PostGIS 2.1 rasters available to Rails code: breadcrumbs

rubyonrails-talk
1

Just wanted to post this in case someone else was faced with the same problem. It’s database-specific code, worse, it’s specific to a particular version of a particular extension to a particular database, wires in SQL, and uses eval on a string of data from the database!

So, you know, I’m getting coal in my stocking from DHH this year. I can deal. (After the whole Jolt Award thing years back, though, he totally owes me, I wanted to keep that thing so bad…)

Anyway, problem is this: I wanted to do some calculations on some geospatial raster data, which are to be stored in a GIS-enabled database. I’d chosen PostGIS long ago for various reasons. But even the fabulous RGeo gem doesn’t know from rasters, or at least not yet AFAIK. So there’s no way to get pixels out of the raster except one-by-one with an ST_Value query; for a 297x582 array, a nonstarter.

Enter PostGIS 2.1 and its lovely lovely ST_DumpValues and ST_SetValues functions. By using the connection’s select_value method, we can return an arbitrary SQL result as a string, in this case a 2D array coded with the PostgreSQL convention of { and } where Ruby uses [ and ].

Likewise, connection.update can take an SQL string, and we can wad a 2D array of values into it with a simple to_s.

So here’s the hack:


# Get and set raster band values out of PostGIS 2.1 rasters as Ruby arrays.
# This example assumes that the raster column is 'rast' and that the primary
# key for the table is 'rid'. Also, NULL pixels will probably tank the getter.
# Fixing those is left as an exercise :-)
class PostGisRaster < ActiveRecord::Base


  self.primary_key = 'rid'
# Grab a band of a raster out of the database and convert it to a 2D array. Defaults to first band (1)
def self.to_a(tablename,id,band=1)
id = id.to_i
str = connection.select_value("select st_dumpvalues(rast,#{band}) from #{tablename} where #{primary_key}=#{id}")
str.gsub!('{','[')
str.gsub!('}',']')
eval(str)
end
# Update a raster band (default: first band) in the database with values from arr. Note that arr can be any
# subset of the size of the full raster. Corner is X,Y (or long,lat), with the upper left corner as [1,1].
def self.update_raster(tablename,id,arr,band=1,corner=[1,1])
update_str = <<-END
select st_setvalues(
(select rast from #{tablename} where rid=#{id}),
#{band},#{corner[0]},#{corner[1]},
array#{arr.to_s}::double precision[][]
)
END
update_str = "update #{tablename} set rast=(#{update_str}) where rid=#{id}"
puts "update_str: '#{update_str}'"
connection.update(update_str)
end
end