[Talk-GB] GB Coastline - PGS vs OS

Colin Smale colin.smale at xs4all.nl
Sun Dec 11 23:27:15 UTC 2016

This discusses the accuracy of the MHW/MLW data 



On 2016-12-11 23:17, Colin Smale wrote:

> Hi David, 
> Looking at the spot you indicate on Bing imagery does indeed look like MHW should be above the salt-marsh areas. Looking at Google[1] it is however possible that the grass doesn't quite get submerged, even at the highest tides, so it might also be possible that it is strictly correct. 
> The Bing imagery is of course just a snapshot, and we don't know the state of the tide at the moment the photo was taken, so it can also be misleading. Even a personal visit is not really enough as MHW is apparently calculated over a 19-year cycle (not sure if OS use this though) and things could change a lot in that time. As MHW is an average, many tides will of course be higher. 
> The OS data looks a definite improvement for steeper coastlines, where combining OS admin boundaries with PGS coastlines produces many anomalies (admin boundary=MLW inland of coastline=MHW). I would definitely suggest applying the OS MHW data to address this kind of issue. But I agree, use of the OS data would need case-by-case judgements. However I still think the OS data is probably a better base to work from than (unimproved) PGS for reasons I mentioned earlier. 
> Could you give a couple of examples of problems you saw in the IoW? 
> //colin
> [1] https://www.google.com/maps/@51.5386032,0.6292606,3a,24.7y,277.12h,84.08t/data=!3m6!1e1!3m4!1sMl8cwBlLLuOVtPES_DfkOQ!2e0!7i13312!8i6656 
> On 2016-12-11 22:30, David Groom wrote: 
> 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 Looking here http://www.openstreetmap.org/#map=19/51.53546/0.60580 an area near Southend, unless the Bing imagery is outdated, the Boundary Line data seems to be an odd representation of the coastline.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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