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= Conflict =

JOSM locally manages copies of OSM objects. It loads them from a file or downloads them from the OSM server and keeps them in memory. If you modify objects or add some to your map, you are working on copies. The reference object is still kept on the server from where other users can retrieve it and where other users can send their updates to. 

This means that you don't ''own'' the objects you are currently working with, you still ''share'' them with others. If you exchange your local objects with the server your version may clash with the version somebody else sent in as update. JOSM does its best to hide these clashes from you, but sometimes it can't resolve the differences between two versions automatically. In this cases it creates a '''conflict'''.

A '''conflict''' is a difference between two versions of an object which JOSM can't resolve automatically. 

== Conflicts when downloading/updating == 
If you download objects from the OSM server into an existing layer, the objects from the server might "collide" with objects already present in the layer. Often, JOSM can handle these collisions automatically but sometimes this isn't possible. Here's an example:

 1. You've downloaded a node with id 1 from the server whose [http://wiki.openstreetmap.org/wiki/Key:name name] tag is set to `"My name"`.
 2. You edit the node and change its name to `"Or is it this name?"`. In the meantime another user downloaded the node too, changed its name to `"That's the name"`, and uploads it.
 3. You select the menu [[Image(wiki:Help/Action/UpdateData:updatedata.png)]]''File->[wiki:/wiki/help/Action/UpdateData Update data]''. The version of the node retrieved from the server will have a tag [http://wiki.openstreetmap.org/wiki/Key:name name] set to `"That's the name"` which collides with `"Or is it this name?"`, the name you've been setting.

JOSM can't resolve this automatically. It creates a '''conflict'''. Typically JOSM displays an error message as follows:[[br]]
[[Image(conflict-update-data.png)]]

== Conflict when uploading ==
If you upload objects to the OSM server, they might "collide" there with updates provided by other mappers. Sometimes, JOSM can handle these collisions automatically. If, for instance, you try to delete an already deleted object, JOSM will sillently skip deleting and won't bother you with an error message. In most cases, though, JOSM can't deal with them automatically. Here's an example:

  1. You've downloaded a node with id 1 from the server whose [http://wiki.openstreetmap.org/wiki/Key:name name] tag is set to `"My name"`.
  2. Another mapper downloads the node with id 1 too, assigns it the name `"That's the name"`, and uploads it.
  3. You change the name to `"Or is it this name?"` and upload to the server.
  4. On the server, you're upload collides with the upload from the other mapper.

JOSM can't resolve this automatically. It creates a '''conflict'''. Typically JOSM displays an error message as follows:
[[BR]]
[[Image(conflict-upload-data.png)]]
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== Conflicts when merging ==
In JOSM you can merge the objects in one data layer into another data layer. Again, this can lead to "collisions" between different versions of the same objects.
If JOSM can't resolve them automatically it creates '''conflicts''' too. 

== Resolving conflicts ==
Each data layer maintains a list of '''conflicts''' which are not resolved yet. You can consult this list in the [wiki:/Help/Dialog/ConflictListDialog Conflict List Dialog]. 
For each '''conflict''' listed there you can (and should) launch the [wiki:/Help/Dialog/ConflictDialog Conflict Resolution Dialog] and resolve it.

Resolving conflicts is important. For instance, you can't upload your updates to the OSM server unless the list of non-resolved conflicts is empty. 

== See also ==

 * [wiki:/Help/Dialog/ConflictDialog Conflict Dialog] - You can use the conflict dialog to resolve a conflict 


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