[Talk-GB] Administrative boundaries (from Re: traffic island mapping / harmful detail?)

[Colin Smale]

>     On 04/05/2021 11:07 PM Tom Crocker <tomcrockermail at gmail.com> wrote:
> 
>         > > On 04/04/2021 13:47, Colin Smale wrote:
> >         > These days I'm sure the boundary is expressed in terms of coordinates on
> >         > a map, which is why OS Boundary-Line is essentially authoritative. For a
> >         > boundary to move, some form of legislation is required.
> > 
> >     > 
>     They may be there somewhere, but it would be really helpful if this was spelled out in the wiki, perhaps in https://wiki.openstreetmap.org/wiki/United_Kingdom_boundaries (would it also be possible for the general boundaries page to link to country-specific details?) As a new mapper I wondered whether it would be helpful for me to remap a boundary while remapping the stream that ran near it. Fortunately I guessed at no, but can imagine others guessed otherwise. The relation:boundary page does suggest this is usually the case but offers a caveat: "An exception is if the boundary is legally defined to be the physical feature."
> 
It is also important to remember (not just for admin boundaries) that there is often an intrinsic positional error in aerial photos, which can be several metres - enough to put features on the wrong side of a road for example. Of course consumer-grade GPS receivers such as phones or sat-navs are also only accurate to a few metres as well. On the other hand the admin boundaries provided by Ordnance Survey are surveyed with extremely expensive kit with an accuracy of around a centimetre. These coordinates are put directly into OSM, without any tracing or surveying by mappers. If you see that a boundary runs parallel to a road's centre line but not on it, there are several possible explanations: It may be that the aerial imagery is correctly located and the boundary is indeed where you see it, but it is seems more likely to me that the imagery is offset by a bit and that in fact we should move the highway to match the boundary, even if that looks wrong when referenced to the aerials. These boundaries are a gift - they give us an accurate reference framework, professionally surveyed. Another reason to leave them alone.

>     I also agree it would be really useful if the boundary could be in some separate layer (even if, in principle, editable by all). I was recently ungluing some landuse that had been stuck to the boundary and realised at the end I had carried the old nodes with the landuse rather than leaving them on the boundary and had to figure out how to swap them back.
> 
Landuse boundaries don't correlate that well with admin boundaries, which happily cut across fields and towns. Even at field boundaries you cannot see if the admin boundary follows the centre line of a centuries-old hedge or the ditch behind it for example. Putting the admin boundary to one side, how well can you determine the actual landuse boundary? Do you want the hedge to have a distinct land use value such as field_margin or will you include it in the field?

>     Out of interest, how often, if at all, are the UK boundaries redrawn in OSM to undo accidental or otherwise 'improvements'?
> 
Not regularly, but I can say this takes up quite a bit of my time.... I monitor all changes to admin boundaries in the UK and judge whether to fix "minor improvements" or not. I try to get actual (legal) changes implemented as soon as possible after the boundary data becomes available and I also have a backlog of things that need "fixing"...
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