[postgis-users] How best to do this?

[Stephen Woodbridge]

Paul, et al,

Any chance that someone would have GEOS 3  built for WinXP that I could 
drop into a system currently running:

"POSTGIS="1.1.3" GEOS="2.2.2-CAPI-1.1.0" PROJ="Rel. 4.4.9, 29 Oct 2004" 
USE_STATS"

"PostgreSQL 8.1.4 on i686-pc-mingw32, compiled by GCC gcc.exe (GCC) 
3.4.2 (mingw-special)"

So I could give this a try.

In the past, I seem to remember people dealing with these problems using 
snaptogrid() - would this potentially help here and how would I change 
my query to use that?

-Steve

Paul Ramsey wrote:
> GEOS 3 does include a number of fixes that will help difference and union.
> 
> P
> 
> On 1-Feb-07, at 9:01 PM, Stephen Woodbridge wrote:
> 
>> OK, one step forward and two steps backward ...
>>
>> I was about to get rid of my gemetrycollections with:
>>
>> update newtable set the_geom =
>>  (select geomunion(geom1) from
>>    (select (dump(the_geom)).geom as geom1) as foo
>>     where geometrytype(geom1)='POLYGON'
>>  )
>>  where geometrytype(the_geom)='GEOMETRYCOLLECTION';
>>
>> This worked very nicely once I figured out that this was the way to 
>> solve the problem. So now each geometry collection is converted into a 
>> polygon or multipolygon and all the linestring and point artifacts of 
>> the intersection are removed.
>>
>> select distinct geometrytype(the_geom) from soils2;
>> POLYGON
>> MULTIPOLYGON
>>
>> select * from soils where not isvalid(the_geom);
>> reports 0 rows
>>
>> insert into newtable (..., the_geom)
>>   select A....,
>>          difference(A.the_geom, B.the_geom) as the_geom
>>   from A, B
>>   where A.the_geom && B.the_geom and overlaps(A.the_geom, B.the_geom);
>>
>> NOTICE:  TopologyException: no outgoing dirEdge found 
>> (287030,892621,892621)
>>
>> ERROR:  GEOS difference() threw an error!
>>
>> Now what do I do??? If I have to upgrade it might be easiest to 
>> upgrade my WinXP laptop if there is an easy way to do that, then dump 
>> and load the data to that.
>>
>> select postgis_full_version();
>> "POSTGIS="1.1.5" GEOS="2.2.3-CAPI-1.1.1" PROJ="Rel. 4.4.9, 29 Oct 
>> 2004" USE_STATS (procs from 1.1.1 need upgrade)"
>>
>> select version();
>> "PostgreSQL 8.0.8 on i386-portbld-freebsd5.3, compiled by GCC cc (GCC) 
>> 3.4.2 [FreeBSD] 20040728"
>>
>> -Steve
>>
>> Stephen Woodbridge wrote:
>>> Hi all,
>>> Sorry the subject line is not more useful, but let me explain.
>>> 1) I have a set of polgons A and another set of polygon B
>>> 2) I need to a new set of polygons C that are the union of
>>>    A intersect B and A difference B
>>> So basically B partially covers A and I want to split polygons in a 
>>> into the covered piece(s) and that that is not covered.
>>> On the surface this is very straight forward, but it isn't ...
>>> insert into newtable (....)
>>>    select ..., intersection(A.the_geom, B.the_geom) as the_geom
>>>      from A, B
>>>      where A.the_geom && B.the_geom and
>>>        intersects(A.the_geom, B.the_geom);
>>> This works, but I get a bunch of GEOMETRYCOLLECTION objects in the 
>>> results and out of 1022 collection I have 2780 additional POLYGONs.
>>> So I could do something like:
>>> insert into newtable (....)
>>>     select * from
>>>       (select ..., (dump(the_geom)).geom as the_geom from newtable
>>>          where geometrytype(the_geom)='GEOMETRYCOLLECTION' ) as foo
>>>     where geometrytype(the_geom)='POLYGON';
>>> delete from newtable where geometrytype(the_geom)='GEOMETRYCOLLECTION';
>>> So now comes the tricky part how do you do the difference? I was 
>>> thinking I could take A difference newtable but because I have 
>>> multiple little pieces instead of a single piece, I think I am getting:
>>>   (A - Part1), (A - Part2), (A - Part3) instead of
>>>   (A - Part1 - Part2 - Part3)
>>> So I think I need to union the polygon pieces from each row that is 
>>> dumped above. Is that as simple as tossing part of the dump into a 
>>> geomunion()? How would that look?
>>> -Steve W
>>> _______________________________________________
>>> postgis-users mailing list
>>> postgis-users at postgis.refractions.net
>>> http://postgis.refractions.net/mailman/listinfo/postgis-users
>>
>> _______________________________________________
>> postgis-users mailing list
>> postgis-users at postgis.refractions.net
>> http://postgis.refractions.net/mailman/listinfo/postgis-users
> 
> _______________________________________________
> postgis-users mailing list
> postgis-users at postgis.refractions.net
> http://postgis.refractions.net/mailman/listinfo/postgis-users