Cropping OSM data
Services producing automated extracts of OSM data like Geofabrik or Nextzen are limited to predefined areas. You'll often need to download an extract for a country or region larger than your true analysis area, then cut it down to size.
Excessively large OSM data can lead to significant increases in computation time and complexity, both while building the graph and handling trip planning requets. You may want to crop the OSM data if they cover an area significantly larger than your transit network. Several command line tools are also able to perform these cropping operations: Osmosis is a multi-platform Java tool that works on Windows, Linux, and MacOS but is relatively slow, OSMConvert is a fast tool pre-built for Windows and Linux and available on MacOS and Linux distributions as part of
osmctools package. Osmium-Tool is a personal favorite that is extremely fast but only straightforward to install on Linux and MacOS platforms. Below are some example crop commands for these different tools:
osmosis --rb input.osm.pbf --bounding-box left=4.34 right=5.84 bottom=43.10 top=43.97 --wb cropped.osm.pbf
osmconvert input.osm.pbf -b=-77.255859375,38.77764022307335,-76.81365966796875,39.02345139405933 --complete-ways -o=cropped.osm.pbf
osmium extract --strategy complete_ways --bbox 2.25,48.81,2.42,48.91 input.osm.pbf -o cropped.osm.pbf
The latter two commands expect bounding boxes to be specified in the format
min_lon,min_lat,max_lon,max_lat. We frequently find bounding boxes using the convenient Klokantech bounding box tool. Selecting the "CSV" format in the lower left will give exactly the format expected by these tools.
Filtering OSM data
The OSM database contains a lot of other data besides the roads, paths, and public transportation platform data we need for accessibility analysis. As of this writing, according to TagInfo 59% of the ways in OSM are buildings, and only 23% are roads or paths. Buildings frequently have more complex shapes than roads, and objects like waterways or political boundaries can be very large in size. It has been jokingly said that OSM should be renamed "OpenBuildingMap" rather than "OpenStreetMap".
Removing unneeded data will reduce file sizes, facilitating copying or moving files around and reducing the size of project backups and archives. It may also speed up the processing stage where the OSM data is converted into a routable street network. Several command line tools exist to filter OSM data. Command line tools for this purpose include Osmosis and Osmium-Tool. Osmium-Tool is extremely fast but is only straightforward to install on Linux and MacOS platforms. Osmosis is often slower at filtering but will also work on Windows as it's a multi-platform Java application. OSMFilter cannot work with PBF format files so we rarely use it. Below are some example commands for retaining only OSM data useful for accessibility analysis. Here are some example commands:
osmosis --rb input.osm.pbf --tf reject-ways building=* --tf reject-ways waterway=* --tf reject-ways landuse=* --tf reject-ways natural=* --used-node --wb filtered.osm.pbf
osmium tags-filter input.osm.pbf w/highway w/public_transport=platform w/railway=platform w/park_ride=yes r/type=restriction -o filtered.osm.pbf -f pbf,add_metadata=false