[gdal-dev] Call for discussion about RFC 61: Support for measures in geometries

[Ari Jolma]

Folks, especially the PSC,

I'd like to call for discussion about RFC 61: Adding support for 
measures in geometries.

Measures (M) are a sort of fourth coordinate for points in geometries. A 
typical use of measures is for distance from origin in line strings that 
represent roads but they can be used for other things as well.

Measures are a part of the simple features model and they are supported 
by many vector data formats.

Since measures are a rather fundamental element, adding them will 
require quite many changes into the basic building blocks of the GDAL 
API. Most importantly, the 'int nCoordDimension' property of OGRGeometry 
is proposed to be replaced by 'int flags', which will have bit flags 
denoting whether the geometry is empty and has Z or M coordinates.

No backwards incompatible changes (syntactic nor semantic) will be done 
but the methods getCoordinateDimension and setCoordinateDimension will 
be deprecated since they do not take into account M coordinate.

The RFC is as usual in the wiki:

https://trac.osgeo.org/gdal/wiki/rfc61_support_for_measured_geometries

There are a couple of decisions to be made (if the overall decision is 
positive).

Should the set3D(b3D) method (and also for example AddGeometry method) 
of GeometryCollection force all child geometries to be 3D (or 2D and 
remove possible Z values)? Maybe just unsetting the flag when 
downgrading is enough?

Should storing data, with measures, into a format, which does not 
support measures, remove M values from the data? This would be similar 
to the current behavior with curved geometries and formats which do not 
support them.

Some smaller issues are mentioned/hinted to in the draft RFC.

I have set up a github repository for the work at

https://github.com/ajolma/GDAL-XYZM

It has an associated travis builder - which however seems to have some 
problems right now. I have committed the API changes from the draft RFC 
and it compiles but there is a lot to do with actually implementing the 
support.

Best regards,

Ari