Re: [Talk-GB] GB Coastline - PGS vs OS

[Jason Woollacott]

Colin,


I've been doing some coastline updates around the South West, and have been 
basing my coastline on the OS_StreetView map layer which also shows MHW.  
however the GPX file does not seem to match this.   In Cornwall, which has a 
very jagged coast, you can never rely on Bing as the angles are quite often 
wrong,  but on flatter ground, say around, Looe, 
https://www.openstreetmap.org/#map=17/50.35216/-4.44840

The GPX around the pier goes well into the sea (about 6m), compared to the OS 
SV layer which links fairly close to the bing image at that point.


Agreed that the coastline extract is much better than the PGS, which is a still 
straight lines in some places.  however, one other area that the GPX extract 
doesn't seem to cover is the islet and rocks which are above sea level even at 
high water level.  (See the Looe link again)


Jason (Unieagle)







From: David Groom 
Sent: 12 December 2016 00:51
To: talk-gb@openstreetmap.org
Subject: Re: [Talk-GB] GB Coastline - PGS vs OS

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" >
To: talk-gb@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 

Re: [Talk-GB] GB Coastline - PGS vs OS

[David Groom]

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" 
To: talk-gb@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 

Re: [Talk-GB] GB Coastline - PGS vs OS

[Colin Smale]

This discusses the accuracy of the MHW/MLW data 

http://eprints.hrwallingford.co.uk/554/1/HRPP523_Error_analysis_of_Ordnance_Survey_map_tidelines.pdf


//colin 

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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> Talk-GB@openstreetmap.org
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Re: [Talk-GB] GB Coastline - PGS vs OS

[Colin Smale]

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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Re: [Talk-GB] GB Coastline - PGS vs OS

[David Groom]

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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Re: [Talk-GB] [OSM-talk] Maps.me editing

[Rob Nickerson]

On 10 December 2016 at 21:01, Martin Koppenhoefer 
wrote:

>
> it seems sufficiently scalable for deleting stuff like settlements.
> Deletions are hard to spot, 
>

Maybe, but if deletions are hard to spot then shouldn't we be making the
change tracker tools better rather than restricting this type of edit?

Rob
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[Talk-GB] GB Coastline - PGS vs OS

[Colin Smale]

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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