Re: [Talk-GB] Boundary_line at the coast

[Colin Smale]

On 2019-09-08 00:09, Colin Smale wrote:

> On 2019-09-07 23:06, Edward Bainton wrote: 
> 
> 3. Also, there are two walls visible on aerial imagery that all but match the 
> doglegged county boundary as it crosses the isthmus. Is it safe to assume 
> that these mark the actual boundary, and can I tug the boundary to match 
> them? Or maybe assume the boundary is definitive, and the imagery is 
> misaligned, so I should move the walls? Or leave well alone?

>> West side: https://osm.org/go/e7tUHqdxn?m==669235281 
>> East side: https://osm.org/go/e7tUHrpqY?m==669235281

Looking a bit closer, the boundary that "almost" aligns with these walls
is an admin_level=10 boundary, which is not based on OS Open Data. I
have no idea where it came from. This admin level is used in Scotland
for Community Councils, and there is indeed one called Northmavine in
this location. The boundary across the isthmus appears to be a straight
line, without the wiggle we can see in OSM. I will see if I can import
the official boundary soon. 

Bottom line: probably best to leave the walls alone as it looks like
they are not intended to align with the community council boundary.___
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Re: [Talk-GB] Boundary_line at the coast

[Colin Smale]

On 2019-09-07 23:06, Edward Bainton wrote:

> I'm interested in boundaries marked at Mavis Grind [1] (thanks to SK53 for 
> the waterway=portage [2] tag - Mavis Grind is an old Norse portage, still in 
> use by Shetland Canoe Club). 
> 
> 1. Does anyone know if county boundary lines at the coast are set at mean low 
> water? There's a gap between coastline (which I understand is MHW) and the 
> county boundary: https://osm.org/go/e7tUHoc5F--?m==669235281

Indeed, GB administrative boundaries are set to MLW(S) and the coastline
is defined in OSM as MHW(S). So normally the coastline should be
"inland" relative to the admin boundary, although in areas with steep
cliffs, harbour walls etc they are shown as coincident. 

> 2. "coastline" is very coarse - is it ok to make it follow the coast more 
> finely, or is it some important legal line where it stands? (I've already 
> done this on the east side of the portage, but then thought perhaps that was 
> a no-no: https://osm.org/go/e7tUNAM7G?layers=D==669235281)

The coastline is in most cases (source=PGS) derived from very old aerial
imagery. A better source is OS Open Data but it is a lot of workI
have done bits and pieces around Scotland, but not this bit of Shetland
yet. The actual physical coastline doesn't have any particular
administrative significance, AFAIK. 

> 3. Also, there are two walls visible on aerial imagery that all but match the 
> doglegged county boundary as it crosses the isthmus. Is it safe to assume 
> that these mark the actual boundary, and can I tug the boundary to match 
> them? Or maybe assume the boundary is definitive, and the imagery is 
> misaligned, so I should move the walls? Or leave well alone? 
> West side: https://osm.org/go/e7tUHqdxn?m==669235281 
> East side: https://osm.org/go/e7tUHrpqY?m==669235281

Note that the OS data was mostly surveyed using high-precision GPS
equipment (±1cm or so); its lat/lon positioning is going to be far more
accurate than what we can get from either aerial imagery (±5m
sometimes?) or consumer GPS (can be ±10m or more). So I tend to accept
that Boundary-Line is pretty much correct, and sometimes things have to
move to fit that. In this case, if it can be established that the walls
are supposed to on the boundary, then I would consider moving the walls
to fit the boundary, and not the other way round. 

Having said that, when it comes to high water / low water marks, I
believe OS re-survey the coast every few years, and make a lot of use of
aerial imagery for this. Don't forget when looking at Bing etc that you
don't know what the state of the tide was at that time, and with a
shallow slope or sandbank it might be difficult to say with any
certainty where the water stops as even sea water is fairly transparent
through short distances. 
  

Links:
--
[1] https://osm.org/go/e7tUHqY_g-
[2] https://wiki.openstreetmap.org/wiki/Tag:waterway%3Dportage___
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[Talk-GB] Boundary_line at the coast

[Edward Bainton]

I'm interested in boundaries marked at Mavis Grind
 (thanks to SK53 for the waterway=portage
 tag - Mavis
Grind is an old Norse portage, still in use by Shetland Canoe Club).

1. Does anyone know if county boundary lines at the coast are set at mean
low water? There's a gap between coastline (which I understand is MHW) and
the county boundary: https://osm.org/go/e7tUHoc5F--?m==669235281

2. "coastline" is very coarse - is it ok to make it follow the coast more
finely, or is it some important legal line where it stands? (I've already
done this on the east side of the portage, but then thought perhaps that
was a no-no: https://osm.org/go/e7tUNAM7G?layers=D==669235281)

3. Also, there are two walls visible on aerial imagery that all but match
the doglegged county boundary as it crosses the isthmus. Is it safe to
assume that these mark the actual boundary, and can I tug the boundary to
match them? Or maybe assume the boundary is definitive, and the imagery is
misaligned, so I should move the walls? Or leave well alone?
West side: https://osm.org/go/e7tUHqdxn?m==669235281
East side: https://osm.org/go/e7tUHrpqY?m==669235281

Thanks as ever, Edward
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Re: [Talk-GB] 'Sources' tags

[Simon Poole]

I don't know how iD does it, but Vespucci includes the imagery used when
you actually made an edit, potentially multiple different sources. You
should still enter a source comment, imagery_used is simply for
additional documentation purposes.

Am 07.09.2019 um 18:54 schrieb Edward Bainton:
> I've noticed the changeset data includes the aerial image used
> (presumably at the moment you hit 'save', if you've referred to several?)
>
> Does this mean I don't need to add a source=aerial_imagery tag before
> I save a changeset? iD doesn't have it available as default; you have
> to choose it under 'add field'.
>
> Thanks.
>
> ___
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Re: [Talk-GB] 'Sources' tags

[Mateusz Konieczny]

Manual addition is still useful as it 
 confirms that editor guessed correctly
(iD will add this data both for user
tracing from imagery and one who
ignored it).

7 Sep 2019, 19:54 by bainton@gmail.com:

> I've noticed the changeset data includes the aerial image used (presumably at 
> the moment you hit 'save', if you've referred to several?)
>
> Does this mean I don't need to add a source=aerial_imagery tag before I save 
> a changeset? iD doesn't have it available as default; you have to choose it 
> under 'add field'.
>
> Thanks.
>___
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Re: [Talk-GB] 'Sources' tags

[David Woolley]

On 07/09/2019 17:54, Edward Bainton wrote:
I've noticed the changeset data includes the aerial image used 
(presumably at the moment you hit 'save', if you've referred to several?)


Does this mean I don't need to add a source=aerial_imagery tag before I 
save a changeset? iD doesn't have it available as default; you have to 
choose it under 'add field'.




This is a feature of particular editors and I think it was introduced 
because people were failing to add sources.  Of course it can result in 
a source being indicated that wasn't really used.


In any case, source=aerial_imagery is too vague, one should really be 
using the actual imagery source.


If it was the only source (i.e. armchair mapping) and is correct, it 
shouldn't really be necessary to add a source, but if there were other 
sources (survey and gps are gold standards) they should be included, and 
if it is wrong, it should be removed.


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[Talk-GB] 'Sources' tags

[Edward Bainton]

I've noticed the changeset data includes the aerial image used (presumably
at the moment you hit 'save', if you've referred to several?)

Does this mean I don't need to add a source=aerial_imagery tag before I
save a changeset? iD doesn't have it available as default; you have to
choose it under 'add field'.

Thanks.
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