Relation:multipolygon

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 Description v · d · e multipolygon The multipolygon relation is used to represent complex areas. Group: Properties - outer - inner Search Wikidata Status: de facto taginfo · GB · IE · IN overpass-turbo
Relations of type multipolygon are used to represent complex areas.

Simple areas are mapped in OSM by creating one circular way and tagging it with something that suggests an area rather than a circular way. For example, a circular way tagged `landuse=forest` will be assumed to be an area, while a circular way tagged `junction=roundabout` will not.

However, this model only works for areas the outline of which consists of one single way, and which do not have holes. Any area that is more complex than that (e.g., because its outline consists of several ways joined together, or because the area consists of multiple disjunct parts, or has holes) requires a multipolygon relation.

In short, a multipolygon relation can have any number of ways in the role outer (the outline) and any number of ways in the role inner (the holes), and these must somehow form valid rings to build a multipolygon from.

Mainly in Germany `type=multipolygon` has also been used instead of type=boundary for boundary relations. This method was not widely accepted and should be considered deprecated.

Tags

Key Value Explanation
type multipolygon Tells applications to use area-building rules on the members.
... ... Any tags describing the multipolygon (names, landuses, ...)

Members

Way or Node Role Recurrence? Explanation
outer one or more The ways making up the outer ring(s) of the area.
inner zero or more The ways making up the inner ring(s) of the area.
none Don't use, deprecated. Tools may handle this as outer.

Usage

The intended use of multipolygons is this:

• Tags describing the multipolygon (e.g., `landuse=forest`) always go on the relation. The outer way(s) must be left untagged[1], unless they describe something in their own right. For example, a forest could be delineated by four fences, in which case the four ways would be tagged with the barrier tag, but could still be used as "outer" members of the forest relation.
• If the inner way represents something in itself (e.g., a forest with a hole where the hole is a lake), then the inner way must be tagged as such.
• Otherwise the inner way(s) should be left untagged.
• The direction of the ways does not matter.
• The order of the relation members does not matter (but properly sorted member lists can help human editors to verify completeness).

Valid Multipolygon conditions

Generally, the multipolygon relation can be used to build multipolygons in compliance with the OGC Simple Feature standard (Graphical examples of OGC validity). Anything that is not a valid multipolygon according to this standard (e.g., polygons with intersecting rings) should also be considered an invalid multipolygon relation, with the notable exception of touching inner rings (see below).

We define a valid (closed) polygon as the combination of a subset of member ways which, when their endpoints are joined, form a closed polygon.

We define an unclosed way as a combination of nodes in which the first node is different than the last node.

The conditions to form a valid multipolygon relation are the following:

• Member ways of a multipolygon relation MUST form one or more closed polygon(s). When the ways belonging to the relation are combined they must form one or more closed chains. [[1]]
• Exactly two unclosed ways, and no more should share an endpoint (e.g. the most extreme nodes of a way represented by the black dot in the images).
• If an endpoint is shared by less than two unclosed ways, the polygon can't be closed and is ill formed. invalid example 1
• If an endpoint is shared by more than two unclosed ways, it's ill formed and a closed polygon can't be reconstructed unambiguously. invalid example 2 (Exception - points shared by an even number of unclosed ways might be part of touching inner rings which is ok.)
• Inner polygons must not overlap with outer polygons, and not touch on common segments (except at isolated nodes as discussed above). Overlapping can be avoided completely by reshaping.

Examples

One outer and one inner ring

 The simplest and most often used case is one outer ring and one (or more) inner rings consisting of single closed ways. ``` ``` Figure 1: One outer and one inner ring

One outer and two inner rings

 ``` ``` Figure 2: One outer and two inner rings

Multiple ways forming a ring

 The advanced multipolygon schema allows any inner or outer ring to consist of more than one way. This is useful for multipolygons encompassing very large areas, where it would be impractical to have one way run around the whole of it: ``` ``` Figure 3: Multiple ways forming a ring

Two disjunct outer rings

 Unlike simple multipolygons, the advanced multipolygon relation allows any number of outer rings and thus is a true multipolygon: ``` ``` Figure 4: Two disjunct outer rings

Two disjunct outer rings and multiple ways forming a ring

 The ability to combine a ring from individual ways is not limited to outer rings, it can also be used for inner rings: ``` ``` Figure 5: Two disjunct outer rings and multiple ways forming a ring

Complex combination of all advanced features

 This example shows a complex combination of all advanced features: three outer rings, two of which have one or more inner rings, and plenty of them consisting of more than one way. ``` ``` Figure 6: Complex combination of all advanced features

Island within a hole

 From the possibility of having multiple outer rings in one relation, it also follows that you can easily model "islands" within a hole: ``` ``` A construct like this would previously have required different multipolygon relations, one with way 1 being outer and way 2 being inner, as well as one with way 2 being outer and way 3 being inner. Such cascading is still needed when the "island" in the middle is something else than the area on the outside, but where the "island" is the same stuff it can just be made a hole in the hole. Figure 7: Island within a hole

Touching inner rings

 Some mappers use the current "multipolygon" relation for combining touching inner rings: ``` ``` An implementation of advanced multipolygons should attempt to render these as if the touching rings were indeed one ring. This is the one case where OpenStreetMap use deviates from standard OGC Simple Features. In Simple Features, touching inner rings are not supported because they are unnecessary (why make two inner rings when you could combine them into one). In OpenStreetMap, they sometimes make sense if tagged individually, for example a forest with a clearing which is half occupied by a lake and half by farmland - you would have two "holes" in the forest, one being tagged as `natural=water` and the other as `landuse=farmland`. This is a convenience shortcut; requiring of mappers to create only one hole in the forest, and then create individual polygons for lake and farmland, saving a step. Avoid building multipolygons where an inner ring touches an outer ring though. There is some discussion about this section on the talk page. Figure 8: Touching inner rings

Invalid examples

Some examples of invalid multipolygon relations to illustrate what should NOT be done

Unclosed polygons

 This is an example of an invalid multipolygon as way #2 and way #3 are not connected. Figure 9: invalid Unclosed polygon

Overlapping, unclosed member ways belonging to the same role

 This is an example of an invalid multipolygon as way #2 and way #3 endpoints share more than two ways. Figure 9: invalid ambiguous multipolygon relation

Implementation notes

• The Drawing style is always taken from the tagging of the relation itself.
• Drawing of inner areas is handled independent from multipolygon - inner ways can form closed rings with area tags or also be outer ways of multipolygons.
• Area styles containing drawing rules for their boundaries can lead to conflicts - An inner way may have a line style (e.g. fence) an own area style (e.g. water) and an multipolygon area style (e.g. forest). Each software may define drawing order different, but a general rule may be to give line style highest priority and multipolygon area style for inner ways the lowest priority.
• There is a suggested Algorithm for processing multipolygons.
• Note that before May 2017, some multipolygons also had their tags on the outer way (if there was only one such way or all ways had the same tags) while keeping the respective multipolygon-relation untagged. But when consuming older OSM data excerpts, it is necessary to handle such old style multipolygons, too.
• Implementation for compatibility - If relation is not tagged, the drawing style of outer ways is used.
• If there is more than one outer way:
The relation has tags
Use the relation tagging. Ignore anything on the ways.
The relation has no tags, but one or more constituting outer ways have an identical set of tags
Valid data, take the tags from the tagged segments and apply them to the complete outer way.
The relation has no tags, and constituting outer ways are tagged differently
This is a problem situation with undefined results.

Mapping Style, best practice

Multipolygons open up the possibility to create geometrically identical objects in different styles: as ways or as multipolygons, using closed or open ways, or with shared or unshared ways.

This raises the question of which mapping style to use. Some styles have advantages over others, and should be regarded as good practice. For others, the choice is more of a matter of preference, or whether one is an experienced or an inexperienced mapper.

Most generally when large areas share the same tag, they can be represented either by a large number of small multipolygons or closed ways, or by a smaller number of large multipolygons. For multipolygons themselves, two possible mapping styles are:

Method A
Inner and outer rings are created from closed ways whenever possible, except when these ways become very large (on the order of 2000 nodes). Ways are usually not shared by different multipolygons
Method B
Every border between two multipolygons is represented by a unique way which is then shared by the adjacent multipolygons. Consequently the multipolygon rings are often composed of several open ways

The question of best practice for multipolygons has been discussed intensively over the years, see Talk:Relation:multipolygon and the forums. A final consensus hasn't emerged yet, but the following observations apply:

• Mapping simple closed areas as multipolygons instead of ways increases the number of database objects and increases rendering time. This additional overhead of complexity should be avoided.
• Sharing way segments between multipolygons (method B) offers a representational efficiency by avoiding redundant representations of overlapping ways.
• Multipolygons consisting of non-closed ways (method B) are harder to handle by inexperienced users, and simple editors such as Potlatch 2. This has often led to the accidental destruction of such multipolygons by unsuspecting users.
• Many experienced users have expressed their discomfort with method B, especially when the multipolygons are very large.
• Huge multipolygons cause a slowdown of the rendering process.
• Editing complex geometries in JOSM is easier, faster and less error-prone with method A. This is because method B requires deletion, creation and insertion of several way segments into the correct multipolygon relations.

So far there are no official restrictions on how to use multipolygons as long as they are geometrically valid. However adopting a considerate mapping style will help to keep the database clean and keep editing easy for every user.

Potlatch 2 example

Here's a grass area within some woodland: