Re: [Talk-GB] Admin Boundaries and OS OpenData BoundaryLine

[Chris Hill]

On 29/05/12 18:10, Tom Chance wrote: On 29 May 2012 17:19, Chris Hill <o...@raggedred.net> wrote: They need to be manually entered as relations sharing nodes with those features. I would say that sharing nodes can lead to problems. Boundaries that get imported or manually traced from OS data often have no visible reference on the ground. If you share nodes with something else, when someone aligns that something else to aerial imagery, or a GPS trace or whatever, the boundary (which was probably right) gets moved too. Why do nodes of one object need to be shared when they are quite different objects? This probably varies according to a number of factors, but where boundaries are just abstract information coterminous with physical features it makes sense to me that the objects in OSM share nodes. Many boundaries in urban areas could just be relations containing lots of roads. My question is: how do you know the boundary aligns with an existing object? If they align, what evidence on the ground do you have for that? How do you *know* that a road, stream or whatever aligns with the boundary, other than using other sources such as complete (copyright) OS maps? I would say import or draw the boundary based on the OS Open data as a separate entity from anything else. Take the example of someone moving a road but not also moving the boundary. That introduces an inaccuracy right away. How do you know that? What rule is there that says a boundary must follow a road? Why, if the OS Boundary data has been used, would you want to move a boundary unless an administrative change has been made? They could also move both but not bother to get the locations exactly right, say by making the gap between them larger, getting a kink in the wrong place, or not having them exactly coterminous. Perhaps they aren't all that interested in boundaries. But then along another person comes to check if a house is in this or what ward and they're misled. I prefer to have the boundaries share nodes so that people are forced to move boundaries with roads/streams/etc. and forced to break them apart if they really aren't coterminous. I think some areas such as some landuse can be described as coterminous and might benefit from sharing nodes. Sharing nodes between disparate object types causes more trouble than it is worth IMHO. I don't see how a road and a boundary can be described as coterminous since there is no evidence of it from a source we can use.  To my mind in these cases boundaries should be treated the same as routes. I don't understand that comparison. Of course if you don't know that a stream and a boundary are supposed to be related and you go and share nodes because they happen to be in roughly the same place, then moving the stream to align with a GPS trace obviously shouldn't move the boundary so you introduce problems. That's my point, how would you know that a boundary is 'supposed' to be related to anything else? Are  boundaries ever anything more than arbitrary? They don't exist on the ground, even the boards saying "Welcome to My Town" are not always placed on the boundary because of traffic sign placement rules, indeed, most parish boundaries are not marked, the village name sign is no where near the parish boundary. People will do as they think best, and I'm sure some boundaries will end up with shared nodes - I just don't see the benefit nor the justification and I do see problems. -- Cheers, Chris user: chillly

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