry type of wkbPoint25D (actually any of the 25D) types, then the
resultant layer ALWAYS has an M dimension present as well (i.e.
ogrinfo reports PointZM type).

Is this expected behaviour for shapefiles?

Nyall
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