[gdal-dev] Support for measures

[Even Rouault]

Le mercredi 27 janvier 2016 11:55:01, Ari Jolma a écrit :
> Folks,
> 
> I'd like to try to implement the XYZM support since I have some free time.
> 
> Before making a RFC, there are some thoughts/questions/ideas:
> 
> * I made a fork for this work at https://github.com/ajolma/GDAL-XYZM so
> I can more easily use travis.
> 
> * I think this is mainly changes in the geometry API and generic methods.
> 
> Are there other drivers than shape, which are affected?

On top of my head: PostGIS, FileGDB, OpenFileGDB, GeoPackage, Spatialite. 
Probably other RDMBs too (Oracle, MSSQL, ... ?)


> Currently shape
> driver creates XYZ data from XYM data, that would change, which may
> break some code.

The way the shape driver currently exposes XYM as XYZ is a hack. I'm not sure 
many people rely on that, and I think it would be OK to change the behaviour 
with a cleaner approach.

> 
> * Currently XYM or XYZM data seems to be accepted (by WKT and WKB
> importers for example) but not stored in generic objects.

Right, M components are skipped/ignored

> 
> * The closeness of XYZ and XYM might need some thought:

Yes, the concept of coordinate dimension is ambiguous for 3.  
getGeometryType() would be needed to know the real interpretation.

> For example C++ API for making a adding a XYZ and XYM point to a curve
> would be the same.
> Currently the method calls Make3D() if the curve is
> 2D, which is not ok for XYM data.
Indeed, we should likely keep some auto-promotion rules for XY->XYZ but 
perhaps not for XYM/XYZM.
> 
> Tickets related to this are
> 
> https://trac.osgeo.org/gdal/ticket/6063
> https://trac.osgeo.org/gdal/ticket/6331
> 
> Are there other things I should know?

GEOS methods ignore M AFAIR.

Other thought: XYM/XYZM support will have some (unavoidable) compatibility 
impact on C API users as they will potentially receive new geometry types.

> 
> The target would be 2.1 I guess.
> 
> Ari
> 
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