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Hello everyone,

In many cases, forest surfaces described by polygons of more or less complex shapes, also contain other surfaces not currently described in OSM. This may be for example a meadow in the middle of the forest which may in some cases include another surface such as a small pond.

An example showing grasslands not yet represented in OSM within the forest, is given below.

https://www.openstreetmap.org/edit#map=17/48.74253/2.07390

I wonder, however, about the best procedure for mapping these areas: I noticed that, in some cases, grasslands, or even water surfaces in these grasslands, were simply added to the surface of the forest (Link to case 1 below). In fact the surfaces are added to each other.

https://www.openstreetmap.org/#map=18/48.72057/2.02757

Is this procedure acceptable? if this procedure is not correct, what is the best way to correct the case 1. Should not we rather use Multipolygon especially when these small areas are numerous? Or use separate small individual polygons to describe each surface?

Regards,
Gerard C.
FRANCE

asked 08 Sep, 10:37

Woinic's gravatar image

Woinic
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accept rate: 0%


Good question, but not easily unanswerable, at least not without debate. If you search the mailing lists and forums you will find endless debates around it. Maybe a few facts:

Actually, there are (at least) three different tags to describe landscapes: landuse=*, landcover=* and natural=*. In their strict meaning the former two are orthogonal and could be both used on the same object. In tagging practice landuse often also carries a landcover meaning. On top landcover is ignored by most renderers today and hence not very popular among mappers.

Now, should a pond as an example be part of a forest? Some argue it is part of the forest and hence we just draw it and a forest around it. Some say, where the pond is forest must end and create a multipolygon for the forest with the pond as an inner. You could also have a large forest (in the meaning: This is an area where mainly trees grow that are used for timber) and somewhere in there there is a patch with landcover=grass (where the game comes out to feed and keeps new trees from growing).

I'd say both practices were quite common until a few years ago when the standard map on openstreetmap.org changed the rendering. At that time they started drawing little trees on top of the water if the pond was not cut out of the forest. Since then mappers are more favoring the multipolygon approach.

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answered 09 Sep, 08:06

TZorn's gravatar image

TZorn
2.9k21659
accept rate: 12%

edited 12 Sep, 10:22

Hi woinic, Did you read these lines https://wiki.openstreetmap.org/wiki/Relation:multipolygon ? Its is getting complicated when the wet area has no border to the surroundings and is embedded. Just start a new Multi. And dont choose a way just to get it visual, as far as I can remember "dont map for the renderer" !

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answered 09 Sep, 09:38

Hendrikklaas's gravatar image

Hendrikklaas
8.5k177210350
accept rate: 6%

Hi,
I have so far only consulted the information given below.
https://wiki.openstreetmap.org/wiki/Multipolygon_Examples

I will read this other information.

Regards,
Gerard C.

(09 Sep, 10:22) Woinic
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question asked: 08 Sep, 10:37

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last updated: 12 Sep, 10:22

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