I've started a wiki page for the status of the drivers regarding
measured geometries:
https://trac.osgeo.org/gdal/wiki/MeasuredGeometriesInDrivers
Ari
10.02.2016, 15:05, Even Rouault kirjoitti:
Le mercredi 10 février 2016 13:53:28, Peter Halls a écrit :
Types on page 2).
PointZ
A PointZ consists of a triplet of double-precision coordinates in the
order
X, Y, Z plus a measure"
I had misinterpreted their meaning of "optional" here and submitted a
documentation query to ESRI on being told I was wrong. The response was
that M being optional means that it can be valueless, but is always
present.
Woo, really ???? That's a genuine scoop.
I reported it to Frank at the time ... but several email systems later, I
no longer have my copies of the correspondence with ESRI nor the OGR bug
report, in which these were copied. Somewhere I may still have a backup of
my work area, to see what I did in my code. However, as you say, this is
20+ years ago .... It was something which, at the time, was causing us
some problems ....
I think you to be correct in your Shapelib summary below:
Unless I'm seriously mistaken, shapelib / OGR has produced XYZ shapefiles
without M values for more than 20 years. I'm surprised we wouldn't have
heard about that if such shapefiles couldn't be read. Or perhaps ESRI
software is robust to missing M too
Would someone looking at this discussion and having access to ESRI software be
willing to generate tiny (meaning just one single shape, with the smallest
number of vertices) shapefiles of type PointZ, ArcZ, PolygonZ and MultiPointZ,
and *without* M values ? So as to enhance the test suite and see in particular
which value is set exactly as the nodata value in the M component
I was wondering if we should change the behaviour of the shape driver to add
that nodata M component for XYZ shapes. This wouldn't cause compat problems
with previous versions of GDAL, but it would cause .shp sizes to grow by ~
1/3. So perhaps go on with current behaviour of stripping M component if most
implementations seem to be OK with that ?
Best wishes,
Peter
On 10 February 2016 at 12:31, Even Rouault <[email protected]>
wrote:
Le mercredi 10 février 2016 13:05:20, Peter Halls a écrit :
Ari, et al,
ESRI handle this in a non-intuitive way: XYM is supported, but Z
always has a Measure, so is XYZM! The formal definition is here:
https://www.*esri*.com/library/whitepapers/pdfs/*shapefile*.pdf
(1998)
Shape Types having Z are defined on pp19ff where it states:
"Shape Types inX,Y,Z Space
Shapes of this type have an optional coordinate
M. Note that "no data" value can be specified 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 a measure"
I had misinterpreted their meaning of "optional" here and submitted a
documentation query to ESRI on being told I was wrong. The response
was that M being optional means that it can be valueless, but is
always present.
Woo, really ???? That's a genuine scoop.
Unless I'm seriously mistaken, shapelib / OGR has produced XYZ shapefiles
without M values for more than 20 years. I'm surprised we wouldn't have
heard
about that if such shapefiles couldn't be read. Or perhaps ESRI software
is robust to missing M too
Best wishes,
Peter
--
Spatialys - Geospatial professional services
http://www.spatialys.com
_______________________________________________
gdal-dev mailing list
[email protected]
http://lists.osgeo.org/mailman/listinfo/gdal-dev
