CartoDB-SQL-API/doc/handling_geospatial_data.md

Handling Geospatial Data

Handling geospatial data through the SQL API is easy. By default, the_geom is returned straight from the database, in a format called Well-Known Binary. There are a handful of ways you can transform your geometries into more useful formats.

The first is to use the format=GeoJSON method described above. Others can be handled through your SQL statements directly. For example, enclosing your the_geom in a function called ST_AsGeoJSON will allow you to use JSON for your data but a GeoJSON string for your geometry column only. Alternatively, using a the ST_AsText function will return your geometry as Well-Known Text.

ST_AsGeoJSON

Call

https://{username}.cartodb.com/api/v2/sql?q=SELECT cartodb_id,ST_AsGeoJSON(the_geom) as the_geom FROM {table_name} LIMIT 1

Result

{
  time: 0.003,
  total_rows: 1,
  rows: [
    {
      cartodb_id: 1,
      the_geom: "{"type":"Point","coordinates":[-97.3349,35.4979]}"
    }
  ]
}

ST_AsText

Call

https://{username}.cartodb.com/api/v2/sql?q=SELECT cartodb_id,ST_AsText(the_geom) FROM {table_name} LIMIT 1

Result

{
  time: 0.003,
  total_rows: 1,
  rows: [
    {
      cartodb_id: 1,
      the_geom: "POINT(-74.0004162 40.6920918)",
    }
  ]
}

More advanced methods exist in the PostGIS library to extract meaningful data from your geometry. Explore the PostGIS documentation and get familiar with functions such as, ST_XMin, ST_XMax, ST_AsText, and so on.

All data returned from the_geom column is in WGS 84 (EPSG:4326). You can change this quickly on the fly, by using SQL. For example, if you prefer geometries using the Hanoi 1972 (EPSG:4147) projection, use ST_Transform,

ST_Transform

https://{username}.cartodb.com/api/v2/sql?q=SELECT ST_Transform(the_geom,4147) FROM {table_name} LIMIT 1

CartoDB also stores a second geometry column, the_geom_webmercator. We use this internally to build your map tiles as fast as we can. In the user-interface it is hidden, but it is visible and available for use. In this column, we store a reprojected version of all your geometries using Web Mercator (EPSG:3857).