# [postgis-users] PostGIS Raster: Two Questions

Simon SPDBA Greener simon at spdba.com.au
Tue Mar 16 17:15:17 PDT 2021

```I have a 1 band raster, srid 4326, (0.001 x 0.001 roughly 100m x 100m) ,
called *gbr100x100d*, whose pixels are coded 0 and 1 where 1 is water
deeper than 100m and 0 water shallower than 100m.

It was created as follows:

*create table gbr100x100d
as
SELECT row_number() over (order by 1) as rid,
ST_SetSrid(ST_MapAlgebra(a.rast, 1, NULL,
'([rast.val]<-100.0)::integer'),4326) as rast
from gbr100x100c as a;*

First question: how can one recode *gbr100x100d *so that all pixels with
value 0 are NODATA?

Next.

Polygons of size 0.02x 0.01 need to be extracted from the 1 valued 0.001
pixels.

I think this should be done as follows.

First we resample to get a raster with cells of size 0.02 x 0.01:

*create table gbr2kmx1kmd1**
**as**
**SELECT row_number() over (order by 1) as rid,**
**ST_SetSrid(ST_Rescale(a.rast,0.02,-0.01,'Bilinear',0.125),4326) as rast**
**  from gbr100x100d as a;**
*

Then we extract the polygons of these 0.02 x 0.01 pixels.

*create table gbr2kmx1kmd1p**
**as **
**SELECT row_number() over (order by 1) as id, (gv).val,
ST_SnapToGrid((gv).geom,0.001)::geography as geog**
**  FROM (SELECT ST_PixelAsPolygons(a.rast,1,true) as gv **
**          FROM gbr2kmx1kmd1 as a**
**       ) gv**
** WHERE (gv).val = 1;*

The result is close as the polygons are 0.02 x 0.01 but they are
organised as a set of striped polygons where the columns don't touch.

See attached.

What was expected was a complete coverage of abutting polygons.

Can anyone spot what I am doing wrong?

regards

Simon

2021-03-17_10-27-06.png

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```