OpenStreetMap is a map of the world, created by people like you and free to use under an open license.
That’s OpenStreetMap’s (OSM) opening lines to their free-to-anyone spatial database.
Yes. You read that right. Free spatial data!
If you’re looking for spatial data you can’t get from an authoritative source (i.e. city, county, state, or federal agency) for free, then OSM data may be worth using, especially if you’re working in a part of the world where getting local data would be difficult due to language barriers or just simply there is a lack of data.
But before you do so, let’s go through the pros and cons of this popular spatial data source.
- It’s free! Complete and truly free to use. You can download a whole world of data (approximately 803GB as of June 2017) by snagging the weekly copy of Planet.osm. Once on your computer, you are free to do with it what you like. Load it into a QGIS or ArcMap document and symbolize as you see fit. You can even edit the data. It is truly all yours.
- Related to the first point; the data comes with no strings attached. When you read my first point, you may have thought: “What’s the big deal? Google Maps, Bing Maps, and other web map services are free, too.” True, but Google Map data is *not* available for download. You can contribute edits to Google Maps, but that doesn’t mean Google will give data back to you.
- OSM often has richer data than other free map sources. For example, someone might have digitized all the hydrants in your city. Or all the best dog-friendly businesses. Or, if you are like me and ride your bike everywhere, all the barriers a bike-rider might encounter when getting around town.
- Data quality is spotty. Because OSM depends on volunteers, literally located all around the wold, data quality and consistency is haphazard. In places where a community of local mappers make a concerted effort to improve the database, you can get some stunningly detailed and accurate base maps (i.e. San Francisco area). Where they don’t…well, let’s just say, you get what you pay for.
- Very little, if any, metadata is associated with OSM layers. Are the bus routes current? Who knows.
- Data is not authoritative, obviously. In the United States, OSM’s road network was derived from U.S. Census data. So on the national level, the road network is fairly accurate, complete, and authoritative. You can rest assured that the data went through some sort of data quality check with the U.S. Census and comes with a statement of accuracy. However, most other OSM layers are not. While someone may have digitized every tree in your town’s parks, there’s no way to tell if they accurately identified a walnut tree from a spruce.
In a nutshell, OSM data is great to have when you’ve got nothing else. Even when you do have access to something better, you may want to download the latest OSM layers. For example, I’ve used OSM’s building outline layer for the San Francisco Bay area, because, frankly, those industrious volunteers have already merged the nine SF Bay area county databases for me.
Another great reason to use OSM is that they support mapping efforts throughout the world. With their Humanitarian OpenStreetMap Team (HOT), they even try to help folks with efforts like their Malaria Elimination Campaign. While some tech companies claim to do no evil, HOT actually does.
Here’s a nifty info graphic of the Pros and Cons to OSM.
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