[GIS] PostGIS: convert point data to raster

postgisraster

I have a table of points and I'm trying to visualize their distributions.
Each point has a POINT geometry (WGS84) and some numeric attributes: a, b, c.

| id | geom       | a  | b | c |
|-----------------|----|---|---|
| 1  | POINT(...  | 12 | 9 | 2 |
| etc ...                      |

The first thing I tried was to project to an equal area projection (EPSG: 3175), use ST_SnapToGrid and then group on the resulting point geometry.

SELECT 
  avg(a), avg(b), avg(c), 
  ST_Transform(ST_SnapToGrid(ST_Transform(geom, 3175), 50), 4326) AS bin
FROM points
GROUP BY bin

This groups the points into 50×50 meter bins with their aggregate values which I can import into qgis to play with. I optimized this by adding another column to the points table called bin_id which was a foreign key to a bins table:

| id | geom      |
|----|-----------|
| 1  | POINT(... |

Then I could write queries like this:

SELECT avg_a, avg_b, avg_c, geom
FROM (
  SELECT 
    avg(a) AS avg_a,
    avg(b) AS avg_b,
    avg(c) AS avg_c,
    bin_id
  FROM points
  GROUP BY bin_id
) AS tmp
JOIN bins ON tmp.cell_id = bins.id

This all works fine but i want to use Rasters instead. All the examples I've seen show how to import existing rasters or shapefiles into PostGIS. But I need to know how to generate one from data already in PostGIS.

Ideally I want a raster with 3 bands: avg_a, avg_b & avg_c

Can someone point me in the right direction?

EDIT 1: I'm not opposed to doing it outside of the database.
EDIT 2: Seems like I can use the GDAL API to do it.

Best Answer

We wrote a couple of articles about this you might find useful: http://www.bostongis.com/blog/index.php?/archives/211-Waiting-for-PostGIS-2.1-ST_SetValues-The-road-to-thematic-maps.html

http://www.bostongis.com/blog/index.php?/archives/222-Using-ST_ColorMap-for-Thematic-Maps.html

You'll want to check out in PostGIS manual -- ST_AsRaster, ST_DumpValues (and other pixel setter functions) http://postgis.net/docs/manual-2.1/RT_reference.html#Raster_Pixel_Accessors raster ST_Union -

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