[OSM-newbies] What's the easiest method to "snap" an area boundary to adjacent ways?

[Phil! Gold]

* Russell Harrison <russell.harrison at gmail.com> [2010-08-30 17:19 -0400]:
> > The way is the centerline plus the road as a whole. It's an abstraction. If
> > you don't like this, then draw a polygon. For instance, if you draw an
> > intersection between a footway and a highway, do you cut the footway 2
> > meters before the intersection and 2 meters after the intersection because
> > the 4 meters are on the road ? If you start micromapping, you have to do
> > that everywhere and consistently.
> 
> I see your point about road center lines.  What about river center
> lines?  I was under the impression that many administrative boundaries
> do correspond exactly.

As with many things, it depends.  I recommend researching the boundary in
question to be sure.

In general, administrative boundaries often follow either the center of
the river or the center of the main channel of the river (which may be
different).  Less commonly, they might follow one bank or the other of the
river, usually at the mean low water mark.  Sometimes the boundaries will
be fixed in place by a particular survey, while other boundaries are
considered to move as the river changes course over time.  The latter
generally make exceptions for man-made diversions of the river or sudden
changes, like when a flood changes the course of a river.

If the boundary really does follow the center of the river, my preference
is to use that section of the river as an element of the boundary's
multipolygon.  The same holds (in theory[0][1]) for boundaries on the
banks of rivers; I'd use the riverbank way as part of the boundary.

[0] I say "in theory" bacause the only riverbank-aligned boundary I know
    of in my area is the souther border of Maryland, which runs along the
    southern bank of the Potomac River.  It, however, is fixed according
    to a particular survey (the Matthews-Nelson survey of 1927), so it
    does not change if the river's banks move.

[1] A further "in theory" is that riverbanks are often mapped along the
    mean *high* water mark, but it's more common for administrative
    boundaries to be along the mean *low* water mark so you don't end up
    with 2-meter strips of land owned by the administration across the
    river.

-- 
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