30 PostGIS

Mickey Rose edited this page 2019-08-12 16:38:44 +02:00

Table of Contents

• Parameters
• Usage
• Advanced Usage
• bbox token
• other tokens
• Usage from Python
• Usage from XML
• Usage from C++
• Further References

Mapnik's PluginArchitecture supports the use of different input formats.

This plugin supports PostGIS, a spatial extension to the popular PostgreSQL database.

See also a performance tuning page: OptimizeRenderingWithPostGIS

Parameters

name = default value	type	description
host	string	name of the postgres host
port	integer	name of the postgres port
dbname	string	name of the database
user	string	username to use for connecting
password	string	user password to use for connecting
application_name	string	connection application_name. If omitted, then fallback_application_name = "mapnik" will be set instead. Available in Mapnik >= 4.0
table	string	name of the table to fetch, this can be a sub-query; subquery has to use syntax of: '( ) as subquery'.
geometry_field	string	name of the geometry field, in case you have more than one in a single table. This field and the SRID will be deduced from the query in most cases, but may need to be manually specified in some cases.
geometry_table	string	name of the table containing the returned geometry; for determining RIDs with subselects
srid = 0	integer	srid of the table, if this is > 0 then fetching data will avoid an extra database query for knowing the srid of the table
extent	string	maxextent of the geometries; if omitted, the extent will be determined by querying the metadata for the table
extent_from_subquery = false	boolean	evaluate the extent of the subquery, this might be a performance issue
connect_timeout = 4	integer	timeout is seconds for the connection to take place
persist_connection = true	boolean	choose whether to share the same connection for subsequent queries
row_limit = 0	integer	max number of rows to return when querying data, 0 means no limit
cursor_size = 0	integer	if this is > 0 then server cursor will be used, and will prefetch this number of features
initial_size = 1	integer	initial size of the stateless connection pool
max_size = 10	integer	max size of the stateless connection pool
multiple_geometries = false	boolean	whether to use multiple different objects or a single one when dealing with multi-objects (this is mainly related to how the label are used in the map, one label for a multi-polygon or one label for each polygon of a multi-polygon)
encoding = utf-8	string	internal file encoding
simplify_geometries = false	boolean	whether to automatically reduce input vertices. Only effective when output projection matches (or is similar to) input projection. Available from version 2.1.x up.
asynchronous_request	boolean	Postgres queries are sent asynchronously : while rendering a layer, queries for further layers will run in parallel in the remote server. Available from version 2.3.x up. DEPRECATED (automatically set internally based on max_async_connection > 1 condition)
max_async_connection = 1	integer	max number of PostGIS queries for rendering one map in asynchronous mode. Full doc here. Used only when asynchronous_request=true. Default value (1) has no effect.

Usage

Note:

• Spatial tables read from PostGIS by Mapnik should ideally have a corresponding entry in geometry_columns.
• Use the geometry_field parameter to specify which field to use if you have >1 geometry in the table/query or if your table does not have a geometry_columns entry.

Advanced Usage

The PostGIS plugin supports several special tokens. You can use them in subqueries and Mapnik will replace them at render time.

bbox token

Under normal circumstances, if you pass to Mapnik table=mytable then, when mytable is ultimately queried, Mapnik will form up a query like:

ST_AsBinary("geom") AS geom,"name","gid" FROM mytable WHERE "geom" && ST_SetSRID('BOX3D(<map bbox>)'::box3d, 3857)

Using the token !bbox! allows you to write a subquery and leverage the spatial filter in a custom way. So, if you wished to override the geom && and do ST_Intersects instead then you could do (in XML):

<Parameter name="table">(Select * from mytable where ST_Intersects(geom,!bbox!)) as mysubquery</Parameter>

other tokens

Other tokens that can be used include:

• !scale_denominator! (Mapnik >= 0.7.0)
• !pixel_width! (Mapnik >= 2.1.0) -- width of a pixel in geographical units
• !pixel_height! (Mapnik >= 2.1.0) -- height of a pixel in geographical units

Usage from Python

Instantiate a datasource like:

    lyr = Layer('Geometry from PostGIS')
    lyr.datasource = PostGIS(host='localhost',user='postgres',password='',dbname='your_postgis_database',table='your_table')

If you want to do complex queries you can nest subselects in the table argument:

    lyr = Layer('Buffered Geometry from PostGIS')
    BUFFERED_TABLE = '(select ST_Buffer(geometry, 5) as geometry from %s) polygon' % ('your_postgis_table')
    lyr.datasource = PostGIS(host='localhost',user='postgres',password='',dbname='your_postgis_database',table=BUFFERED_TABLE)

If you want to add something after the query (for example ORDER BY) you must use !bbox! dynamic map variable:

    lyr = Layer('Order by st_length from PostGIS')
    BUFFERED_TABLE = 'table_line where way && !bbox! ORDER BY st_LENGTH(way) DESC'
    lyr.datasource = PostGIS(host='localhost',user='postgres',password='',dbname='your_postgis_database',table=BUFFERED_TABLE, srid='your_srid', geometry_field='way', extent='your_extent')

• Note: because mapnik depends on the geometry_columns entry be careful not to use sub-selects that change the geometry type.
• Further references: See Artem's email on using the PostGIS from Python
• Example code at the Mapnik-utils project: http://mapnik-utils.googlecode.com/svn/example_code/postgis/postgis_geometry.py

Usage from XML

If you are using XML mapfiles to style your data, then using a PostGIS datasource (with a sub-select in this case) looks like:

• Note: if you use a sub-select that changes the extents of your features, make sure to use estimate_extent=false otherwise Mapnik will return no features. Otherwise you don't need to use the estimate_extent or extent parameters at all.

    <Layer name="countries" status="on" srs="+proj=latlong +datum=WGS84">
        <StyleName>countries_style_label</StyleName>
        <Datasource>
          <Parameter name="type">postgis</Parameter>
          <Parameter name="host">localhost</Parameter>
          <Parameter name="dbname">geodjango_geographic_admin</Parameter>
          <Parameter name="user">postgres</Parameter>      
          <Parameter name="password"></Parameter>
          <Parameter name="table">(select ST_Buffer(ST_Centroid(geometry),2) as geometry, name  from world_worldborders) as world</Parameter>
          <Parameter name="estimate_extent">false</Parameter>
          <Parameter name="extent">-180,-90,180,89.99</Parameter>
        </Datasource>
    </Layer>

Note: If you use a custom projection, you might need to change the extent parameters to the area for which the projection is defined. For example, the Dutch grid (EPSG:28992) is only defined around the Netherlands. It does not make sense to try to project South America onto it. You need to change the extent parameter to something like this:

    <Parameter name="extent">3.09582088671,50.6680811311,7.41350097346,53.6310799196</Parameter>

If you don't do this, you might not see data from this data source at all, even if it does not contain data outside of the valid region. Also note that you always specify the extents in the coordinates of the source system.

Usage from C++

Plugin datasource initialization example code can be found on PluginArchitecture.

A PostGIS datasource may be created as follows:

#include <mapnik/version.hpp>
#include <mapnik/datasource_cache.hpp>

    {
        parameters p;
        p["type"]="postgis";
        p["host"]=database_hostname;
        p["port"]="5432";
        p["dbname"]="gis";
        p["user"]=your_username;
        p["password"]="";
    
        Layer lyr("Roads");
#if MAPNIK_VERSION >= 200200
        set_datasource(datasource_cache::instance().create(p));
#else
        set_datasource(datasource_cache::instance()->create(p));
#endif
        lyr.add_style("roads");
        m.addLayer(lyr);
    }

For other PostGIS parameters, see the postgis_datasource constructor in postgis_datasource.cpp#L48

TODO -- more PostGIS query usage

Further References

Using Mapnik and PostGIS with OSM