[Talk-GB] UK coastline data
edloach at gmail.com
Thu Jul 11 20:01:21 UTC 2019
You'll probably get comments about import guidelines but I did similar for Tendring about 9 years ago before there were any. I think your use of the word import in this scenario may be misleading as you're not bulk importing the whole coastline but selectively improving sections of coastline by manually improving existing data by using a small subset of available opendata.
If you're using JOSM you can remove excess nodes (which I didn't know at the time and have tried to clear up a bit since).
Coastlines take some care when editing so you don't flood the country; from your post and the lack of any recent issues you've proved you can handle this.
Coastlines change over time - locally a coastal protection scheme added a few fish tailed groynes to MHW so I replaced that short section when the data became available (too recent to trace from imagery).
OSM is a process of continual improvement. I would say if you are doing small areas manually with care rather than bulk importing the whole coastline then carry on doing areas if you're willing to maintain them too.
From: Borbus <borbus at gmail.com>
Sent: Thursday, July 11, 2019 8:38:39 PM
To: talk-gb at openstreetmap.org
Subject: [Talk-GB] UK coastline data
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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