A changeset consists of a group of changes made by a single user over a short period of time (see also: editing). One changeset may for example include the additions of new elements to OSM, the addition of new tags to existing elements, changes to tag values of elements, deletion of tags and also deletion of elements.
Edits can only be added to a changeset as long as it is still open; a changeset can either be closed explicitly (see your editor's documentation), or it closes itself if no edits are added to it for a period of inactivity (currently one hour). The same user can have multiple active changesets at the same time. A changeset has a maximum capacity (currently 50,000 edits) and maximum lifetime (currently 24 hours).
The History function on www.osm.org shows the changesets of the currently viewed area. They have an "extent" (bounding box) which is a rectangle surrounding all the changes. For "bots" that make small changes around the world this can be quite big. This is why many changesets show up for an area even when that changeset does not seem to be relevant. Better tools are available.
Tags on changesets
name=value pairs attached (Tags!). The vast majority of changesets will have these two tags:
- comment=* – describing why a mapper made that group of changes, or what was changed. By some software (e.g. www.osm.org) this tag is not displayed as tag but as changeset summary/headline instead.
While optional, mappers are encouraged to make full use of this tag by setting a meaningful, human-generated description (not an automated message), as it will show up almost everywhere where the changeset is listed, and is likely to be read by fellow mappers to try and understand what has happened. See also Good changeset comments.
- created_by=* – specify the editor or script which made the changes
Some other commonly used tags include:
- imagery_used=* – indicates which imagery was being displayed in the editor
- source=* – specify the source for the edits that have been made in this changeset
- bot=yes – for automated edits, performed by a program (a.k.a. script or bot)
It is not possible to change a changeset after it has closed (in editors or via the API), and we do not store a history of changes to tags on changsets.
Changesets were introduced with API v0.6 in April 2009. Changesets were "synthesised" for edits before that date. For technical details, see the API 0.6 documentation which contains extensive documentation on them. See also the Get Capabilities documentation.
Here is a proposal to define more common changeset tags that can be used to describe edits: Proposed features/changeset_tags (including the previous)