[Talk-GB] GB Coastline - PGS vs OS
reviews at pacific-rim.net
Sun Dec 11 21:30:17 UTC 2016
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area near Southend, unless the Bing imagery is outdated, the Boundary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.
On 11/12/2016 10:43, Colin Smale wrote:
> 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
> Any comments?
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