[Imports] Vermont Town boundaries from VCGI

Andrew Guertin andrew.guertin at uvm.edu
Wed Aug 8 14:49:15 BST 2012


On 08/08/2012 07:15 AM, Greg Troxel wrote:
> 
> I would suggest looking at the way town boundaries in mass are.  I
> believe that each boundary segment is a way, and town boundaries are a
> relation.  Is that what you are intending to do?

Yes.

> Have you confirmed that the state boundary in osm matches the outer
> boundary of towns?

The state boundary in the VCGI file (which exactly matches the outer
boundary of the towns in the VCGI file) does *not* exactly match the
state boundary in OSM.

In most cases involving straight lines it is reasonably close--within
about 10 meters. In a few areas of the middle-of-the-lake border with
New York, they diverge by over 100 meters. In on the river border with
New Hampshire (the border is specifically the low water mark of the west
bank of the river), there are some places where the river has shifted
(e.g. to cut off an oxbow) and this is reflected in one version but not
the other. There are also some places where the the two versions match
but do not match the bing satellite imagery. Interestingly, on the
US-Canada border, most of the nodes seem to be exactly the same--OSM has
them marked with survey point reference numbers, so this would make
sense--but shifted a few centimeters. Probably something to do with
projections.

I plan to keep the existing state and national borders, and manually
connect the new town boundaries with the existing state border.

> Is Vermont like Mass, in that every bit of land is in exactly one town?
> 
> Do the Vermont state people think that the state boundary is exactly the
> outermost town boundary, or is there somehow land in Vermont not in any
> town?

To the best of my knowledge--I don't have an authoritative source--every
bit of Vermont is accounted for. Either it is a city or a town, which
has a local government, or it is a township, gore, or grant, which
doesn't have a local government.

Each city or town sets its own boundaries in its town charter, but as I
understand it in most(?) cases, that's in conjunction with a state law
defining what it has to be. I haven't found official definitions of the
boundaries of the gores, but the reading I've done (wikipedia) suggests
that they exist, and match the boundaries of the adjacent towns (or
adjacent gores).

That said, each town does its own survey of the boundaries, and there
have been disagreements about the results of the surveys. In the
metadata of the VCGI file, it mentions a number of cases where
neighboring towns have official maps that overlap or leave empty space.
It also mentions that some towns aren't even using digital files for
their official data. Thus VCGI has made an effort to provide its guess
at the correct boundaries. From what I've seen, they've done a better
job than anyone else.

--Andrew



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