Tagging for subsea banks

I’d like to get some sort of tag documented for subsea/ocean banks. These are areas of shallower ocean that don’t necessarily get close enough to the surface to count as reef or shoal in their own right. They often aren’t shallow enough to count as hazards to navigation themselves, but may be frequently avoided anyway if they are prone to shifting shoals or are home to multiple actual reefs.

Name Mapped as
1. Grand Banks of Newfoundland (280,000 km2) Missing
2. Agulhas Bank (116,000 km2) Exists just as a protected area and a restricted zone
3. Great Bahama Bank (95,798.12 km2, Inc. islands) See note below.
4. Saya de Malha (35,000 km2, excluding the separate North bank, least depth 7 m) Missing
5. Seychelles Bank (31,000 km2, including islands of 266 km2) Missing
6. Georges Bank (28,800 km2) mapped as a reef
7. Lansdowne Bank (4,300 km2, least depth 3.7 m) Missing
8. Dogger Bank (17,600 km2, least depth 13 m) See note below.
9. Little Bahama Bank (14,260.64 km2, inc. islands) See note below.
10. Great Chagos Bank (12,642 km2, inc. islands) Missing
11. Reed Bank, Spratly Islands (8,866 km2, least depth 9 m) mapped as reef
12. Caicos Bank, Caicos Islands (7,680 km2, inc. islands) mapped as reef
13. Macclesfield Bank (6,448 km2, least depth 9.2 m) mapped as reef/atol
14. North Bank or Ritchie Bank (5,800 km2, least depth <10 m) Missing
15. Cay Sal Bank (5,226.73 km2, inc. islands) Mapped as reef
16. Rosalind Bank (4,500 km2, least depth 7.3 m) Missing
17. Bassas de Pedro (2,474.33 km2, least depth 16.4 m) Missing

Table notes:

  • Dogger bank (of shipping forecast fame), which currently exists in OpenStreetMap twice, once with the undocumented natural=fishing_bank and once with natural=reef (which seems like a bit of a stretch to get it rendered).
  • The Bahama Banks, which show up very clearly on satellite imagery due to water clarity and white sandy bottom, but are in parts deep enough for deep water harbours and cruise terminals. They contain lots of reef and shoal, but aren’t really one big reef no matter how many armchair mappers decide it would be fun to trace random chunks of them. (I’d quite like to get these mapped with something more sensible.)

In use we have:

  • the previously mentioned natural=fishing_bank in use 5 times and a bit too focused on a single use to me.
  • subsea=bank in use 8 times, usually because of an import, probably the most straightforward tag.

I’m leaning towards just documenting subsea=bank in the wiki as I don’t think there will be enough interest to go through the whole proposal process, but I wanted to see if there’s any big issues with that that I haven’t spotted.

Edit: somehow I forgot mention that the list in the table is from the linked Wikipedia article. I was hoping that one of the mapped ones would reveal a tag that I’d missed and found the results interesting enough to include.


Makes sense, and I’d be happy to support the rendering of Dogger Bank as something sensible once it is changed from being a reef, which it surely is not.

subsea seems like a good home for banks, and is due for some documentation. Looking at the common values, subsea=channel is a tag I have been longing for, with the meaning “a linear way following the deepest part of a strait”. Also used as a straight duplicate of strait it seems.

Bank-like features inherited theough the seamark schema:

Key:subsea - OpenStreetMap Wiki is mostly redundant with natural=reef now, and it was unstructured by using different =*_reef . While it can be about ideologies and ideals, there’s no practical reason not to use natural= unless you have something as extensive as geological= etc.

I did think there would be a relevant seamark category in there somewhere, but didn’t manage to find it.

This tag seems to be used more often, but it 114/118 uses seem to be by “:heart_on_fire:_import” as a way of importing LINZ data so I’m not sure it’s any more popular in practical terms.

The origin of most of the subsea=bank ones seem to be “GNS” too so there isn’t much in it in user count, but I’m still leaning towards the more straightforward tag.

The most practical reason I can see for using subsea= rather than natural=subsea_bank or natural=ocean_bank here is that if there is a bank that is particularly well known for being a good place to fish you can still use the existing natural=fishing_bank in combination with it.

It’ll also be less typing as I don’t think it will be a huge priority for preset makers.

The current wiki definition for natural=reef appears very broad (“rock, sandbar or other feature”). Before introducing other tags for “non-reefs”, the definition of a reef needs to be more specific.

The way I have generally understood it reefs are traditionally considered hard underwater features that are shallow enough to be a hazard for navigation and banks are generally notably shallower than their surrounding but not shallow enough that they are a hazard. The use of coral reef for deeper accumulations of coral has blurred this distinction a little, but I think it generally holds.

The navigation distinction is present in the documentation for the seamark:sea_area:category which appears to be lifted from modern charting practice (not that anyone should be using OSM for the kind of navigation where getting shipwrecked is a possibility). For convenience:

Category seamark:sea_area:category Definition
Bank bank An elevation over which the depth of water is relatively shallow, but normally sufficient for safe surface navigation.
Reef reef Rock lying at or near the sea surface that may constitute a hazard to surface navigation. See also natural=reef
Shoal shoal An offshore hazard to surface navigation that is composed of unconsolidated material.

I would be happy to add a note about this to the wiki page for reef as well as a bullet point under mapping caveats saying that deeper objects might be better as considered subsea=bank when I document the usage of that tag.


OK, I have now:

  • added a page to the wiki here (with a link back her on the talk page)
  • added a mapping caveat to the reef page to mention that this might be a more appropriate tag in some cases.
  • added a link to it from the general subsea page
  • added a link to it from the seamark:sea_area:category page.