I suspect that even though much of the coastline is tagged "source=PGS"
is has been amended by reference to Yahoo and after that Bing imagery,
but the subsequent editors did not remove the "source=PGS" tag.
Certainly comparing your gpx file for the Isle of Wight with the
coastline currently in OSM there appear a number of places where the gpx
file does not accurately represent MHW.
I certainly would not want to see a wholesale replacement of what is in
currently in OSM with OD Boundary Line data.
Looking here http://www.openstreetmap.org/#map=19/51.53546/0.60580 an
area near Southend, unless the Bing imagery is outdated, the Boundary
Line data seems to be an odd representation of the coastline.
David
On 11/12/2016 10:43, Colin Smale wrote:
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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