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(For more resources on OpenStreetMap, see here.)
It’s important to remember that there are few fixed ideas of what is “wrong” data in OpenStreetMap. It should certainly be an accurate representation of the real world, but that’s not something an automatic data-checking tool can detect. There may be typographical errors in tags that prevent them from being recognized, but there are also undocumented tags that may accurately describe a feature, yet be unknown to anyone except the mapper who used them. The latter is fine, but the former is a problem.
It’s tempting to use the two map renderings on openstreetmap.org as a debugging tool, but this can be misleading. Not every possible feature is rendered, and many problems with the data, such as duplicate nodes or unjoined ways, won’t be obvious from a rendered map. If a feature you’ve mapped doesn’t render when a similarly tagged one does, there’s an issue, but a feature appearing in the map doesn’t mean it’s free of problems, and a feature that doesn’t appear isn’t necessarily wrong.
Ultimately, you will have to use your own judgment to find out whether or not an issue reported by one of these tools is really an error in the data. You can always contact other members of the OpenStreetMap community.
This is only a selection of the more widely used quality assurance tools used by mappers. For a more complete list, refer to http://wiki.openstreetmap.org/wiki/Quality_Assurance.
Inspecting data with openstreetmap.org’s data overlay and browser
The openstreetmap.org website has a range of tools you can use to inspect the data in the database, both current and past. Some of the tools aren’t obvious from the front page of the site, but are easily found if you know where they are.
The tools, which consist of the data map overlay and the data browser pages, allow you to see the details of any object in the OpenStreetMap database, including coordinates, tags, and editing history, without the need to launch an editor or read raw XML. As these tools work directly with the data in the OpenStreetMap database, they always show the most up-to-date information available. However, they simply provide raw information, and don’t provide any guidance on whether the geometry or tagging of any feature could be problematic.
The easiest way of inspecting data is to start with the data map overlay. Go to the map view and find Compton (or any other area you want to inspect). Open the layer chooser by clicking on the + sign at the top-right. Click the checkbox labeled Data, and a box will appear to the left of the map view. After a short delay, the data overlay will appear, and a list of objects will appear in the box.
Once the data for the area you’re inspecting has loaded, you’ll see something like the following image:
In the preceding image, on the left you can see the Object list, which gives a text description of every feature in the current map view, giving its primitive type and either its ID number or a name, if the feature has one. On the right is the map with the data overlay, which highlights every feature in the current area, whether they’re rendered on the map or not. This last point is worth repeating: Not every type of feature gets rendered on the two map renderings used on openstreetmap.org, and those that do can take some time to appear if the load on the rendering engines is high. Any feature in the database will always appear in the data overlay.
Inspecting a single feature
To inspect an individual feature, either click on its entry in the object list, or on its highlight in the map view. Both the object list and the overlay will change to reflect this. Occasionally, an area feature may get drawn on top of other features, preventing you from selecting the ones underneath, but you’ll still be able to select them from the list.
Let’s select The Street and inspect its data. Either click on its name in the object list, or on the way in the map view, and the object list should change to show the tags applied to the feature, and you should see something like the following in the object list:
This gives a list of the tags attached to the feature. If you click on Show History, a list of the edits made to the current feature is added to the list. To get more information, click on the Details link next to the feature’s name, and you’ll be taken to the data browser page for that object, as follows:
Here you see far more details about the feature we’re inspecting. Apart from the object ID and its name, you can find the time when the object was last edited and by whom, and in which changeset. There are clickable links to any related objects and a map showing the feature’s location.
At the bottom of the page are links to the raw XML of the feature, the history page of the feature, and a link to launch Potlatch—the online editor—for the area surrounding the feature.
Checking a feature’s editing history
The OpenStreetMap database keeps every version of every feature created, so you can inspect previous versions and see when and how a feature has changed. To look at a feature’s history, click on the link at the bottom of its data browser page. For the Watts Gallery in Compton, you should see something like the following:
You can see each version of the object listed in full, including which mapper created that version in which changeset, and what the tags for that version were. There’s currently no way of showing any previous version or the changes between versions on the map, but third-party tools such as OSM Mapper provide some of these features.
Along with looking at individual features, you can see how the map gets changed by looking at changesets. Since version 0.6 of the OpenStreetMap API went live in April 2009, every change to the map has to be part of a changeset. A changeset is a list of related edits made to OpenStreetMap data, with its own set of tags. What goes into a changeset is entirely up to the mapper creating it.
You can view the list of recent changesets by clicking on the History tab at the top of the map view. This will show a list of the 20 most recent changesets whose bounding box intersects your current map view. Note that this doesn’t guarantee that any changesets listed include any edits in your current view, and any changesets covering a large area will be marked with (big) in the list.