Howdy.
I’ve been mapping a lot of tidal areas over the past year. Mainly estuaries with large tidalflats and saltmarshes.
One of the things I have noticed while doing so is that the tagging schemes for the intertidal areas are a bit of a mess.
There are 3 main ways intertidal areas are being tagged currently natural=beach + tidal=yes natural=sand + tidal=yes natural=wetland + wetland=tidalflat
and sometimes also just natural=wetland
The wiki pages don’t seem to agree on how to tag the intertidal parts of beaches:
The beach wiki page says to use natural=beach for ‘inclined, wave formed areas at the coast’. And for larger flat areas use either natural=sand or wetland=tidalflat. https://wiki.openstreetmap.org/wiki/Tag:natural%3Dbeach
The tidal key wiki says “Don’t use natural=beach for an area under water at high tide, the value beach is only for the part above the coast line” https://wiki.openstreetmap.org/wiki/Key:tidal#Tidal_area_at_sea
Both agree that wetland=tidalflat is only for flat muddy areas.
But this is not how these areas are being mapped currently. At the moment natural=beach and wetland=tidalflat are the two most popular tagging scheme’s I come across.
With Germany, Denmark, the Netherlands and Belgium basically exclusively using wetland=tidalflat.
while France uses wetland=tidalflat in the north, but then switches to a mix of natural=beach and natural=sand further south.
and in the UK its all over the place. OpenStreetMap Over here you can find all three options next to each other.
How I think it should be tagged:
I believe that having natural=beach for smaller ‘inclined, wave formed areas at the coast’ makes sense. I’m unsure whether tidal=yes is strictly necessary, considering that the coastline already makes it clear where the Mean high water line is.
For larger flat intertidal areas I believe natural=wetland + wetland=tidalflat + surface=* would be a better solution than having both sand and tidalflat.
The only differentiating factor between natural=sand and wetland=tidalflat is whether the surface is muddy or sandy. This is pretty vague and open to interpretation in my opinion. Plus we already have a handy surface=* key for that. Then there is also the fact that multiple countries already exclusively use wetland=tidalflat. And finally, the name “tidalflat” already implies a function, a tag for flat intertidal areas.
I would like to hear other peoples opinions on this.
(disclaimer, my site**) makes an attempt to show what’s actually mapped where. It’d be interesting to ask the person who mapped this and this to join in here to help people understand why they went with one tag over another. They mapped both at around the same time, so presumably had some reasons.
Edit: a bit of info about what is what - beach is yellow, sand is gold, mud is brown, and blue dots on any of those mean tidal. Tidalflat is shown as tidal mud, saltmarsh is green vertical lines and blue horizontal ones, reedbed similar but with taller green, wet meadow as more like a meadow, but wet.
** not the gold standard by any means - the relative amounts of blue and yellow suggest that Hilbre is less tricky to reach than it actually is.
I haven’t mapped beaches yet partly because the topic seems incredibly complex (if you notice one is missing in the map do you then also move the coastline? what if it was imported? what if an admin boundary is glued to it? how do you even tell mean high water and mean low water from aerial imagery?) but I’m interested in the answer. There is an interesting discussion on the talk-gb mailing list about this topic.
Just a few thoughts:
I wonder if this has anything to do with the climate, and how people use the sandy bits along their coastline. The word tidal flat doesn’t exactly bring up images of sandcastles, deckchairs and lifeguards.
There are beaches that are completely submerged at high tide, yet any child can tell you at low tide that they are looking at a beach. So I’m not surprised that mappers tag them as beaches (with tidal=yes).
Reading the Wiki I get the impression that this was written by someone who was assuming that there is always a part that’s exposed at high tide, and therefore only that is tagged as the beach.
The shore along the North Sea is basically one very large sandy beach that sees tonnes of beach tourist. The Waddensea, the narrow sea going from the Netherlands to Denmark, on the other hand consists entirely of mudflats. With the barrier island only having sandy beaches on the North Sea side.
Yeah, it also makes total sense to tag those areas as beach to me. The issue here is more when is a beach no longer a beach. Both wiki pages seem to agree that a beach needs to be “inclined”, and that at some point a beach becomes either natural=sand or wetland=tidalflat. But thats another definition thats open to interpretation. If I have a small isolated beach that extends maybe even as far as a few hundred extra metres at low tide, then yeah, thats totally a beach. But what if it extends several Kilometres? What if one part of the beach is fairly small, but another part extends for miles? What if a beach starts sandy, but then parts of it transition in to mud?
I feel like the wiki can use an update. Especially the part where the key:tidal and tag:natural=beach both say contradictory things. But before that happens, its probably a good idea to get a better consensus here.
Is it worth trying to get the respective authors of both wiki pages to contribute here so that they can discus their opposing views together?
The normal OSM answer to that would be to “map it in two pieces, with different OSM tags on each”. An example of that in a very different context (and ignoring the boundary relation there) is here and here.
Shame this post isn’t getting more traction. It’s an interesting topic.
So your suggestion is to stop using natural=sand at the shore entirely? So that everything is either a natural=beach (= inclined, wave formed, including any intertidal areas that this description applies to) or a wetland=tidalflat (= flat, could be sandy or muddy)?
The natural=shoal tag is used to mark an area of the water floor which nears the water surface (literally, becomes shallow) and is exposed from time to time (at low tide or when a river/lake is almost empty in summer but higher at spring for example). This tag is used for distinct raised features like sand or other unconsolidated material (shingle, gravel, pebble stones…) but not for tidal flats (i.e. muddy flats) or rocks.
I’m not familiar with real mudflats, I only know Windwatt on the Baltic Sea. And here it doesn’t seem to be clear whether these are mapped as tidal flats or as shoals because most of them are surface=sand.
Reading a 2012-2018 discussion on the natural=beach talk page, I get the impression that the practice of dividing beaches into a polygon above the coastline and one below, with tidal=yes, is a holdover from a time when Carto would otherwise render both the same. This changed in 2019, so the distinction is no longer necessary.
Two more highly relevant discussions on the topic - partly about tagging, partly about rendering - both in the OSM Carto repo:
The practically dominant use of tags we try to reflect in our rendering is that beaches (i.e. wave formed coastal stretches, which can be grain size from fine sand to boulders) are tagged natural=beach both above and below the coastline, tidal flats (i.e. flat areas of loose material in the tidal zone predominantly shaped by tides) are tagged wetland=tidalflat and distinct protrusions from the surrounding ground in the tidal range (like sandbars) are tagged natural=shoal. And that natural=sand is not widely or consistently used for tidal sand areas (and its adoption for such purpose would remove existing consistent geomorphological distinctions).
Beaches are formed by waves, therefore consist of sand or coarser material and always have a significant incline. Tidal flats are formed by tides, are typically very flat and depending on the conditions are often structured with tidal channels of various size (which are absent on a beach because they can’t form in a wave dominated environment).
In the UK you can find in some areas use of natural=beach for tidal flats with a sand surface - based on a local historic practice from a time when wetland=tidalflat was not yet established. But because this is an atypical local anomaly unique to the UK we have not adjusted rendering to accommodate this.
In other words: natural=beach and wetland=tidalflat are distinguished by geomorphological characteristics and not by surface material. To differentiate surface material use surface=sand/surface=mud.
Just one person’s views but would appear to support your argument against natural=sand.
I meant that the approach of splitting off a part with tidal=yes was only ever mapping for the renderer, that no longer serves a purpose.
Relevant quote from the discussion:
Surely a crucial factor is to distinguish between above and below high tide mark (coastline) - OS maps do this clearly, so should OSM! Thus unless/until rendering copes it is misleading to have the same appearance (and polygon!) for above and below high tide coastline.
If this was the main reason, historically, that some (not all!) beaches are split and there is a part with tidal=yes then why continue doing that?
Sorry for the very late reply. I’ve gotten very busy over the past few weeks and this all kinda slipped my mind
Yes, this is basically what I’m suggesting.
Honestly, I had forgotten all about this tags existence due to never seeing it. I’m not familiar with its uses in river/lake mapping. For tidal mapping however the only thing this tag tells you is that its raised compared to other nearby features.
A very interesting read. Thanks for sharing. Not sure what to take from it yet though.
