Michal ZimmermannPieces of knowledge from the world of GIS.

PostGIS Case Study: Vozejkmap Open Data (Part II)

In the first part of my little case study }}) I downloaded vozejkmap.cz dataset and imported it into the PostGIS database. Having spatial data safely stored the time comes to get it onto the map. Libraries used are:

I teach cartography visualization classes this semester and this map should serve well as an example of what can be done with online maps.

Retrieving data from the PostGIS database

Our goal is to build the whole map as a static HTML page without any backend logic. Thus, data needs to be extracted from the database into the format readable with Leaflet - GeoJSON.

That’s fairly easy with the postgresonline.com tutorial. It took me quite a time to find out what the following query does. Splitting it into smaller chunks helped a lot.

SELECT row_to_json(fc)
FROM (
SELECT 'FeatureCollection' AS type,
    array_to_json(array_agg(f)) AS features
    FROM (SELECT 'Feature' AS type,
        ST_AsGeoJSON(lg.geom)::json As geometry,
        row_to_json((SELECT l FROM (SELECT id, title, location_type, description, author_name, attr1, attr2, attr3) AS l
  )) AS properties
FROM vozejkmap AS lg ) AS f )  AS fc \g /path/to/file.json;

To get all rows with type, geometry and properties columns (these are the ones defined in GeoJSON specification, see the link above), run this:

SELECT 'Feature' AS type,
            ST_AsGeoJSON(lg.geom)::json As geometry,
            row_to_json((SELECT l FROM (SELECT id, title, location_type, description, author_name, attr1, attr2, attr3) AS l
      )) AS properties
    FROM vozejkmap AS lg

array_agg() squashes all the rows into an array while array_to_json() returns the array as JSON.

SELECT 'FeatureCollection' AS type,
    array_to_json(array_agg(f)) AS features
    FROM (SELECT 'Feature' AS type,
        ST_AsGeoJSON(lg.geom)::json As geometry,
        row_to_json((SELECT l FROM (SELECT id, title, location_type, description, author_name, attr1, attr2, attr3) AS l
  )) AS properties
FROM vozejkmap AS lg ) AS f

In the last step (the whole code as shown above) row_to_json returns the result as JSON.

Caveats

If you run this code from the psql console, be sure you

If you don’t, you’ll have lots of hyphens and column names saved to the json file.

Leaflet map

Map JavaScript is rather simple with ~30 lines of code (not taking styles into account). Thanks to the great plugins it is easy to show ~7,600 points on the map real quick.

I didn’t do much customization apart from styling markers and binding popups.

What’s next

  1. Turf which means I need to think of what could be fun to do with this data
  2. Layers switching
  3. Map key (by extending L.Control)

The code is still available at my GitHub.