Shira,
No clue and haven't looked at the code to see the difference. The only thing I would guess is maybe in 2.0.1 if a pixel was only partially covered by a geometry then it doesn't include the pixel (thus the pixels you may be missing in 2.0.1 are those only partially covered by the geometry) And in 2.3, maybe it includes it if it's partially covered. I think we had discussions about that and if we needed another argument to denote which behavior should be used. I forget what was decided if anything. Hope that helps, Regina From: postgis-users [mailto:postgis-users-boun...@lists.osgeo.org] On Behalf Of Shira Bezalel Sent: Wednesday, January 18, 2017 2:20 PM To: PostGIS Users Discussion <postgis-users@lists.osgeo.org> Subject: Re: [postgis-users] ST_Clip - Different results between PostGIS 2.0.1 and 2.3.1 On Mon, Jan 9, 2017 at 10:18 AM, Shira Bezalel <sh...@sfei.org <mailto:sh...@sfei.org> > wrote: Hi there. I'm testing an upgraded database and seeing different results in a query that's using the ST_Clip function. old database: PostgreSQL 9.1.14 with PostGIS 2.0.1 new database: PostgreSQL: 9.6.1 with PostGIS 2.3.1 I know ST_Clip was rewritten in C in PostGIS 2.1. Could this be responsible for different results? Or did the clipping algorithm change? I consulted the docs, but didn't see anything noted to this effect. Query: SELECT (pvc).value, SUM((pvc).count) AS total FROM ( SELECT ST_ValueCount(st_clip(rast, c.the_geom),1) AS pvc FROM nlcdcal20_2011, counties c WHERE st_intersects(rast, c.the_geom) and c.NAME = 'Alameda' ) AS foo GROUP BY (pvc).value order by (pvc).value If I remove the clip, the results are identical. In terms of the actual difference, here's the sum total of all pixels found by this query: Total pixels in 2.0.1 = 2,362,444 Total pixels in 2.3.1 = 2,418,017 It's not a huge difference, but enough to be curious about. We can live with it, but it would just be nice to know the cause. Thank you for any insight you can provide. Shira p.s. The faster performance of the new ST_Clip is awesome! Didn't hear back from the list on this, so thought I'd resend. Just looking to know if anyone has run into this, what might be the cause, and maybe most importantly (and ideally), if the new results would be considered more accurate? Thank you, Shira
_______________________________________________ postgis-users mailing list postgis-users@lists.osgeo.org https://lists.osgeo.org/mailman/listinfo/postgis-users
