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Undo / Discard changes
JOSM generally doesn't read or write data to the OpenStreetMap database except when you ask it to, but at any time before uploading, you can discard all your changes by closing down JOSM (or by deleting the active data layer). You can also undo individual editing actions by clicking the undo toolbar button () or by pressing Ctrl+Z. So relax! If you make a mess of your editing at any point, you can always undo a few steps.
(S)elect and (A)dd
There are two main modes of operation when editing:
Because you will be flipping between these modes quite a lot, it's worth remembering the keyboard shortcuts: 'S' and 'A'
You will also need to delete things sometimes (Keyboard shortcut is the 'delete' or 'del' key):
Adding a way
When creating a way, you need to think about what is currently selected.
Adding a stand-alone node
To create a standalone node, repeat the same steps above, but this time we'll stop adding after only one node:
Panels appear as a strip on the right hand side of the JOSM screen, but you can show & hide each one. Do this by clicking the various buttons in the bottom-left:
They are toggle buttons, meaning you can click the same button to show the panel and then hide it and then show it again (try it). On the panels themselves, you have the option to minimise them, or un-pin (undock) a panel to have it as a separate window.
Some panels are more useful than others. You'll definitely want to have the "properties/memberships" panel visible ().
Tags are displayed in a panel called "properties/memberships" in the top-right. When you select an element, its tags will be displayed there in a table with keys on the left, and values on the right. Go into (S)elect mode and try selecting an existing way or node to see its tags. In the areas of existing map data which you downloaded, you should find a lot of tags. Some tags have quite complicated meanings, while others like the 'name' tag, are quite straightforward. Refer to Map Features for tag documentation (worth having a look at, but it's quite a big list. You're not expected to know all of these tags!)
To edit an existing tag, double-click on it, or click once to select it and then click the 'edit' button, or you can press the keyboard shortcut Alt+S.
When typing your tag into the key and value boxes, you'll find JOSM snaps to suggested values from a drop-down. This is a time-saving feature based on all keys and values appearing within the downloaded area and all common tags that JOSM knows about.
A note on selecting: When you select elements to modify the tags on, take care to select the right thing. Normally you might only select a single element (a way or a node), but in JOSM you are also able to select multiple elements and apply tags across the entire selection. One approach is to drag a selection box, but for more control over your selections, try holding down the Ctrl key while clicking.
Some nodes have tags on them, while others exist purely for connecting to form ways. Standalone nodes (almost) always have tags on them, meanwhile nodes making up a way may have tags, but often do not. JOSM displays nodes which have tags differently: It either displays an icon which describes the meaning of the tag (e.g. traffic lights) or if JOSM does not recognize the tag combination, the node is drawn as a filled, light blue square.
Adding and adjusting nodes of a way
You can move nodes and ways by selecting them and, while still in (S)elect mode, dragging them. The vast majority of the time you will be interested in moving only one node at a time. Clear your selection before dragging the node. If you accidentally move a whole way use Ctrl-Z to undo.
You can add nodes to the middle of a way by going into (A)dd mode, and clicking on a way segment at the desired place. Return to (S)elect mode afterwards, unless you are building a connecting way. To add another node along this way segment, be sure to clear your selection before adding again. Take care when adding nodes in a situation where lots of ways are nearby (criss-crossing or running in parallel) It's quite easy to accidentally add a new node which bunches together several ways. Undo! The easiest way to avoid this, is to temporarily zoom right in, so that things are spaced out nicely.
There is another trick for adding nodes to an existing way. Look out for the little crosses appearing midway along the segments. You can drag these to quickly add a node without leaving select mode.
Delete a node by selecting it, and pressing Delete key. This will delete the selected objects, but keep you in (S)elect mode.
Splitting and combining ways
You can split one long way into two smaller ways. Select the node where the split should occur, and select 'Tools' menu > 'Split Way'. You can do this several times to chop a way in several places. The resulting ways will share the same tags initially, but they are separate ways now. You can give them different tags.
Sometimes you need to do some clever selecting to indicate exactly which split JOSM should perform. If a node is connected to multiple ways, you must select the node and the way that you wish to split. To do this, select the way and then while holding shift, click on the node . Another trick is to select two nodes before clicking 'Split Way'. By doing this, a way can be split at the two different nodes at the same time. This can be a time-saver, but you are actually required to do this when splitting a way which is forming a loop.
The opposite kind of operation, is to combine two ways together to form one. You can only do this with two ways which are arranged end-to-end (they are lined up ready to be combined into one way) Select both of the ways at the same time. Again this can be achieved by holding down shift to add to your selection. Click 'Tools' menu > 'Combine Way'. At this point JOSM will create a single way. If the two ways had different tags, then the combined way will share all tags of both. You may be prompted to "solve conflicts" with tags, where the same key is set to different values on the two different ways. Remember tags always apply along the entire length of a way. If you need different tags, you need different ways.
The reverse way action will change the direction of the way. Every way has a direction which is indicated with arrows when the way is selected. As mentioned above, ways must be arranged end-to-end before they can be combined, and this includes pointing in the same direction (although it will prompt you to automatically do a reverse if they are not correctly end-to-end). Often the direction of a way doesn't matter, but it can be used to indicate directional features. Some important examples include:
You can adjust the topological layout of nodes and ways with many alternative sequences of clicks and keypresses. This can be confusing at first, but you'll master it with a bit a practice. Use Ctrl-Z to undo. Have a play!
Remember you can #Undo / Discard changes at any time, but what if you're happy with your changes? It's time to upload the changes to the OpenStreetMap server.
You will be prompted to enter a changeset comment. Write a brief description of what kind of edits you are making. This short comment is recorded along with your changes, and displayed to other people if they are looking through the history displays. It's good to explain why you are making the changes rather than just what you are changing, but don't worry about it too much. A brief comment will suffice.
The first time you upload, you will also be prompted to enter a username and password as registered on the main OpenStreetMap.org website (see Creating an Account). Save this to avoid typing it every time, unless you are sharing a computer.
When you upload to OpenStreetMap you agree to release your data contributions under the OpenStreetMap License. Please only contribute data which you are allowed to release (no copying!). Now it's in the OpenStreetMap database, it's free! Your changes will only appear on the map after a delay. It typically takes a few minutes, but can take longer (See Slippy Map#Tile rendering). While you're waiting, why not do some more editing? It's a good idea to upload your editing work quite regularly.
What about conflicts? What if somebody else has been editing in the same area since the time when you downloaded? If you think there is a chance of this (e.g. if you downloaded several hours ago, or if you're editing close to somebody else at a mapping party) then it is good idea to do another download before doing an upload. If new elements have been added, you can review these before uploading. If there is a direct clash with somebody else changing the exact same elements, then JOSM will report "conflicts" and allow you to correct them. Some more details of this are described in JOSM/Advanced editing#Resolving conflicts
Once you're familiar with basic editing you might want to learn about some more advanced editing.