[Talk-us] Coastal Maine needs love

Peter Dobratz peter at dobratz.us
Wed Oct 15 18:25:12 UTC 2014

On Wed, Oct 15, 2014 at 9:33 AM, Eric Kidd <emk.lists at randomhacks.net>

> Thank you for your response!
> I've made two test edits:
>    - I imported TIGER County Subdivision files for four town boundaries.
>    - I modified an existing TIGER Place outline to be admin_level=9,
>    because it's a actually a subdivision of the real town. That's the brown
>    blob on my example map.
> I think you are on the right track with the TIGER County Subdivision
files.  I did a lot of work on the town boundaries in New Hampshire and it
probably had the same sort of mess you are seeing in Maine.  Boundaries for
towns were imported multiple times by different users and the county
boundaries were imported using a much lower precision source.  At least in
New Hampshire, the county boundaries are most often coincident with town
boundaries, so it makes sense to have these in OSM as Relations (not as
closed Way polygons) and sharing the Way objects among multiple
boundaries.  Also, the TIGER CDP shapes were erroneously used in place of
the actual town boundaries.  The CDP for a town is often generally where
the most of the people live, but the actual boundary for the town is a much
larger area and this can be verified against town-line signs on the ground.

The TIGER boundary data seems to want to share nodes with TIGER road data
even though it doesn't actually make sense.  For example, town boundaries
are often straight lines, but in TIGER these lines are slightly jagged so
that they can share points with roads that are close to the town line.  If
you look at resources like property tax maps that some towns make
available, you can see that in many cases the TIGER boundary data should
just be made into straight lines.  And straight lines in OSM should just be
represented by a Way connecting 2 Nodes (your simplify step in QGIS will
get you most of the way there).

Tagging on the Ways is completely optional as that information should all
be in the admin Relations.  However, the consensus is that the Ways should
have the admin_level be the lowest number (for example admin_level=6 for
Ways that make up both a town and a county boundary) rather than trying to
have multiple values separated by semicolons.

Generally, I have used admin_level=9 for areas inside of towns that appear
to be separately administered.  In some larger towns, I used admin_level=9
for the wards or districts which correspond to seats on the local

Towns have also been imported as single nodes, which could be influencing
your Nominatim results.  These place Nodes should be added to you admin
Relations with role "admin_centre" or "label".  You should also add
wikipedia tags to the boundary relations as this will help Nominatim
determine the place hierarchy.

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