[postgis-users] Concern about GEOMUNION() results

[James G Wilkinson]

Strk,

>>I am using PostgreSQL v.7.4.2, PostGIS 0.9.0, PROJ v.4.4.7, and GEOS 
>>v.2.1.1.
>>I have prepared two spatially enabled tables called "test" and "test2."  
>>"test" is a
>>single polygon, and "test2" is a single line segment (see below for 
>>their geometries).
>>When I perform a union of the two geometries (also shown below), I 
>>expected to
>>get a GEOMETRYCOLECTION with three line segments and the polygon.  Instead,
>>I received only two line segments and a polygon (results shown below).  
>>The third
>>line segment that I expected was as follows:
>>
>>LINESTRING(-97 42.04,-97.05 42.1)
>>
>>Is my expectation wrong?
>>    
>>
>
>Yes.
>The missing segment of the line is 'absorbed' by the
>polygon it overlaps.
>
>If you want your third line you need the intersection of you the
>input.
>  
>
Thanks for the insight.  I am still stuck in my Arc/Info frame of mind 
(i.e., use of the
ARC/Info IDENTITY function...I think that is the function...oh, well).  
At least I am
getting a bit more savy in the use of PostGIS.  Thanks again.

Regards,

Jim

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