I though I might as well throw out the definition of the intertidal area according to the Norwegian Mapping Authority and describe how it might differ from what is currently used in OSM, just to give some context to all of this, and give you all a better feel for what this might mean in practice.
According to the the NMA, the zero meter contour (the intertidal boundary) is the:
boundary for intertidal area, curve with depth value of 0.5 m below K0, the zero level of the nautical chart Note: Corresponds to DEPCNT in S-57 -57 with standard depth value of 0.5 (m)
So, in summary, this line is intended to visualise the zero level of the nautical chart, and is defined 0.5 meters lower than that, presumably to have some margin of error. This margin of error is important because one wants to be certain that one doesn’t promote areas above the K0 level as below it by accident. I’ll come back to why this is important.
Then, in order to understand what this line represents, one needs to first understand how K0, the zero level of the nautical chart is defined. The NMA doesn’t have an english-language source on this that I could find, so I’m translating from The Norwegian Maritime Pilot, p. 15:
Reference level for depths, Sea chart zero
The zero level in sea charts is, for safety reasons, set so low that the water level rarely falls below this level. On 1 January 2000, most of the North Sea countries introduced the Lowest Astronomical Tide (LAT) as reference level for depths in the charts. In areas where tidal variations are small in relation to the meteorological contribution to the water level, the new chart zero can be set lower than LAT. In Norway, this particularly applies to the southern coast and the Oslofjord, where the water level for long periods (at times 1–2 weeks) can lie lower than LAT. For this reason, chart zero is placed 30 cm lower than LAT in the inner Oslofjord (within Drøbaksundet) and 20 cm lower than LAT along the coast from the Swedish border to Utsira. In the rest of the country, Svalbard included, chart zero coincides with LAT.
To summarise, K0 is basically LAT, with an additional safety margin in areas where needed, such that the practical meaning of K0 becomes “the tide will rarely fall below this level”. The zero contour slaps another 0.5 meters of safety margin on top of this, so that one avoids displaying false negatives on a map, and presumably because only ACVs has zero draft.
The significance of this definition, including the added safety margins, is this: If you’re traveling in a low-draft vessel, and you stay out of the area outlined, you should be confident that the probability of grounding is acceptably low, even at low tide. Sailing within this area is still possible, but you risk hitting bottom, even in a low-draft vessel.
It is precisely because of the practicality of this definition with regards to navigational purposes that I feel it is deserving of inclusion in OSM somehow.
EDIT: I forgot to describe how this differs from OSM. Here it comes.
The wiki article on the tidal key claims:
Areas in the tidal range between
mean high water springs (natural=coastline) and
mean low water springs can be tagged with tidal=yes
to explicitly indicate this.
When comparing the zero contour line (including the one or two safety margins, depending on area), to the MLWS level, one discovers that it is about 75 - 120 cm deeper, varying along the coast.
An open question is how set in stone this definition used in OSM is, and how it is motivated. Defining the lower bound of the tidal zone isn’t straight forward, and as the reasoning above shows, it might be set differently depending on purpose. I guess what I want to import can be thought of as “the intertidal zone for navigational purposes”, whereas, from my understanding, MLWS could be more important from the perspective of land ownership.