In JOSM, how do you draw an polygon that is adjacent to another polygon without having to retrace the shared boundary? I'm looking for the same functionality that is found in Auto-Complete Polygon tool found in Esri software: http://desktop.arcgis.com/en/arcmap/10.3/manage-data/creating-new-features/creating-adjoining-polygons.htm

What I've tried to do is draw the new polygon boundary (that doesn't have a shared boundary). Then use the Split Adjacent Ways tool. If I select the new boundary, the split boundary, and then combine them, it works but I break the other polygon. I also tried to Copy/Paste or Duplicating the split boundary, so there are two ways. I then would select the new Way to combine. But that doesn't work because the copy/paste/duplicate doesn't put the geometry in the same place.


A polygon with 2 or more ways needs to be a multipolygon. So you will have 2 multipolygons and the shared way becomes part of each of the multipolgons members. There is a wiki explaining openstreetmap multipolygons. http://wiki.openstreetmap.org/wiki/Relation:multipolygon

Select each of the ways that make up one of the polygons, then Tools>Create Multipolygon. You should then see it highlighted in the Tags/Memberships window at the right side, select the edit button on that window and you should be able to add tags to the new multipolygon relation and also put the correct role for each segment. In your case the roles are probably 'outer'. Then do the same method for the other multipolygon and this time you will select the shared way again and that will be common to both multipolygon relations.

To split a polygon you would normally select the polygon plus select one or more of the nodes where you want the split to occur then select Tools>Split Way

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  • In my case, the other polygon is already a multipolygon with multiple outer ways (and many inner ways forming holes). Wouldn't doing it this way create this invalid relationship? wiki.openstreetmap.org/wiki/… – dvdhns Nov 28 '16 at 3:15

If using Josm to split the outlines of the multi-polygon, Josm will preserve the MP and just add more member lines to the outer polygon. You do not have to re-create the MP. Then you create your new adjacent MP re-using some of those same lines as necessary. I would still open the edit window of each MP on completion to check that all seems ok.

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Here's the steps that did what I wanted to do.

  1. Draw your new line that touches your adjacent polygon
  2. More Tools > Split Adjacent Ways
  3. Select the line you just drew and the newly split line from the adjacent polygon
  4. Tools > Create multipolygon
  5. In the Tags / Membership Window Right-Click on the NEW multipolygon (while likely just have two members)
  6. Click on "Select Relation". The new polygon you want will turn purple.
  7. Use your presets to choose the new key:values you want (or add them manually)
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