[Talk-GB] GB Coastline - PGS vs OS

Colin Smale colin.smale at xs4all.nl
Sun Dec 11 10:43:16 UTC 2016


Hi, 

Most of the coastline is currently tagged as "source=PGS". As part of
the Boundary-Line open data set OS provide MHW lines which look to be
significantly better than the PGS data: 

 	* Much newer - updated twice a year, although I am not sure how old
the actual underlying survey data is (PGS coastlines seem to be from
2006)
 	* Better resolution - more nodes, smoother curves
 	* Consistent with admin boundary data, so MLW never appears above MHW
(often a problem on rocky coastlines like Wales and Cornwall)

There are a couple of caveats when working with the OS data: 

 	* Where MHW=MLW, i.e. the MHW is colinear with the admin boundary at
MLW, there is a gap in the MHW data
 	* The MHW data goes miles inland in tidal estuaries, which is correct
from the MHW standpoint, but for coastlines I think we need to cut
across the estuaries at the right point to form the correct baseline
 	* The MHW data is organised by area - down to constituency level.
Every time the line crosses the area boundary, it simply stops and you
need to load the adjacent area to continue the line

I have uploaded GPX versions of the October 2016 OS MHW data to
http://csmale.dev.openstreetmap.org/os_boundaryline/mhw/ with a file per
county / unitary area (I have not produced the files for the
higher-level regions or the lower-level constituency areas). 

In the Thames estuary around Southend and on the north Kent coast I have
replaced the PGS data with the new OS data and to me it looks much
better (in Potlatch) although the changes are not yet showing through on
"the map". I think coastline changes are processed less frequently. 

Any comments? 

//colin
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