On 29/05/12 16:00, Tom Chance wrote:
Most boundaries follow existing features like roads, rivers, etc.
Not always by any means. Many urban boundaries follow roads, but many
rural ones run alongside roads and have little jinks in them where they
cross to the other side of the road. This allows a stretch of road to be
firmly the responsibility of one administration, not shared. Many
boundaries follow an old course of a stream, when the stream moved the
boundary did not.
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?
In my experience this is often a nice opportunity to spot other
problems with very old features using aerial imagery and GPS tracks,
e.g. poor alignment, or complicated junctions that aren't fully
modelled for routing. So much better done manually than by dumping a
load of new ways into the database.
I agree that manual scrutiny is vital. Local knowledge and control is
also important. Documenting the existing practice would help too.
2) Many boundaries already exist, but are often slightly incorrect,
e.g. not sharing nodes with existing features but being a little offset.
That offset might be right. See above.
[...]
--
Cheers, Chris
user: chillly
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