[Talk-GB] GB Coastline - PGS vs OS

David Groom reviews at pacific-rim.net
Mon Dec 12 00:51:18 UTC 2016


Colin

I was more talking about the actual shape of the MHW rather than its 
position; if that makes sense.

some examples of problems in the Isle of Wight

1)  There's a section here  
http://www.openstreetmap.org/#map=17/50.66636/-1.48566, where the Bing 
imagery seems reasonably aligned to the gps tracks of the main road, but 
the gpx file for MHW seems to be too far to the north on the cliff area, 
and too far to the south on the area to the east.  this beach shelves 
relatively steeply so there is unlikely to be much difference between 
MHWS & MHWN

2) Even clearer is an area 
http://www.openstreetmap.org/#map=17/50.69439/-1.09414, OSM is much more 
accurate here than the OS Boundary Line

3)  The car park and ice rink here 
http://www.openstreetmap.org/#map=18/50.73237/-1.15736  were built 
sometime around 1990, but Boundary line  MHW would show these as flooded

4)  More inaccuracies here   
http://www.openstreetmap.org/#map=19/50.76650/-1.30029

David



------ Original Message ------
From: "Colin Smale" <colin.smale at xs4all.nl>
To: talk-gb at openstreetmap.org
Sent: 11/12/2016 22:17:44
Subject: Re: [Talk-GB] GB Coastline - PGS vs OS

>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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