I have the following 3 polygons: ABCD, DEFI, IFGH. These are arranged as shown here:
I want to create a multi polygon with ABCD as the outer and DEFGHI as the inner. If I select all three polygons and use the "Create Multipolygon" tool (Ctrl+B) I then get a validation warning saying "Intersection between multipolygon ways". I understand why this occurs, the two inner polygons share the edge FI. How do I correctly create the desire multipolygon? asked 13 Jul '16, 14:19 BarnsleyOli |
There is an example of this in http://wiki.openstreetmap.org/wiki/Relation:multipolygon . answered 13 Jul '16, 14:52 nevw 1
@nevw Thanks, I had already read this and the talk page (which gives the example of a lake with a beach both inside a wood). However I couldn't find any advice on what to do where the phrase "The boundary between farmland and lake does not belong to the forest" applies.
(13 Jul '16, 15:20)
BarnsleyOli
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Hi BarnsleyOli, try to make 2 complete inners DEFI & FGHI or try the trick and draw them at a large scale or zoom in and align FI and IF next to each other closely. I presume that you read the Multi pages instructions ? answered 13 Jul '16, 14:59 Hendrikklaas |
It works perfectly in JOSM. Draw a polygon ABDCA tag it as natural=wood. Inside that polygon draw polygon DEFID tag it as natural=beach, also draw polygon IFGHI tag it as natural=water, water=lake. Select all three polygons click 'create multipolygon. Check the relation editor to make sure the inners and outer are correct (adjust if neccessary). Job done! answered 13 Jul '16, 16:31 BCNorwich "adjust if neccessary, Job done!" is a nice solution for various kinds of problems ;)
(14 Jul '16, 08:19)
scai ♦
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A possible solution to avoid touching inner ways is to split inner polygons into three segments, creating three multipolygons as follows: 1) ABCDA - outer IDEF - inner FGHI - inner 2) IDEF - outer FI - outer 3) FGHI - outer FI - outer answered 13 Jul '16, 20:19 Alecs01 While this uses 3 relations, it would work fine, so I don't understand the down-vote.
(14 Jul '16, 18:29)
alester
I didn't vote it down, but I have found that relations mapped that way are pretty unmanageable. The problem is the abstraction - "IDEF" and "IF" are just line segments, and don't actually match anything in the real world, whereas DEFID, FGHIF and even DEFGHID all do represent physical features.
(14 Jul '16, 19:51)
SomeoneElse ♦
@SomeoneElse it adds complexity for sure, it's an edge case to me, slightly better than two touching inner rings. I wasn't aware before there was some discussion on the topic here, and not much consensus: http://wiki.openstreetmap.org/wiki/Talk:Relation:multipolygon#Touching_inner_rings
(14 Jul '16, 20:39)
Alecs01
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