[Gdal-dev] what about OGRCircle , OGRArc and OGREllipse

[Frank Warmerdam]

Jacob Bensabat wrote:
> hello to all
> I have seen the need to draw arcs, ellipses and circles using ogr. The
> problem is that these
> geometric features are not directly supported by OGR. There seems to be no
> problem adding
> them so my question is why these are not supported. Is there any reason
> (conceptual or practical
> one) ?

Jacob,

Basically, they aren't there because they aren't part of the simple features
geometry model, though they do have obvious (and intended) places to fit as
add-ons to that geometry model.  While I wouldn't necessarily mind having
OGR classes for them, it is important to note that alot of stuff wouldn't
work properly for them.  For instance, there would presumably be hardly
any drivers that would support them.  Also, translation to WKT and WKB would
not be compatible with stuff like GEOS or PostGIS.

Basically, my concern is that they wouldn't be well supported and that in many
cases it would be better to have just converted them into polyline or polygon
approximations.

An important step in any system design is coming up with a data model.   I
chose the OpenGIS Simple Features geometry model as a basis for OGR and it
has provided me with a great deal of leverage in terms of compatibility with
other systems.

Of course it does not lend itself to accurate or efficient representation of
some feature from CAD oriented systems.  However, that hasn't been a huge issue
for me till this point.

In short, if you are interested I wouldn't necessarily mind having classes for
ellipse, circle and so on, but I would want to include big disclaimers on them
that they are not widely supported.  Furthermore, I would likely not want to
make drivers like DGN actually return these geometries even where it would make
sense as they would not be translatable to other drivers.

Best regards,
-- 
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I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
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