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Saving and loading .osm files
For most purposes you'll probably want to stick with downloading data and uploading your changes. When you upload your changes you have effectively "saved" your changes. Better than that, you've made your changes available for other people to see on the map, and to to make further changes to. OpenStreetMap is all about internet collaboration! However JOSM can also work with files on your local filesystem, saving and opening files to/from your local disk. This works much like any other normal desktop application.
Use 'file' menu > 'Save As...' to save data (and your changes) to a file.
Use 'file' menu > 'Open...' to open a previously saved file for editing or uploading.
The JOSM file format is an XML representation OpenStreetMap data in the standard .osm format. JOSM extends this slightly to store details of changes to OpenStreetMap data, that is, any changes which were not uploaded already at the time when the file was saved. This is a powerful feature which means JOSM can be used as an offline editor as follows:
While you have an internet connection...
While you're offline without an internet connection...
When you're back to an internet connection...
You can filter OpenStreetMap data (e.g. by tags, to give a thematic extract) either by manipulating .osm files on your disk using tools such as osmosis, osmconvert or osmfilter, or by requesting filtered data from a specialised API such as XAPI or Overpass API. You then can load filtered data into JOSM. This can be the best way to get a look at it, but take care when editing. You should not delete or move any nodes without first doing a normal unfiltered download around it, to get all connected elements.
While you are editing with JOSM, there's a chance other people could be editing the same area of data at the same time. JOSM only downloads data when you ask it to, which means you can have quite long editing sessions without necessarily noticing potential clashes with other contributors. In general it's a good idea to upload regularly if you are able to do so, to reduce the chance and also the impact of any conflicts. But it can also be a good idea to download prior to uploading. The download action does not overwrite your changes with data from the server. There's no need to worry about losing any of your changes, but it does mean you have the chance to see any changes which other users may have made (or new data they have added) before you do your upload.
Clearly these changes made by other users are something you should check over prior to uploading. If another user has changed exactly the same element which you have also changed, then JOSM reports a "conflict". At this stage you have not yet uploaded your changes. You are forced to resolve conflicts before you upload. This feature is designed to help you to find areas where you have clashed with other users. It allows you to pick and choose whether to overwrite other people's changes with your own in each case. Once you have resolved the conflicts you can upload your changes.
Unglueing and untangling
In the JOSM/Basic editing section we described how to edit nodes and ways, and also operations such as split/combine/and reverse ways. Things can get a little more tricky when data is intertwined, overlapped or tangled in a complex manner, but remember you can always try to make changes and then 'undo' a few steps if things don't work out. You can also temporarily move nodes or even delete elements to understand linkages and overlaps, before undoing to restore the data. Here are a few additional editing techniques:
Overlapping ways, where two or more ways are drawn exactly on top of each other between the same two nodes, this can happen by accident or may be a correct representation of something (Two highways should not normally overlap each other like this, but you may come across landuse areas with nodes shared with the bordering highways for example) In this situation you may need to select one or the other of the overlapping ways. There are various ways of doing this:
Unglue ways is an option on the 'Tools' menu. If two (or more) ways are connected at one node, this option will create a second node, freeing one way from the other. Note that this normally results in a 'duplicate node' (two different nodes sitting exactly on top of one another) with both nodes being selected. As a follow-up, or just to understand the situation, you may want to clear your selection and then drag just the top node off to one side. You should do this to avoid leaving a duplicate node in the data. They are usually regarded as a bad thing, often resulting in confusion and routing connectivity problems. There should be no need for them in the data. The 'unglue ways' action can also be used with a way element selected, to unglue it from any nodes it shares along it's entire length. If you want to merge such nodes together again, use 'Tools' > 'Merge nodes' - but note that this merges all nodes that are very near.
The features described here are particularly useful for working with complex data. There are several other actions on the tools menu, and more can be installed via plugins.
Relations are a type of element which can be used to represent some more advanced kinds of map features and geodata, such as routes and turn restrictions. Elements like "nodes" and "ways" can be members of relations. Relations can be members of relations too (a hierarchy) For each of these memberships the "role" can be named, and the overall relation is given tags. See the Relations page for more details.
JOSM provides an interface to give you raw editing power over relations. There are several ways of accessing the main relations dialog.
The "properties/membership" panel, which you normally use to view and edit tags, will also show relations, this time just those of the selected element (the element's memberships).
For creating brand new relations you need to use the relations panel, but both the panels give access to the dialog for editing existing relations. The main relation editing dialogue is split into three areas. At the top you can edit the tags of the relation. At the bottom-left is list of elements which are members, with a column for the roles. The list is ordered (which may or may not be significant) On the bottom-right there is a pick-list of elements. This list is linked to the element selection you currently have in the background data view. Adjust your selection in the background to bring elements onto the list which can then be added to the relation.
Create a new relation
Add new members to a relation
JOSM allows you to sort members, and this is recommended for some types of relations. e.g. route relations, multipolygons, .... Sorting the members allows you to ensure the members are connected, and to locate any unconnected ways. To sort the members click the A-Z button in the relation editor.
Official JOSM relation documentation
That's a very brief description of relation editing in JOSM. Refer to the 'Relations' help documentation for a more exhaustive description of these features.
JOSM supports distributed tagging presets. This is a XML config file that describes the GUI for entering tags (or properties, annotations, sometimes called map features) available when using the Presets menu in JOSM. The XML files can be placed anywhere in the web or at the users local file system. Presets can be added by the users in JOSM's Preferences under Map Settings on the Tagging Presets tab. After they have been added, the presets (menus or single buttons) can be added to the toolbar in JOSM's Preferences under Toolbar customization.
For more detailed information visit the following links at http://josm.openstreetmap.de: