[Talk-GB] GB Coastline - PGS vs OS

David Groom 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area near Southend, unless the Bing imagery is outdated, the Boundary 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:
> 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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