Geometry Boost List: [ggl] WKT empty

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.

Barend

-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://lists.osgeo.org/pipermail/ggl/attachments/20090613/c2394ab7/attachment.html