[talk-au] Gordon Lake - Multipolygon relations - I just do not understand

Stephen Backway stev391 at email.com
Fri Aug 2 02:45:05 UTC 2013


Don't overthink the multipolygon.

It is just a collection of nodes and ways, which is then (almost) tagged as if it is a way.

Lake Gordon looks ok at the moment, so you might have worked it out...

For lakes in JOSM,
1) Draw the outer edge of the lake as one way or a series of connnected ways.
2) Draw the edge of the islands as one way per island or a series of connected ways per island.
3) Select all the ways you have drawn for the lake and go to the 'Tools' menu and select 'Create Multi-polygon CTRL+ALT+A'. This will get JOSM to create a multipolygon relation, this is where you have all the lake related tags, not tags specific to the islands, the natural=island goes directly on the island way.
4) To add tags to the multipolygon, select one of the ways then under the normal properties side bar you should see a 'Member Of' area, double click on the multipolygon shown.
5) The tags for a multipolygon relation (lake) are:
 - type=multipolygon
 - natural=water
 - name=Lake Gordon
 - source=...
6) For each way/node in a multipolygon you need to define its role. JOSM will attempt to do this automatically for you if you had all the ways selected when creating the multipolygon. If not:
 - All ways that form the outer edge of the lake have the role of 'outer'
 - All ways that form the islands in the lake have the role of 'inner'

Note that when you are in the relation editor window, do not add or delete nodes in the map window behind, as it sometimes confuses it (will it sometimes asks you some questions which confuses me...). So always click OK (or cancel) on that window prior to changing nodes in the main window.

Hope that helps, let me know if you need more details. I suggest you read the multipolygon and relation wiki pages.

----- Original Message -----
From: Brett Russell
Sent: 07/30/13 10:45 PM
To: OSM Australia mailing list
Subject: [talk-au] Gordon Lake - Multipolygon relations - I just do not understand


Must admit that multipolygon relationships have me confused. Lake Gordon is a good example. Thought, great I will refine the shoreline by selecting two points and breaking them and then deleting the section and rebuilding it with both ends joined. But I notice weird lines across the lake and incomplete inner and out mentioned in the relationship. Tried deleting them incomplete ones but all I get is confused. 

Rather than just fix my stuff up can some one walk me through fixing it as I loathed to touch them. Generally it is only the 2000 point limit and islands that forces me to use them. I am using JOSM.

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