[Talk-GB] GB Coastline - PGS vs OS

Colin Smale colin.smale at xs4all.nl
Sun Dec 11 22:17:44 UTC 2016

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? 



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 Looking here http://www.openstreetmap.org/#map=19/51.53546/0.60580 an area near Southend, unless the Bing imagery is outdated, the Boundary Line data seems to be an odd representation of the coastline.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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