[gdal-dev] Support for measures

[Peter Halls]

Ari,

     I do not think that Z and M can ever be truly synonymous: whilst z is
partially defined in terms of, for example, a datum, there is no equivalent
for M.  Also, by convention, X, Y and Z frequently employ the same units;
there is no equivalent convention for M (as yet).

     There may be a philosophical case for using M for altitude
measurements instead of Z where X and Y use spherical or other non-linear
units.

     Currently, there are a number of disparate uses of the M value, for
example, in terms of variations in feature width, cant, segment length,
etc.  I have (once) seen M used for an integer code to report variations in
surface texture.  I do not know how many specialised uses there may be.

Best wishes,

Peter


On 27 January 2016 at 10:55, Ari Jolma <ari.jolma at gmail.com> wrote:

> 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? Currently shape
> driver creates XYZ data from XYM data, that would change, which may break
> some code.
>
> * Currently XYM or XYZM data seems to be accepted (by WKT and WKB
> importers for example) but not stored in generic objects.
>
> * The closeness of XYZ and XYM might need some thought:
>
> 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.
>
> 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?
>
> The target would be 2.1 I guess.
>
> Ari
>
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-- 
----------------------------------------------------------------------------------------------------------------
Peter J Halls, PhD Student, Post-war Reconstruction and Development Unit
(PRDU),
                      University of York

Snail mail: PRDU, Derwent College, University of York,
                Heslington, York YO10 5DD
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