OSM History Viewer
With OSM History Viewer, you can visualise the changes that were made in one changeset compared to what things looked like before. It shows all tags that have been removed or added to elements and visualises how nodes and ways have been modified, moved, created or removed. Later it will also be possible to see how a relation was changed in the changeset.
To visualise the changes, all ways affected by the changeset are turned into 'segments', where one segment represents the connection of two nodes. Each segment is then analysed to determine whether it was moved, created or deleted in that changeset. If a segment, or its location only existed before the commit, it is marked red on a map. If a segment, or its location only exists after the commit, it will be marked green. If a segment did not change, it will be marked blue.
This method has the following effects on the visualisation:
- If you moved a way, the old way will be on the map in red, the new way in green.
- If you only move one node in a very long way, only the part that you actually have changed will be marked modified.
- If you split a way or combine multiple ways, they will be visualised as unmodified (blue), as you have not changed their position on the map.
- If you have only changed the tag of a way, it will also be shown blue.
- If you see something brown on the map (green and red mixed together) you might have moved a node only by a small distance. Zoom in to see that the green line will be distinct from red one instead of being at the same position.
This method has the great advantage over the simple displaying of the changeset content that you can see how the objects on the map actually have changed (and not how their data representation in the OSM database has changed).
See also: the tool achavi has a very similar feature.
With “Relation Blame” you can find out which part of a relation has been added to it by whom. The relation (including all its sub-relations) is split into segments (segments being connections between two nodes) and then the history is searched for the first occurrence of these segments in the relation. By this you can find out who added a certain part to the relation in which changeset (and then “blame” him ;-)). This segment-based method has the big advantage that people just splitting ways will not appear to have added a way to the relation (even though they technically did). A “wrong” author might still be displayed though if someone added some nodes to a way of the relation (for example by connecting another way to it or by improving its accuracy).
Easy ways to use OSM History User
On  there is a Greasemonkey script that adds links to OSM History Viewer to the relation and changeset pages on www.openstreetmap.org. That script is currently broken in opera/chrome. Fixed version
Use a bookmarklet to conveniently access OSMHV! By clicking on the new bookmark (or opening it via a keyword) while visiting a changeset page such as 6798641, you will be redirected to the "OSM History Viewer" analysis page for that changeset. (Credits to Eugene).
Use the following code instead to open the OSMHV analysis page in a new window/tab (allows you to switch between the changeset and OSMHV view):
- When you moved a node in a changeset that was part of a way and is not part of that way any more during analysis, the way might not be shown as affected. This cannot be fixed due to restrictions of the OSM API.
- When two objects belonging to a relation have been changed in the same second in different changesets, only one of these changesets will be considered. So the wrong user might be displayed in the Relation Blame mode. As two things changing in one second are problematic anyway (because you cannot tell which one changed first) and the effect of this is very low, it is not planned to be fixed.
- Hovering over changed property list is jumpy. Workaround: start hovering on bottom of the list instead of the top and move subsequently up the list towards the top.
A bugtracker is available on bugs.cdauth.eu. Bugs Reports and Features Requests can be submitted there.
Screenshot & example
This is how Changeset 6798641 looks in OSM History Viewer.