[postgis-users] Raster SQL to separate high value areas

[Regina Obe]

You probably want to use ST_Reclass  and then ST_DumpAsPolygons.

 

http://postgis.net/docs/manual-2.2/RT_ST_Reclass.html - this will reduce
your pixels into a binary of above 70% , not above 70%

 

 

http://postgis.net/docs/manual-2.2/RT_ST_DumpAsPolygons.html would then give
you two sets of polygons with the two new pixel values.

 

 

 

From: postgis-users [mailto:postgis-users-bounces at lists.osgeo.org] On Behalf
Of liglio at pobox.com
Sent: Tuesday, March 08, 2016 3:10 PM
To: postgis-users at lists.osgeo.org
Subject: [postgis-users] Raster SQL to separate high value areas

 

Hi,

 

I am trying to separate areas of high value of lightning probability (above
70%) in a Raster and calculate their centroids. I successfully loaded a
GeoTiff raster image (one band, gray scale, 982x1108 pixels) in PostgreSQL
9.5 (Postgis 2.2.1). I see the SQL results in OenJump 1.9.

 

I used the next SQL, but the function ST_PixelAsPoints result in a geometry
points which are of the pixel's upper-left corner. I am not sure that is the
right way to do, because of that displacement. Can I do without this
transformation raster-geometry ?

 

Thanks.

 

SELECT row_number() over () AS id,

  ST_NumGeometries(gc),

  gc AS geom_collection,

  ST_Centroid(gc) AS centroid

FROM (

  SELECT unnest(ST_ClusterWithin(geom, 0.1)) AS gc

  FROM (SELECT (ST_PixelAsPoints(rast, 1)).* 

    FROM tb_lightningprob) foo

    WHERE val > 0.7

) f;

 

 

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