Intertidal Area Import - How to tag?

[harahu]

@InsertUser

I believe the term generally used in OSM for this sort of thing is troll tagging.

After having read the article you refer to, and having read more about reefs, I can’t say my impression is that it applies in this case. It is stated that:

In general, any tag that must be processed to avoid serious misinterpretation of the feature is a trolltag.

So, the question is; would what I propose lead to any serious misinterpretation? After giving it some honest consideration, I’m leaning towards a no.

Here’s my take:

1. The possible extent and consequences of misinterpretation are very small. If you, as a data consumer, adhere to the strictest possible understanding of the natural=reef tag, that is, understanding the tag as “always fairly solid, always below MLWS”, you will be mislead in the sense that you might observe exposed land in an area where you did not expect to see exposed land. Does this matter in any practical sense? I’d argue that it doesn’t. A reef is still a danger area, even if submerged, so you shouldn’t be more likely to navigate into the area just because the dangers stay submerged. In general, I struggle to find any practical use case where mistaking an intertidal area for a barely submerged area would matter much.
2. Also, since the definition, as it stands on the wiki, is already impure, and allows for intertidal sections, you already can’t use these areas for any purpose where this distinction matters.
3. To me, it hardly seems like there is consensus that natural=reef should not contain intertidal areas.

   • Although the wiki tries to state that this is how it is, there is not given any reasoning for it, beyond the fact that one could use natural=shoal or similar where one knows an area to be above MLWS.

   • One of the prime examples in the wiki article, Bikini Atoll, is used exactly as I propose, with the reef going all the way up to the coastline.

   • There is an inconclusive discussion on the matter on the article talk page, where also no good arguments are put forward for this limitation.

   • When the feature was proposed, it seems to have been defined as

     A reef is an area of sea bed which can be made from rock or possibly a sandbar. It is usually just beneath the surface and may be exposed at low tide levels.

     This is perfectly in line with what I intend to use it for, and nowhere can I find a discussion that counters this definition. This leads me to conclude that the reason the wiki states otherwise at the moment is simply because someone at some point thought it would be a good idea to unilaterally move away from this definition.

@InsertUser

and using it to tag just the bits of something that do get exposed

This is stretching it a bit, I feel. Note that the area I’m proposing to import goes down to around 1 meter below MLWS. In terms of area this can be pretty significant. This additional area, beyond the MLWS-line, is truly natural=reef, no matter how strictly you decide to define it.

@InsertUser

This varies based on region. Where I grew up there is often a weed line that give a fairly clear indication of how far up the beach the water reached and if the imagery providers are all using different snapshots you can often get a feel for where the top is. Some of us also like to walk the tidal bits and have gps traces. In any case inadvertent inaccuracy isn’t really an excuse for a deliberate mistagging.

Not sure I understand what you’re saying with this. Here it seem like you’re talking about the ease of identifying the high tide line, which indeed can be fairly simple. I’m talking about identifying the low tide line, fore some specific definition of it. This is a lot harder to do from aerial imagery, if possible at all.

@InsertUser

Coral reefs confuse the issue as it causes some to call all places with the presence of coral “reefs” even when they are not hazardous and so not really reefs in the boating sense.

Here I absolutely agree with you. What I’m arguing is that it might be perfectly fine to consider the intertidal area as a reef in the boating sense, where no further detail about it is accessible. It is below the water line (at high tide), and it is so by a small margin, and thus serves as a hazard to boats.

Here, I’m not arguing that you should tag any part of the intertidal zone as a reef if you a) know that a particular part is above MLWS, b) know the surface of it. I’m arguing that you’re not doing something wrong by using natural=reef if you don’t know a) or b).

@InsertUser

This is why I think we need a new tag for the low water mark if no rare “in use” ones come out of the woodwork

I don’t mind this at all. Some have already been proposed. See:

• Proposal:Tide lines - OpenStreetMap Wiki
• Proposal:Tag:natural=mean low water springs - OpenStreetMap Wiki

These aren’t a perfect fit in this case, because what I’m proposing to import is techically slilghtly below low tide, but I would presumably tag the lower bound of the area considered for import with some info about what if represents. Perhaps depth:lat=0.5, following the pattern of specifying datum-specific elevations in a similar way.

@InsertUser

The big caveat though is that OSM generally discourages people from using OSM for applications where a bad map could cause someone to get hurt so if “navigational safety” were someone’s concern a more authoritative source should probably be used.

The fact that it shouldn’t be used for “navigational safety” doesn’t stop us from mapping with this in mind. On land we map natural=cliff, we add sac_scale=* to paths and so on. I don’t think this needs to be different in the marine environment.