Here are some Frequently Asked Questions about OSM.
If you have a question not on this list, feel free to ask it in one of the Contact channels.
Why are you making OpenStreetMap?
Geographical data (geo data) is not free in many parts of the world. Generally these places have given the task of mapping to various government agencies who in return get to make money by selling the data back to you and me. If you live in one of these countries, then your taxes pay for the mapping and then you have to pay again to get a copy of it. In the USA, crude data (such as TIGER) is in the public domain, however refined data and finished maps are generally commercially copyrighted.
Data from commercial maps sometimes contain lies, or Copyright Easter Eggs, to catch anyone copying it. These easter eggs take the form of fake or missing streets, or features like churches and schools that don't in fact exist. If you make a map using their data, they can say "ah-ha! Gotcha!" from looking if you also copied these fake pieces of map. The map may also just be incorrect because for example you bought it a year ago and a path has been dug up in your local park since, or someone just made a mistake.
If you accept all of this then you still can't do anything with the data but photocopy it. In lots of places that's illegal too if you go beyond your fair use rights. You can't correct a street name, or add the pub/bar over the road, or use the data in a computer program without paying a lot of money. More money than you probably have. What about sending it to a friend, enclosing it in an invitation or posting it on a notice board? A lot of these are less legal than you might think.
Advances in technology like cheap GPS units mean you can now create your own maps, in collaboration with others and have none of the restrictions outlined above. The ability to do so allows you to regain a little bit of the community you live in - if you can't map it, you can't describe it.
Why don't you just use Google Maps/whoever for your data?
Because that data is copyrighted and owned by multiple organisations like the Ordnance Survey. Google/whoever just licenses it. Sometimes data itself powering this services is not available at all, or blocked by unfriendly licensing or extreme fees.
Most hackers around the world are familiar with the difference between "free" as in "free beer" and as in "free speech". Google Maps are free as in beer, not as in speech.
If your project's mapping needs can be served simply by using the Google Maps API, fine. That's not true of every project, though. We need a free dataset which will enable programmers, citizen scientists, social activists, cartographers and the like to fulfill their plans without being limited either by Google's API or by their Google Maps/Google Earth Additional Terms of Service:
The data used in Google Maps is either owned by Google itself, or sourced from NAVTEQ and Tele Atlas, two big mapping companies. They, in turn, have obtained some of this data from national mapping agencies (such as the Ordnance Survey). Since they've made significant financial investments to gather this data, these organisations are understandably protective of their copyright.
If you collect data from Google Maps in this way, you are creating a "derived work". Any such data retains the copyright conditions of the original. In practice, this means your data is subject to the licensing fees, and contractual restrictions, of these map providers. That's exactly what OpenStreetMap is trying to avoid.
(It's not yet clear whether it's ok to create a derived work from aerial photography : some readings of UK law suggest that you can do this without 'inheriting' the copyright in the photography. A definitive ruling on this could open up new avenues for OpenStreetMap and similar projects, but in the absence of such a ruling, we're continuing with the approach of sourcing our own, 100% independent data.)
- Google Maps/Google Earth Additional Terms of Service.
- The OpenStreetMap mailing list archives. You may want to search them for phrases like "derived works", and for a thread in October 2005 called "London locations".
- Can I use Google Streetview ? on help.openstreetmap.org
How can a project like this create accurate maps?
By the very nature of the wiki-style process there is no guarantee of accuracy of any kind. Then again, few proprietary maps carry a guarantee of accuracy, either.
The essence of a wiki-style process is that all users have a stake in having accurate data. If one person puts in inaccurate data, maliciously or accidentally, the other 99.9% of people can check it, fix it, or get rid of it. The vast majority of good-intentioned participants can automatically correct for the few bad apples.
As they say, though, your mileage may vary. The Wikipedia project has shown that a large amount of good quality data can be collected but it can be difficult to weed out the inevitable errors.
A full editing history is stored for each user. Since April 21, 2009, users can attach Wikipedia-like edit summaries to their edits, and there is a History tab on the main page that shows recent edits to the selected area.
At the moment the best way to answer this question is to judge for yourself. One way is to pick an area that you know well and use the OpenStreetMap viewer to see how well the map data corresponds to your own knowledge. Maybe you will see something wrong or inaccurate. More likely you will find there's nothing there yet. At this stage, our main challenge is to extend our coverage, without copying from existing maps. As on Wikipedia, it's easy to edit, so you can help!
You seem to have a lot of existing map data. Where did it come from?
Many keen contributors, and sporadic imports of data from open-licensed sources. In areas where there are no such data sources (most areas) we have to start from a blank slate, and head out there to survey the streets ourselves. Despite starting from scratch, we have achieved a good level of completion in many places.
Why is the data sometimes inconsistent?
"OpenStreetMap is a free editable map of the whole world. It is made by people like you." Which means the database will always be subject to the whims, experimentation, and mistakes of the community; this is precisely OSM's strength since, among other things, it allows our data to quickly accommodate changes in the physical world.
In OpenStreetMap it is normal that one town is mapped in extreme detail, another has mapped road and bicycle infrastructure and nothing else while another is not mapped at all (for now!).
Why are some things missing on the map?
OSM relies entirely on its contributors for the data that is shown on the map.
Nobody ever contributed the data that's missing before, so is up to you to contribute that data now!