Intertidal Area Import - How to tag?

[IanH]

@harahu

I don’t have to identify said line. As mentioned, the line is defined in a dataset of quite high accuracy. The plan would be to tag the area between this line and the coastline with something that makes it sufficiently clear that this is an area of water that might either be shallow or not water at all, depending on time and weather. I/we just need to make a decision about how this tagging will look like first.

That line is likely exists for those building structures on the coast. They probably have to be able determine which parts of their structure will likely be underwater. Its probably more for legal reasons like flood maps are used decied which low lying areas need insurance to build on. Anyone else will consult tide charts to determine water level.

[InsertUser]

These examples have highlighted to me why why reef feels to me like such a misleading tag for this import. For me a reef is something you might be surprised to run aground on if the map didn’t warn you whereas a coastline is something you would expect to run aground on even without a map. Most of these “attached” reefs are either around barely submerge peninsulas or where a tiny looking rock is in the middle of a much bigger hazard that could lead to a “surprise” grounding.

As an aside though way/941094297 looks to me like something I’d tag as man_made=breakwater but I haven’t seen it in person to be sure. Some of the others feel like tagging for the renderer but there are tonnes of attached reef around the British coast so individual objections are less helpful.

@harahu

Have you seen the Norwegian coastline? It’s littered in smaller and larger islands protruding from the surrounding area. Would you be ok if I tagged the areas surrounding islands as natural=reef, but avoided the mainland coastline?

I’ve seen some of it, although my visits have been brief and most of the time I’ve spent in a car near the coast was at a time of year where light’s a bit scarce. Some of these might be good candidates for reef, others less so. I don’t know how I’d distinguish “attached reef” algorithmically, it probably depends on the angle of the slope for the submerged part vs for the exposed part? Also you’d have to have a good cutoff for draft which is probably deeper than the proposed import.

Of the tags linked so far I would lean towards natural=mean_low_water_spring although it’s a bit wordy and if the data has further safety factor might be a bit misleading. Perhaps a less definitive natural=low_tide_line that could then be documented with the appropriate jurisdiction specific (but broadly similar) definitions? The specific definition used could then be subtagged so renderers can use either one specific value or any value they like if they’re trying to display an area.

I’d tend to avoid area tagging in the usual sense. I think the multipolygons would become unmanageable at a country or continental level and we can’t always rely on a convenient cliff or seawall to act as a breakpoint where high and low water become indistinguishable for our purposes.

[harahu]

@InsertUser

For me a reef is something you might be surprised to run aground on if the map didn’t warn you whereas a coastline is something you would expect to run aground on even without a map. Most of these “attached” reefs are either around barely submerge peninsulas or where a tiny looking rock is in the middle of a much bigger hazard that could lead to a “surprise” grounding.

Again, I somewhat get you, but I guess “surprise” is hardly a verifiable quantity. The rate of descent of the “slope” from the coastline varies. Also, sailing against the shore you might be surprised by grounding sooner rather than later.

@InsertUser

Perhaps a less definitive natural=low_tide_line that could then be documented with the appropriate jurisdiction specific (but broadly similar) definitions? The specific definition used could then be subtagged so renderers can use either one specific value or any value they like if they’re trying to display an area.

This, or some varaition of it, I think makes perfect sense for the line itself. I’m glad you suggest it.

@InsertUser

I’d tend to avoid area tagging in the usual sense. I think the multipolygons would become unmanageable at a country or continental level and we can’t always rely on a convenient cliff or seawall to act as a breakpoint where high and low water become indistinguishable for our purposes.

This I’m not worried about at all, really. We’ve been tagging huge rivers and forested areas as multipolygons for a long time. The trick is to just break them up at regular intervals, either at natural or artificial points. Not let them grow too big. This is manageable. The intertidal zone is by any accounts tiny compared to areas we already map in this way. The question is just what it would make sense to tag the area as.

[InsertUser]

@harahu

“surprise” is hardly a verifiable quantity

No, but I think the the idea of a reef being (mostly) surrounded by deeper water is and I don’t think you can reasonably ignore a landmass that dwarfs the reef part when evaluating “surroundedness”. I think “reef”, “shallows” and “shore” are different related concepts and this import can only really covers the shore and not reefs without a bunch of further processing and more depth contours.

It might be worth a fresh thread to solicit suggestions around the low water line tagging as the length of the back and forth here is has become quite a bit to wade through.

[IanH]

@harahu

@InsertUser

For me a reef is something you might be surprised to run aground on if the map didn’t warn you whereas a coastline is something you would expect to run aground on even without a map. Most of these “attached” reefs are either around barely submerge peninsulas or where a tiny looking rock is in the middle of a much bigger hazard that could lead to a “surprise” grounding.

Again, I somewhat get you, but I guess “surprise” is hardly a verifiable

A reef is a very specific thing. Whether you find it in the ocean or an aquarium, it is a “free standing” piece limestone that comes in a number of common shapes. Wikipedia has a great entry describing what is and how it forms.

Coral reef

If you read the article you will quickly realize the whether what what you are talking about is actually considered a reef or not.

[harahu]

Fair enough. Although I don’t really agree with your reasoning, I definitely can see where you’re coming from, and I do respect it.

Before moving on, I have another idea to air. After some discussion with the Norwegian community, and alternative proposal came up: Using tidal=yes, possibly in combination with natural=wetland, with no further specification. The latter tag makes sense, given its definition as

A natural area subject to inundation […]

See: Key:natural - OpenStreetMap Wiki

Using something like wetland=tidalflat has a too specific meaning IMO, as it assumes a lot about the surface material and inclination, but just natural=wetland, I feel, is generic enough. It also seems that this tagging pattern is used in this sense over in Canada. See: Relation: 2275793 | OpenStreetMap

[InsertUser]

@IanH

A reef is a very specific thing.

There is a broader definition of reef which has its own separate Wikipedia page (quoted in post 10). Historically it seems to have been used for any ridge of seabed you might accidentally ground a ship on while quite a distance from shore.

[InsertUser]

I think tidal=yes would apply to these areas. From what I’ve read it mostly seems to be used as a secondary tag though so might be a little odd in isolation? My other slight reservation is that it is also used to indicate that an inland patch of water varies in depth with the tide without actually becoming ‘dry’ so it might be a bit ambiguous near rivers or streams without a primary tag.

Using wetland this generically doesn’t quite feel right to me. The OSM wiki doesn’t mention them as being exclusively “soft”, but that is quite prominent on the definitions in the Cambridge and Collins dictionaries. I think it might end up being a little misleading for rocky patches.

[dieterdreist]

I think tidal=yes would apply to these areas. From what I’ve read it mostly seems to be used as a secondary tag though so might be a little odd in isolation?

tidal=yes is clearly a property and doesn’t make sense „in isolation“, one has to add a tag that defines the thing that it tidal

[harahu]

@InsertUser

Using wetland this generically doesn’t quite feel right to me. The OSM wiki doesn’t mention them as being exclusively “soft”, but that is quite prominent on the definitions in the Cambridge and Collins dictionaries. I think it might end up being a little misleading for rocky patches.

I see this point, but I also think it makes sense to describe an area that on a daily basis changes between being inundated and laid bare as “wetland”. For most practical purposes it fits. Perhaps there’s a need for a wetland=intertidal tag to clarify that this area represents inter-tidal terrain in the most general sense.

@dieterdreist

tidal=yes is clearly a property and doesn’t make sense „in isolation“, one has to add a tag that defines the thing that it tidal

In the case of freshwater the intermittent=yes tag is usually tied to the water object, e.g. natural=water + water=pond + intermittent=yes. Similarly, one could understand tidal=yes to be bound to the ocean cover in a particular area. We don’t have an object in OSM representing the world’s oceans, even though this is clearly a physical object, so in this interpretation tidal=yes would have to stand on it’s own, pointing to an implied ocean cover.

If one rather imagines that tidal=yes binds to the land that is exposed at low tide, it is also the case that OSM doesn’t have explicit tagging for “land, as opposed to ocean”, even though this is a physical object. In this interpretation tidal=yes can also stand on it’s own. When not combined with a typical land-cover tag, it would simply be pointing to the very real “land in general” that is there.

That is my interpretation at least.

[dieterdreist]

there is natural=coastline, is this not explicit enough? There once was a tag for just “land”, it was natural=land http://taghistory.raifer.tech/#***/natural/land

[InsertUser]

@dieterdreist

there is natural=coastline, is this not explicit enough?

This thread is specifically about importing the low water mark, so not really.

(Edited for clarity)

[InsertUser]

@harahu

If one rather imagines that tidal=yes binds to the land that is exposed at low tide,

It doesn’t though. It is also used for water bodies that vary in depth with tide but whose bottoms do not get exposed at low tide. A primary tag is still needed to determine if it is something that’s always wet or something that is wet part time.

@harahu

I see this point, but I also think it makes sense to describe an area that on a daily basis changes between being inundated and laid bare as “wetland”. For most practical purposes it fits. Perhaps there’s a need for a wetland=intertidal tag to clarify that this area represents inter-tidal terrain in the most general sense.

What makes sense and what words actually mean aren’t always the same thing. Having said that we have stretched definitions before if that seems to make the most sense.

Could we have competition between wetland=intertidal and other wetland tags? TBH I don’t really know how to distinguish between permanently flooded but tidal coastal wetland and intertidal wetland. I’m also not familiar enough with botany to be sure if that even happens, it seems conceivable for the lower ends of some kind of mangrove but I don’t know if they are always on something that briefly dries. It might be worth introducing intertidal=yes/no to make this crystal clear.

[SomeoneElse (Andy Townsend)]

@InsertUser

@dieterdreist

there is natural=coastline, is this not explicit enough?

This is specifically the low water mark, so not really

This might be just a typo, but https://wiki.openstreetmap.org/wiki/Coastline says (and I’ve always understood) " The natural=coastline tag is used to mark mean high water springs (also known as Mean High Water Level / MHWL)". Or does “this” in your sentence not refer to what you quoted but the rest of the “intertidal” discussion?

[InsertUser]

@SomeoneElse

Or does “this” in your sentence not refer to what you quoted but the rest of the “intertidal” discussion?

The post was meant to be interpreted in the way you have interpreted it after the “Or”. My “this” in post 32 was “this thread” and not “this tag”. I’ll edit for clarity.