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John Zhang | 6 Oct 22:33

Snap and Dissolve function in PostGIS?

Hi there,

I am writing to check how to perform SNAP and DISSOLVE function by PostGIS.

I have a point dataset table and a MultipleLineString dataset table. I need to snap all the points in the dataset to the Line Strings when the distance is within 100 meters.

I also need union some polygon features and dissolve the polygons within a polygon.

Any inputs would be appreciated.

--
Sincerely,


John Zhang,   M.Sc., B.Sc., A.Dipl.T.
email:   JohnZhang06 <at> gmail.com
Phone:  604-998-1874

URL:  http://ca.geocities.com/john_z04/JOHN_ZHANG_GIS.html
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Dane Springmeyer | 7 Oct 00:24
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Re: Snap and Dissolve function in PostGIS?

Hi John,

Regarding snapping points to lines, postgis does not have a built-in function to do that, but this nice post gives you a good guide on how to accomplish it: http://blog.cleverelephant.ca/2008/04/snapping-points-in-postgis.html

Regarding how to do a 'Dissolve' in postgis, I assume you are referring to the Dissolve command equivalent in ArcGIS where polygons with shared boundaries are merged or otherwise turned into multipolygons wen they share a common attribute? In postgis this is done by the combination of the ST_Union() function on the geometry and a Group By on an attribute. Something like:

SELECT ST_Union(the_geom) INTO dissolved_polygons FROM your_table GROUP BY your_attribute;

If you want to run that step with a faster function then read the wiki post here to learn how to use the new/proposed cascaded union function:


Dane


On Oct 6, 2008, at 1:35 PM, John Zhang wrote:

Hi there,

I am writing to check how to perform SNAP and DISSOLVE function by PostGIS.

I have a point dataset table and a MultipleLineString dataset table. I need to snap all the points in the dataset to the Line Strings when the distance is within 100 meters.

I also need union some polygon features and dissolve the polygons within a polygon.

Any inputs would be appreciated.

--
Sincerely,


John Zhang,   M.Sc., B.Sc., A.Dipl.T.
email:   JohnZhang06 <at> gmail.com
Phone:  604-998-1874

URL:  http://ca.geocities.com/john_z04/JOHN_ZHANG_GIS.html
_______________________________________________
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
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