User:B1tw153/Atoll
This page is a work in progress.
Introduction
An atoll is a ring-shaped coral reef enclosing a lagoon. Some or all of the reef may be exposed above the water as individual islands or a single ring-shaped island.
This page describes how the natural and civic elements of atolls are mapped in OpenStreetMap.
Open and Closed Atolls
Many atolls have broad and deep channels through the reef connecting the ocean with the lagoon. These atolls are "open." E.g., Haraiki.
Some atolls have a single ring-shaped island that fully encloses the central lagoon. These atolls are "closed." E.g., Niau.
The techniques for mapping open and closed atolls differ as described below.
Natural Elements
The geometry of atolls is complex and is best represented by several elements within OSM.
Reef Boundary
This way outlines the outer boundary of the reef that forms the atoll. This way should enclose the entire atoll and not cross through any of the channels into the inner lagoon.
The reef boundary should be tagged with natural=atoll + name=* + wikidata=* + wikipedia=* with the name of the entire atoll and references to the Wikidata and Wikipedia entries for the atoll.
This way should be the outer member of the multipolygon relation for the reef that forms the atoll.
In some cases, the reef boundary may also be an member of an archipelago relation to include the entire atoll in the archipelago (see below).
Lagoon
This way outlines the lagoon within the atoll and is the inner boundary of the reef if the reef is present in the lagoon. This way should enclose the entire lagoon and not cross through any of the channels into the ocean.
The lagoon should be tagged with natural=water + water=lagoon + salt=yes (except in rare cases where the water is fresh). Fully-enclosed lagoons rarely dry up, but if a lagoon does dry up intermittent=yes may be appropriate.
If the atoll is open, this way should be an inner member of the multipolygon relation for the reef that forms the atoll.
If the atoll is closed, there may be no distinct reef within the lagoon. In this case, the lagoon is not a member of the multipolygon relation for the reef.
Coastline
The single ring-shaped island or multiple islands and islets should be mapped as individual ways tagged with natural=coastline + place=island / place=islet. Many small islets are unnamed, but named islands and islets should be tagged with name=*.
Coastline ways around islands and islets must be drawn counterclockwise.
All of the coastline ways should be inner members of the multipolygon relation for the reef that forms the atoll.
Channels
Atolls often have small, shallow channels (or hoa) that connect the ocean to the lagoon. If these channels are flooded continuously they separate the individual islands and islets in the atoll. Look for the boundaries of wet and dry sand to identify where a hoa is flooded.
Often, small islets of sand or scrub (called motu) are present between hoa. These islets may be mapped as separate coastline ways and if so, should be inner members of the reef relation. Since natural=coastline cannot be combined with other natural=* tags, areas of sand or scrub on a motu must be mapped as separate ways.
Atolls may have many small hoa and motus. Rather than mapping these areas as individual islets, it may be better to consolidate them with a larger island and map the hoa with natural=water + water=pond + tidal=yes + salt=yes. With this approach, if a hoa remains flooded at low tide, it should be mapped with natural=reef, and if it is dry at low tide, with intermittent=yes. This method simplifies the coastline ways that need to be included in the relations associated with the atoll. It also better represents hoa that are not continuously flooded.
A partially formed or partially blocked hoa may not fully connect between the ocean and lagoon. A blocked hoa may be mapped either as an indentation in the coastline, or as a tidal pond with natural=water + water=pond + tidal=yes + salt=yes + intermittent=yes.
Some hoa are dry even at high tide. These dry hoa are part of the island and should not be mapped as coastline. Often, they can be mapped as areas of natural=sand.
Atolls often also have large, deep channels (or ava) that connect the ocean to the lagoon. These channels also separate the islands and islets of the atoll. An ava can be mapped using natural=strait, with name=* if the ava is named. At an ava the outer boundary of the reef and the inner boundary of the lagoon may "touch" without crossing, sharing two nodes and one line segment.
Reef
For a closed atoll, the reef should be mapped as a multipolygon relation with the reef boundary as the outer member and the coastline of the (single) island as the inner member. This creates an area of reef that surrounds the island but does not extend over the island or into the lagoon.
For an open atoll, the reef should be mapped as a multipolygon relation with the reef boundary as the outer member and all of the islands, islets, and the lagoon as inner members. This creates an area of reef that surrounds and separates all the islands, but does not extend into the lagoon.
Civic Elements
Atolls are typically members of larger administrative regions and countries. As islands, they establish portions of the maritime boundaries for these regions and countries.
Archipelago
Although atolls are often located in arcs of volcanic islands, the clustering and naming of an archipelago is a civil determination.
An archipelago should be mapped as a relation with type=multipolygon + place=archipelago + name=* + wikidata=* + wikipedia=*.
Ideally, all the component natural=coastline ways of the islands in the archipelago should be outer members of the relation. However, as atolls may have many small islands and tiny islets, it may be impractical to map the archipelago relation using the natural=coastline ways. As an alternative, consider using the outer boundary of the atoll's reef or the baseline way (below) to capture the entire area of the atoll with a single member of the archipelago relation.
An archipelago may also be an administrative boundary, in which case the extent of the relation and choice of member ways in the relation should be based on information from local government sources.
Baseline
The maritime 
Baseline is the line (or curve) along the coast from which the seaward limits of a state's territorial sea and certain other maritime zones of jurisdiction are measured. Ideally, the baseline is the low-water line along the coast or reef. This differs from the definition of Coastline in OSM which is placed at the high-water line. However, the determination of the baseline location is an administrative and political process, so this element in OSM should be based on official sources and not necessarily on physical features "on the ground."
The baseline may be mapped as a way enclosing the atoll. Depending on the official definition, it may or may not be the same as the Coastline or the outer boundary of the reef.
The baseline way should be tagged with boundary=maritime + border_type=baseline.
The baseline way is commonly tagged with place=atoll, the name of the atoll in name=* and variant name tags, and with wikidata=* and wikipedia=* tags that refer to the atoll.
Note: Do the name, Wikipedia, and Wikidata tags belong on the baseline element or do they belong elsewhere?
The baseline way should not be tagged with natural=atoll since it is not a natural feature (except if the baseline way corresponds to the outer boundary of the reef).
The baseline way should likely be a member of one or more administrative relations.
Administrative Areas
Atolls will be members of various administrative boundaries assigned by the state. Refer to authoritative local government sources for the location of these boundaries. In some cases, the baseline boundary or natural elements such as the coastline or outer boundary of the reef may be members of administrative relations.
Land Area
Land areas are administrative boundaries that group together the islands in a civil area. Within a particular civil area, all the natural=coastline ways should be outer members of the land area relation.
Land areas should be tagged as type=boundary + boundary=administrative + land_area=administrative.
Alternate tagging schemes such as type=land_area + land_area=administrative or type=boundary + boundary=land_area + land_area=administrative are not widely used. The type=land_area tag for relations is rare and is unlikely to be supported by data consumers. Even if boundary=land_area is used, the relation should be tagged with type=boundary.
As with other administrative areas, land areas should rely on authoritative local government sources for the extent of the boundaries.