ckanext-spatial - Geo related plugins for CKAN

This extension contains plugins that add geospatial capabilities to CKAN. Currently, there are a WMS previewer (wms_preview) and a spatial query API call (spatial_query) available.

Dependencies

You will need CKAN installed. The present module should be installed at least with setup.py develop if not installed in the normal way with setup.py install or using pip or easy_install.

The extension uses the GeoAlchemy and Shapely libraries. You can install them via pip install -r pip-requirements.txt from the extension directory.

If you want to use the spatial search API, you will need PostGIS installed and enable the spatial features of your PostgreSQL database. See the "Setting up PostGIS" section for details.

Configuration

Once you have PostGIS installed and configured, you need to create the necessary DB tables running the following command (with your python env activated):

paster spatial initdb [srid] --config=../ckan/development.ini

You can define the SRID of the geometry column. Default is 4326. If you are not familiar with projections, we recommend to use the default value.

Problems you may find:

LINE 1: SELECT AddGeometryColumn('package_extent','the_geom', E'4326...
       ^
HINT:  No function matches the given name and argument types. You might need to add explicit type casts.
 "SELECT AddGeometryColumn('package_extent','the_geom', %s, 'GEOMETRY', 2)" ('4326',)

PostGIS was not installed correctly. Please check the "Setting up PostGIS" section.

:: sqlalchemy.exc.ProgrammingError: (ProgrammingError) permission denied for relation spatial_ref_sys

The user accessing the ckan database needs to be owner (or have permissions) of the geometry_columns and spatial_ref_sys tables

Plugins are configured as follows in the CKAN ini file:

ckan.plugins = wms_preview spatial_query

If you are using the spatial search feature, you can define the projection in which extents are stored in the database with the following option. Use the EPSG code as an integer (e.g 4326, 4258, 27700, etc). It defaults to 4326:

ckan.spatial.srid = 4326

Command line interface

The following operations can be run from the command line using the paster spatial command:

initdb [srid]
  - Creates the necessary tables. You must have PostGIS installed
  and configured in the database.
  You can privide the SRID of the geometry column. Default is 4326.

extents 
   - creates or updates the extent geometry column for packages with
    an extent defined in the 'spatial' extra.

The commands should be run from the ckanext-spatial directory and expect a development.ini file to be present. Most of the time you will specify the config explicitly though:

paster extents update --config=../ckan/development.ini

API

The extension adds the following call to the CKAN search API, which returns packages with an extent that intersects with the bounding box provided:

/api/2/search/package/geo?bbox={minx,miny,maxx,maxy}[&crs={srid}]

If the bounding box coordinates are not in the same projection as the one defined in the database, a CRS must be provided, in one of the following forms:

• urn:ogc:def:crs:EPSG::4326
• EPSG:4326
• 4326

Geo-Indexing your packages

In order to make a package queryable by location, an special extra must be defined, with its key named 'spatial'. The value must be a valid GeoJSON geometry, for example:

{"type":"Polygon","coordinates":[[[2.05827, 49.8625],[2.05827, 55.7447], [-6.41736, 55.7447], [-6.41736, 49.8625], [2.05827, 49.8625]]]}

or:

{ "type": "Point", "coordinates": [-3.145,53.078] }    

Every time a package is created, updated or deleted, the extension will synchronize the information stored in the extra with the geometry table.

Setting up PostGIS

Configuration

• Install PostGIS:

  sudo apt-get install postgresql-8.4-postgis

• Create a new PostgreSQL database:

  sudo -u postgres createdb [database]

  (If you just want to spatially enable an exisiting database, you can ignore this point, but it's a good idea to create a template to make easier to create new databases)

• Many of the PostGIS functions are written in the PL/pgSQL language, so we need to enable it in our database:

  sudo -u postgres createlang plpgsql [database]

• Run the following commands. The first one will create the necessary tables and functions in the database, and the second will populate the spatial reference table:

  sudo -u postgres psql -d [database] -f /usr/share/postgresql/8.4/contrib/postgis-1.5/postgis.sql
  sudo -u postgres psql -d [database] -f /usr/share/postgresql/8.4/contrib/postgis-1.5/spatial_ref_sys.sql    

• Execute the following command to see if PostGIS was properly installed:

  sudo -u postgres psql -d [database] -c "SELECT postgis_full_version()"

  You should get something like:

  postgis_full_version                                          
  ------------------------------------------------------------------------------------------------------
  POSTGIS="1.5.2" GEOS="3.2.2-CAPI-1.6.2" PROJ="Rel. 4.7.1, 23 September 2009" LIBXML="2.7.7" USE_STATS
  (1 row)

  Also, if you log into the database, you should see two tables, geometry_columns and spatial_ref_sys (and probably a view called geography_columns).

  Note: This commands will create the two tables owned by the postgres user. You probably should make owner the user that will access the database from ckan:

  ALTER TABLE spatial_ref_sys OWNER TO [your_user];
  ALTER TABLE geometry_columns OWNER TO [your_user];

More information on PostGIS installation can be found here:

http://postgis.refractions.net/docs/ch02.html#PGInstall

Setting up a spatial table

Note: If you run the initdb command, the table was already created for you. This section just describes what's going on for those who want to know more.

To be able to store geometries and perform spatial operations, PostGIS needs to work with geometry fields. Geometry fields should always be added via the AddGeometryColumn function:

CREATE TABLE package_extent(
    package_id text PRIMARY KEY
);

ALTER TABLE package_extent OWNER TO [your_user];

SELECT AddGeometryColumn('package_extent','the_geom', 4326, 'POLYGON', 2);

This will add a geometry column in the package_extent table called the_geom, with the spatial reference system EPSG:4326. The stored geometries will be polygons, with 2 dimensions (The actual table on CKAN uses the GEOMETRY type to support multiple geometry types).

Have a look a the table definition, and see how PostGIS has created three constraints to ensure that the geometries follow the parameters defined in the geometry column creation:

# \d package_extent

   Table "public.package_extent"
   Column   |   Type   | Modifiers 
------------+----------+-----------
 package_id | text     | not null
 the_geom   | geometry | 
Indexes:
    "package_extent_pkey" PRIMARY KEY, btree (package_id)
Check constraints:
    "enforce_dims_the_geom" CHECK (st_ndims(the_geom) = 2)
    "enforce_geotype_the_geom" CHECK (geometrytype(the_geom) = 'POLYGON'::text OR the_geom IS NULL)
    "enforce_srid_the_geom" CHECK (st_srid(the_geom) = 4326)