Geometry Boost List: Re: [geometry] dimension type of geometries

Subject: Re: [geometry] dimension type of geometries
From: Adam Wulkiewicz (adam.wulkiewicz_at_[hidden])
Date: 2013-07-26 09:57:20

Hi,

2013/7/25 Mateusz Loskot <mateusz_at_[hidden]>

> On 25 July 2013 02:41, Adam Wulkiewicz <adam.wulkiewicz_at_[hidden]> wrote:
> >
> > Does OGC allow negative dimensions? If not, should we assume that they
> won't
> > be used in the future?
>
> In OGC (and ISO too) specifications of Simple Features, all attributes
> related to
> geometric dimensions are defined using signed integer.
> In SQL/MM Spatial, an extension or predecessor of the latest OGC/ISO specs,
> there is notion of ST_PrivateDimension for which the following rule is
> specified:
>
> a) If the ST_Geometry value corresponds to the empty set, then the
> dimension is -1.
>
> So, I'd draw conclusion, that OGC/ISO at least do not disallow
> negative values here.
>
> Behaviour of empty geometry vs empty set as in set theory might be
> confusing
> (some details here http://trac.osgeo.org/postgis/wiki/DevWikiEmptyGeometry
> )
>
>
According to this, the empty geometry is useful in cases where the result
of the operation isn't any geometry. Our dimensions are static and we
handle those cases in a different way, so we probably wouldn't need to use
the empty geometry. However we probably shouldn't assume that we won't,
especially with Boost.Variant on the horizon.

So should we leave it as it is and assume that dimension may be signed or
unsigned? It shouldn't be a problem to add empty geometries in the future,
even if core_traits were using size_t and the user were using some signed
int of different size.

Regards,
Adam