The import process is separated into multiple steps. You can combine most steps in one command, but we will explain each one in detail here.
This is step zero, since you have to do it only once. This step creates a PostgreSQL database with the PostGIS extension enabled.
Imposm comes with a little tool that creates all command required to create a new database.
imposm-psqldb --help
You can save the output of imposm-psqldb and use it as a shell script for the postgres user:
imposm-psqldb --postgis-version 2.0 --pg-version 9.1 > create-db.sh
sudo su postgres
vim ./create-db.sh # cross check if all path are set
sh ./create-db.sh
For detailed step-by-step information, read the excellent OpenStreetMap PostGIS tutorial.
The first step is the reading of the OpenStreetMap data. Building the way and relation geometries requires random access to all nodes and ways, but this is not supported by the XML or PBF data formats. Imposm needs to store all nodes, ways and relations in an intermediary data store that allows random access to all elements. It does this on-disk to keep the memory usage of Imposm low. Imposm uses TokyoCabinet key-value databases for this, which are fast and compact.
imposm --read germany.osm.pbf
Imposm distributes the work across multiple processes to leverage modern multicore CPUs. It can set the number of processes with the --concurrency option. It defaults to the number of CPU cores on the host system.
imposm --read --concurrency 2 europe.osm.pbf
Imposm stores the cache files in the current working directory. You can change that path with --cache-dir. Imposm can merge multiple OSM files into the same cache (e.g. when combining multiple extracts) with the --merge-cache option or it can overwrite existing caches with --overwrite-cache.
The second step is the writing of OpenStreetMap features into the database. It reads the features from the cached data from step one, builds all geometries and imports them into the according tables. It overwrites existing tables, see below how to work with existing datasets.
After the import, it creates the generalized tables and views.
You need to tell Imposm the configuration of your database.
imposm --write --database osm --host localhost --user osm --port 5432
Imposm uses localhost for the database host, 5432 for the port and osm for the database and user by default. The database name is always required. You can also configure the database with a single --connection option:
imposm --write --connection postgis://osm:passwd@localhost/dbname
You can combine reading and writing:
imposm --read --write -d osm hamburg.osm.bz2
You can limit the imported geometries to polygon boundaries. You can load the limit-to polygons from Shapefiles, GeoJSON or any other OGR compatible source. The geometries need to be in EPSG:4326.
imposm --read --write -d osm europe.osm.pbf --limit-to germany.shp
This step is optional and it does some optimization on the created tables. It clusters each table based on the spatial index and does a vacuum analyze on the database. The optimizations only work with the import tables, but not the production tables (see below).
imposm --optimize -d osm
You can combine reading, writing and optimizing:
imposm --read --write --optimize -d osm hamburg.osm.bz2
Since Imposm overwrites existing tables on import, it is recommended to use different table names for import and for production. Imposm supports this with table name prefixes.
Imposm adds osm_new_ as a prefix to all tables names, e.g. motorways becomes osm_new_motorways. You can change the prefix with the --deploy-production-tables.
To rename all osm_new_* tables to osm_*:
imposm -d osm --deploy-production-tables
This renames any existing osm_* table to osm_old_* and then renames any osm_new_* tables to osm_*. Existing backup tables (osm_old_*) are removed before. You can then check the results with a PostGIS client or your WMS server.
You can revert to the previous osm_* tables:
imposm -d osm --recover-production-tables
And you can remove backup tables (osm_old_*):
imposm -d osm --remove-backup-tables
You can change the prefixes with --table-prefix, --table-prefix-production and --table-prefix-backup
You can change the default mapping the the -m/–mapping-file option. See Data Mapping for more information.
Imposm uses the the web mercator projection (EPSG:900913) for the imports. You can change this with the --proj option.
Imposm can ignore large multipolygon relations. You can set the maximum number of rings with the IMPOSM_MULTIPOLYGON_MAX_RING environment variable. Set to 0 process all sizes. The multipolygon builder improved since 2.2.0, so this is not needed anymore.
It will log complex multipolygon relations that take more than 60 seconds to build. You can change this time with the IMPOSM_MULTIPOLYGON_REPORT environment variable for debugging.