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

[Huntington, Margaret (US SSA)]

Hello, 

    Currently I'm using PostGIS 1.3.3.  From past discussions and from
testing, both the st_intersection st_extend methods return 2D results
with 4D input geometries.  As a temporary work-around, I had hoped
st_intersection might work with 3DM geometries.  (Plan was to
interpolate the altitude value within the function call if PostGIS could
calculate the time dimension).   I found time components are also
dropped by st_intersection with 3DM geometries.  I abandoned usage of 4D
bounding boxes since these too effectively degrade 4D geometries down to
2D geometries (altitude and time are zeroed out).    

    polyGeometry geometry;
    bbGeometry geometry;
    intersectionGeometry geometry;    
    coorddims smallint;  

           -- both polygon and linestring have an expected zmflag value
of 1
           polyGeometry := 'SRID=4326;POLYGONM((0 0 0, 0 10 4, 10 10 4,
10 0 0, 0 0 0))'::geometry;  
           bbGeometry := 'SRID=4326;LINESTRINGM(0 0 1.5, 10 10
2)'::geometry;                                   

           intersectionGeometry := st_intersection(polyGeometry,
bbGeometry);

           -- st_intersection method drops the time dimension; zmflag
value of 0
           coorddims := st_zmflag(intersectionGeometry);

If I were to download the subversion snapshot, the current 1.4 version,
might st_intersection work with either 3DM or 4D geometries?  Are
bounding boxed or st_extend improved for either 3DM or 4D geometries?  I
had incorporated polygons only as a possible work-around to the bounding
box and st_extend 2D limitations.

Margie  
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