|Specification of a sinkhole type|
|Used on these elements|
|Tools for this tag|
This tag is used to specify what type of sinkhole is tagged with natural=sinkhole, and must be used only in conjunction with it, apart from exceptions specified below. When the latter tag is omitted and the tagging matches exceptions below, the renderer should use the rendering of the main tag.
|sinkhole=estavelle||A sinkhole connected to a water stream or a water body, either intermittent or permanent, which drains the watercourse or body, or feeds it, according to the relative levels of the water stream or body and the aquifer. This is one of the sinkholes a polje can contain.||When an estavelle is not in the bed of a stream but connected to it with a side stream, this side stream, as it can flow both ways according to the current levels of the aquifer and the stream, should be tagged with flow_direction=both, to highlight the unusual flow variations. As the stream connected to an estavelle is often intermittent, this stream should be tagged intermittent=yes accordingly.|
|sinkhole=bluehole||A vertical, underwater sinkhole, often used for scuba diving, named after the deep blue color they mainly show.||This tag, when used on an area, does not need to be an inner polygon in a multipolygon relation, or otherwise disconnected or excluded of the surrounding waters, as it is included in them, owing to its nature.|
|sinkhole=doline||A sinkhole clogged up, sometimes only partly, with earth or debris, and which, when filled, generally looks like an old crater with flat, fertile ground; when fully clogged up, it often contains a pond from trapped rainwater, no longer able to escape by the clogged bottom; when only partly clogged up, the pit still reaches the surface; a doline can form around an estavelle, a ponor, a karst spring or a blue hole. This is one of the sinkholes a polje can contain.||
|sinkhole=ponor||A sinkhole draining water, either temporarily or permanently, from a water body or a water stream; can be associated with polje as the draining orifice, and with rivers and streams, leading to their complete, permanent, or partial, temporary disappearance. This is one of the sinkholes a polje can contain.||Ponors does not always have a well-defined orifice, and often are few square meters areas filled with permeable alluviums or simply punctured by dozens of holes, whose location will only be clear by the sucking noises they make while water drains through them, or by the orientation of aquatic plants, curved in their direction by the water flow. Such area being also a ponor, it should me mapped as one unoccupied orifice.|
|sinkhole=pit||A sinkhole without any of these characteristics; a simple hole, opened in the ground, which may present a waterbody at any depth; that includes cenotes, whose waterbody is often larger than the orifice in the ground.||When the downward orifice is the only feature, i.e. there is no water flow and it is not filled with debris, even partly; if the sinkhole is known to be a cave entrance, map it as natural=cave_entrance in lieu of natural=sinkhole in order to reflect that the cave entrance is a pit, but keep it also tagged sinkhole=pit to reflect its dual nature. If the sinkhole is a cenote, map the underground waterbody as it is, with natural=water+layer=-1+whatever.|
For Dragon Hole area tags would be as follows:
When several sinkholes are known to be linked, i.e. they belong to the same named ensemble, or mining sinkholes are caused by the same network of cavities, these elements should be grouped with relation:site. For instance, ponors and karst springs are known to form karst fensters, i.e. a karst spring whose emissary is quickly, often after a few meters, swallowed by a ponor. Such features, as the Trampot karst fensters (fr_FR), are known sites, which justifies using a relation:site.
This is also to be used on separate features belonging to the same karst system: even when spread over kilometers, such features still belong to the same system, which is what a relation:site is made for; in this case, use name=* on the site relation with the name of the karst system, if named. Once modelled, these relations between karst features can be used to explain, for instance, why a flood in a losing stream predates the activation of an intermittent karstic spring.
There can exist, along the "underground way of a river" (this kind of designation being merely a convenient, simplistic way of representing a karst system for mere mortals), shafts allowing one to observe the karstic system, either for monitoring water levels inside or to allow speleologists to visit caves belonging to the karst system; such shafts should be tagged according to their nature, and be included in the relation:site of the karst system to reflect their mutual connection.
Note that, as the effect of the members of the relation:site modelling the karst system directly stem from their nature, there is no need of using the relation roles; for instance, a natural=sinkhole+sinkhole=estavelle will inherently be both an inlet and an outlet of the karstic system described by the relation:site it belongs to, so giving it a role in this relation would be pointless.
When the stream connected to an estavelle is merely a connection of the estavelle with a neighbor waterway, this side stream should be included in the linked stream relation:waterway with the side_stream role; if applicable, the estavelle itself is to be included to a relation:site describing the linked karst system.
Note that you should not try to map the underground streams between ponors and resurgences, as
- if the goal is to describe the connections between them, this aspect is to be taken into account by relation:site, so the modelling of these streams would merely be a duplicate;
- they practically can't be accurately mapped, being underground, without physical access for speleologists on many kilometers, and often flowing through a permeable stratum, which doesn't make it a single channel flow to be mapped;
- their paths, belonging to a karst system, are complex, ever changing, and probably not permanent enough to justify an inclusion in OSM.
Notes on tagging
Distinguish cause and consequences
A doline can contain any of the other features, like a pit or a ponor, as it frequently forms around them; if the doline is large enough to be considered a separate feature, map the other feature inside as the sinkhole and as the last or first node of the linked waterstream, in order to link it with the sinkhole in the OSM database. Then, map the surrounding of this sinkhole as it is, for instance using natural=cliff.
When a water-related sinkhole feature is inside a water body, map it at its precise location if possible with natural=sinkhole+sinkhole=…; if the water body is itself the sinkhole, then tag it also with sinkhole=… to highlight the fact that it is in the first place a water body, the sinkhole being a secondary characteristic; anyway, the sinkhole feature, either itself or the body containing it, should be properly linked with the related waterways.
Layout altered by human hand
When the sinkhole has been refitted, for example with a grid preventing unauthorized access, or a culvert to prevent its clogging with debris, it should be also tagged with refitted=yes to reflect these man-made changes.
Anthropogenic effects on the environment include sinkhole formations, for instance pinges which are caused by collapsing abandoned underground mines, or ruptured sewer pipes washing away soil until a sinkhole forms. These can be pretty spectacular, such as the Lake Peigneur or the 2007 and 2010 Guatemala City sinkholes, these two last being examples of piping pseudokarst. For these sinkholes, use anthropogenic=yes; if their source is known and modelled in OSM, link the sinkhole with its cause using relation:site.
Relation with landuses
Typically, as a sinkhole is neither a landuse=* nor a natural=* feature as natural=wood or natural=scree, it should be an inner member of the relation:multipolygon describing the surrounding area, which would also ease its rendering.