OpenStreetMap is a collaborative mapping project established in 2004 to create a free editable map of the world. It is the most comprehensive source of volunteered geographic information.
OSM is a great resource for open, rights-free geographic data. However, like any other volunteer-contributed data, OSM data coverage and data quality varies. In general, data are more complete in developed countries and urban areas.
- Visualization of OSM node density around the globe.
- View methods and techniques for validating OSM data.
Authoritative data from governments, institutions and data vendors should be used when possible. These data are created with standards and augmented with good metadata.
Carleton has sourced data on U.S. gas stations as well as on Israeli water well locations from OSM because the former was too costly to source from vendors, while the latter was not found in any other database.
OSM data are saved as geometry types (listed below), and tags are used to describe attributes of the features being shown by the geometries.
- Nodes: A node is a single point
- Ways: a way is an ordered list of nodes. A closed way can be interpreted as an area or as a closed polyline.
- Relations: A relation is an ordered list of one or more nodes, ways and/or relations as members to define logical or geographic relationships between the members. For example, a lake with an island can be saved as a relation with a way representing the outer boundary of the lake and a way representing the boundary of the island.
A particular map feature can be saved as more than one geometry type in OSM. For example, a park can be saved as a node or as a closed way.
Extracting Data from OpenStreetMap
The following types of OSM data can be extracted:
- data for a region
- specific map features for a region
- specific map features for a user-defined area.
Extraction Tutorial: Map Features for a Region
There are multiple websites that allow you to extract OSM data. Here is a great resource to figure out which extraction program will be the most helpful for your needs.
- Identify the map features you want to extract and how they are tagged in the OSM database. Some helpful resources are the OpenStreetMap Wiki page that contains a list of accepted tags, or TagInfo, a site which collects statistics about the usage of different OSM keys and tags.
- For example, if we wanted to access data on gas stations, we would find them in OSM through the tag amenity=fuel.
- Go to download site and under the “LayerName” column, locate the layer(s) that correspond to the map features you want to extract. There will likely be more than one layer for the features you want, one for each point, line, and polygon feature. Make sure to download all of them.
- Continuing with our example of gas stations, you would choose all the layers beginning with amenity – in this case, amenity_ply and amenity_pnt.
- After locating the right layer(s), navigate to the column representing the desired geographical region (North and South America, Africa, Oceania, or Eurasia) and click the “GetLink button(s)” to download the shapefiles.
Working with Downloaded OpenStreetMap Data in ArcGIS
Because a map feature type can be saved as more than one geometry type in OSM, you will likely have to combine the downloaded point, line and/or polygon features into a single shapefile which stores only one geometry type. You can convert map feature geometry to a lower dimension (points: 0 dimension, line: 1 dimension, polygon: 2 dimensions), but not the other way around (with the exception of properly closed polylines). For example, you can convert a polygon to a point (the centroid of the polygon), but you cannot create a polygon from a point.
- Download the shapefiles with compatible software in order to inspect the data.
- Delete unnecessary columns (attributes) from the attribute table.
- Filter down the data by using a definition query or by exporting the desired features to a new shapefile. This will leave you with just the features of interest.
- Use the “Feature To Point” tool to convert line and polygon shapefiles to point shapefiles in ArcGIS.
- If lines are properly closed as polygon boundaries, you can use the “Feature to Polygon” tool in ArcGIS to convert them to polygons. You need to manually trace the lines that are not properly closed to create the polygons.
- Use the “Merge” tool to merge multiple shapefiles of the same geometry type into a single shapefile in ArcGIS.