[postgis-devel] Dropped DM (Time) dimension with intersections

[Havard Tveite]

According to the (FCD) version I have of SQL/MM Spatial, the
ST_ConvexHull shall ignore z/m values, and the z/m values shall
not be included in the ST_Points contained in the resulting
ST_Geometry (I guess this means that 2D geometry shall be
returned).

Citation from SQL/MM FCD (from 2005), ST_ConvexHull:
*************************************************************
c) If SELF.ST_Is3D() is equal to 1 (one), then:
i) The z coordinate values are not considered in the calculation.
ii) The ST_Point values contained in the returned ST_Geometry value do not include z
coordinate values.
d) If SELF.ST_IsMeasured() is equal to 1 (one), then:
i) The m coordinate values are not considered in the calculation.
ii) The ST_Point values contained in the returned ST_Geometry value do not include m
coordinate values.
*************************************************************

The same applies to the other operations that shall return
regions in space: ST_Buffer and ST_Envelope (with 3D
support, these should return volumes when 3D geometries are
supplied as input).
It also applies some other operations that return geometry
(at least ST_Boundary, ST_Intersection, ST_Union,
ST_Difference, ST_SymDifference, ST_MidPointRep, ST_Centroid
and ST_PointOnSurface).

So, the PostGIS answer is not according to SQL/MM Spatial (and
is wrong, anyway).  As long as PostGIS does not support more
than 2D for most(?) operations, and can not return higher
dimension regions, the SQL/MM behaviour seems reasonable.


Håvard

Chris Hodgson wrote:
> I think the answer is, nobody really knows, but it seemed to make as
> much sense as possible given the operation being requested. If it
> doesn't work for your use case then feel free to make a solid argument
> for why we should give a different result and provide a robust method to
> calculate it :-)
> 
> Chris
> 
> 
> Obe, Regina wrote:
>> Which begs the question - what exactly does this tell me?
>>
>> SELECT
>> ST_AsEWKT(ST_ConvexHull(ST_Collect(ST_GeomFromEWKT('LINESTRING(1 2 3,3
>> 4 5, 3 6 7)'), ST_MakePoint(0, 2, 0))));
>> Result
>> "POLYGON((0 2 0,3 6 7,3 4 5,1 2 3,0 2 0))"
>>
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-- 
Håvard Tveite
Department of Mathematical Sciences and Technology, UMB
Drøbakveien 31, POBox 5003, N-1432 Ås, NORWAY
Phone: +47 64965483 Fax: +47 64965401 http://www.umb.no/imt/