[postgis-users] ST_Clip - Different results between PostGIS 2.0.1 and 2.3.1

[Shira Bezalel]

Thanks Regina. That does help.

Shira

On Thu, Jan 19, 2017 at 12:06 AM, Regina Obe <lr at pcorp.us> wrote:

> 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-bounces at lists.osgeo.org] *On
> Behalf Of *Shira Bezalel
> *Sent:* Wednesday, January 18, 2017 2:20 PM
> *To:* PostGIS Users Discussion <postgis-users at 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 <shira at 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​
>
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-- 
Shira Bezalel
Database Administrator & Desktop Support Manager
San Francisco Estuary Institute
www.sfei.org
Ph: 510-746-7304
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