[Talk-GB] UK coastline data
borbus at gmail.com
Thu Jul 11 19:38:39 UTC 2019
I've recently done an import of coastline data from OS VectorMap into OSM
around The Wash. I did this because I'm interested in coastal regions and
the coastline was a complete mess in that area. I'm sure it's similar in
other parts of GB as well.
The mess often happens because mappers don't necessarily know what a
"coastline" is (I didn't before I researched it). For land-based maps the
coastline that is shown is generally shown is mean high water level. The
other "coastline" that is also shown on land-based maps is the mean lower
water level. The bit between these lines is the intertidal zone. This is
admittedly a bit less interesting, but it's certainly useful when there are
causeways and other features in the intertidal zone. The actual high and
low tides can be higher or lower than the means. The tide varies throughout
the month and the highest highs and lowest lows are called spring tides.
Nautical charts will show the lowest low, not mean low.
This seems like quite difficult data to obtain so using OS seems to be the
obvious choice here. I'm pleased with how the import went in The Wash. It
integrated well with the existing OSM data around the coastline. It's
certainly a lot easier to integrate than groundwater but it does require a
lot of manual processing.
But before I start importing other areas (I'm looking at the Blackwater
estuary next), I want to discuss it with others because I'm concerned that
the way I've done it could negatively impact other mappers.
The data as it comes is essentially the two coastlines as described above:
MHW and MLW. The MHW can just replace the existing coastline in OSM. It
adds many, many more nodes to the coastlines, and possibly more ways too.
The MLW along with MHW then can form multipolygons containing the
intertidal zone, which is mapped as a wetland=tidalflat.
Using the coastline to make multipolygons means the coastline is broken up
into many, many small ways. One concern is that the GB island multipolygon
will become very hard to maintain. On my computer JOSM is very slow to
operate when I load this multipolygon.
So before I continue I'd like to give people the chance to tell me to stop
and, if necessary, suggest a better way to do this import. Or maybe people
wouldn't like to see this import done at all. Personally I think there is
value in integrating the data but some may disagree.
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