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Subject: [ggl] WKT empty
From: Barend Gehrels (Barend.Gehrels)
Date: 2009-06-13 15:48:39

>> Empty linestrings and polygons can thus probably be avoided. An empty
>> point makes no sense to me.
> But it's valid in terms of OGC SFS.
> A point entity that has no location in space.
> Similarly, empty linestring does not make sense - what to define line
> with no vertices for?
Sure, we don't need empty linestrings or empty polygons. But users can
supply them to all algorithms, we don't know that beforehand. For points
it is different. See further previous mail.

>> GGL lives in the template world and the result of an intersection should
>> be a typed geometry. Therefore we've modelled the intersection with an
>> output iterator. So two polygons may result in an output iterator,
>> outputting zero, one or more polygons. In practice this is close to
>> using a multi-polygon. However, if the user would specify that the
>> output would be an output iterator of points, it makes sense as well.
> Here, I think we are trying to find a compromise between how to support
> OGC concepts as they are defined and how to give flexibility to non-OGC
> oriented users.
> For OGC-oriented users, if intersection results in N>0 polygons,
> returning them as a list of points but not as list of polygons or a
> multi-polygon, is ambiguous.
Sorry, I didn't describe it well enough. I mean that in the case of an
output iterator on points, the Intersection Points are returned (so not
the complete polygon). For a linestring the same, the Linestring
elements where two polygons intersect are then returned.
OK, maybe it is not that useful. But it is used internally: first the
intersection points, then the rest, then complete polygons.


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