I have the following 3 polygons: ABCD, DEFI, IFGH. These are arranged as shown here:

A-------------B
|             |
|   D-----E   |
|   |     |   |
|   I-----F   |
|   |     |   |
|   H-----G   |
|             |
C-------------D

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's gravatar image

BarnsleyOli
916611
accept rate: 0%


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!

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answered 13 Jul '16, 16:31

BCNorwich's gravatar image

BCNorwich
4.4k43264
accept rate: 20%

edited 13 Jul '16, 16:33

"adjust if neccessary, Job done!" is a nice solution for various kinds of problems ;)

(14 Jul '16, 08:19) scai ♦

There is an example of this in http://wiki.openstreetmap.org/wiki/Relation:multipolygon .
See 'touching inner rings' and can be a valid osm polygon.
I haven't mapped one yet, so unsure what problem needs to be overcome to be accepted by the validator. Possibly you need to complete each polygon by reusing the nodes on the shared boundary.

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answered 13 Jul '16, 14:52

nevw's gravatar image

nevw
8.9k2586160
accept rate: 10%

edited 13 Jul '16, 15:02

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

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 ?

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answered 13 Jul '16, 14:59

Hendrikklaas's gravatar image

Hendrikklaas
8.9k185217360
accept rate: 6%

-1

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

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answered 13 Jul '16, 20:19

Alecs01's gravatar image

Alecs01
1.4k1323
accept rate: 25%

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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question asked: 13 Jul '16, 14:19

question was seen: 4,073 times

last updated: 14 Jul '16, 20:39

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