Re: [Jts-topo-suite-user] Union returning invalid geometry ?

[Martin D. <mtn...@te...>]

Ah, you had me going there - until I noticed that the input polygon is 
invalid!  (In exactly the same way that the output polygon is 
invalid...  not surprisingly).   Note that if you union the input 
polygon with itself, you get a TopologyException, as expected.  Union 
with points is short-circuited, so the invalidity is not detected.

As always, union requires valid input to be provided in order to create 
valid output.  None of the operations check validity of their operands, 
because this is an expensive operation which may not be required.  So 
it's left up to the user to decide whether a validity check is required.

UnaryUnion uses union "under the covers" to actually perform unions - so 
it will exhibit exactly the same behaviour in this case.

Martin

strk wrote:
> Is it normal for Union operation to return invalid geometry ?
>
> select ST_isvalid(
> 	ST_union(
> 		'POINT(0 0)',
> 		'POLYGON((0 0, 10 0, 10 10, 10 0, 10 10, 0 10, 0 0))'
> 	)
> );
> NOTICE:  Self-intersection at or near point 10 0
>
> The invalidity here is a overstroke segment (10 0)-(10 10)
>
> Would UnaryUnion dissolve that ?
>
> --strk;
>
>   ()   Free GIS & Flash consultant/developer
>   /\   http://foo.keybit.net/~strk/services.html
>
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