[gdal-dev] Support for measures

[Peter Halls]

Ari, Even,

      one potential solution where M data are present but not supported by
the Geometry type would be to add M as a user defined attribute, as is done
for Z values in some drivers / packages.  This preserves both the geometry
integrity and the data.  Should the user wish to 'upgrade' the dataset to a
full xyzm structure at some future date, this is then their explicit
decision and can take proper account of format capabilities, etc.

       There may need to be a debate about what the replacement M attribute
be called, of course!

Best wishes,

Peter



On 28 January 2016 at 08:23, Ari Jolma <ari.jolma at gmail.com> wrote:

> 28.01.2016, 00:05, Even Rouault kirjoitti:
>
> Thanks for the pointer to the geometry codes in SF Common Architecture,
> somehow I overlooked it.
>
> my point with adding the new capabilities was that drivers that wouldn't
>> advertize the M capabilities would never see a M or ZM geometry / geometry
>> type passed. See what the (non-virtual) methods
>> OGRLayer::SetFeature(),CreateFeature() and GDALDataset::CreateLayer() do
>> for curve geometries. Similar things could be done for M.
>>
>
> Hm. For example shape driver raises an error if the geometry type is not
> supported. I don't know what scenario would lead to that.
>
> Assume that the user has data in GDAL with measures. She decides to save
> that using a driver, which does not support measures. The impact is that
> the measures are stripped out, which is ok if the data is discarded
> immediately after that but not if she wants to still do something with the
> measures. I'm not sure if documenting the side-effect is enough. However,
> the convert to non linear geometries is already there and this would follow
> that pattern. I guess this is a part of the bigger discussion whether we
> consider GDAL as a data storage for interactive tools for example or as a
> data processing library.
>
> 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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