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Changeset

Changeset

Nodes, ways, and relations created or modified in JOSM are only available locally unless they are published on a server to make them useful for other mappers. The process of publishing data is called uploading.

Because there are hundreds of mappers publishing their geo-data on the OSM server it is important to track who published what data in which context.

Changesets simply group data creations, modifications and deletions - with additional properties comment, created_by, imagery_used and several others (this information stored as simple tags). Similar to other wiki software, one changeset can affect one object (a single character edit in the text), but may contain big changes (removals of paragraphs, partial or complete rewrite of the text) with the only difference that geo information is edited instead of text. Investigating history of one character is equally hard to comparing text at given point in time. Since complete "text" in OSM is 30-600GB big therefore users speak of history within several changesets and not about complete history every time.

A more convenient approach is to package a group of related objects and assign the package a comment, not the individual objects. This way, if a mapper maps his or her neighborhood, he or she can upload a package of nodes and ways and assign it a comment "mapped and reviewed some ways in foo city".

In JOSM such a package is called a changeset. A changeset is a collection of related changes (new objects, object modifications, or object deletings) applied to OSM data.

Changesets and upload operations

A changeset is independent from an individual upload operation.

You can run multiple uploads with the same changeset. The changeset will then package the changes published in all uploads. A mapper can even stop JOSM, start it a day later and continue to publish to the changeset he or she has been using a day before, provided he or she didn't close the changeset.

Properties of changesets

A changeset is initially opened and it stays open until it is closed. A JOSM user can explicitly close one or more of his open changesets. If he doesn't close them, the OSM server will close open changeset automatically after a timeout. A changeset which isn't open is called closed. It stays closed forever and can't be reopened.

A changeset you open is owned by you. No other mapper can publish data in this changeset and you can't upload your data in other users changesets.

Upload comment and other changeset tags

Similar to nodes, ways, and relations, it is possible to decorate changesets with tags.

The most important and only compulsory tag is the so called upload comment. When you publish data in JOSM you are required to enter an upload comment with at least three characters. It is this comment which describes the published data for other mappers.

Advanced Preferences

You can set the advanced preference upload.comment.max-age to 0 if you do not want to get a pre-filled comment text field. You still can select a previous comment from the drop down box.