[Talk-us] NHD import: what data quality is acceptable?

Kevin Kenny kkenny2 at nycap.rr.com
Mon Jul 23 00:36:47 BST 2012

A few months ago, I tried to get started on trying to resume the NHD
import in my area - and some of the places where I hike. I'm trying
to check results with both P2 and JOSM, and tripping over a lot of
things, which made me put the project back on hold for a while. (I
had some other things to be about.)

But now I'm starting to reconsider again, after hearing that there are
others out there thinking the NHD import is desirable for an "Open Trail
Map." I mentioned this in a message earlier today.

So, let me review some of the things that have me scratching my head.

(1) The mapping from NHD feature codes to OSM tags is incomplete (and
not quite consistent). That's fine; for all the FCodes in my area, I've
been able to find features that I'm familiar with that I can describe.
So I think I have that problem fairly well licked. (I may have to invent
a tag or two, like "waterway=rise" and "waterway=sink" to describe
streams that go underground and appear again in karst terrain.)

(2) One historic complaint I've seen about the NHD import is that it
clutters the map, and that the assignment of "river" and "stream" is
difficult.  What I'd propose is to tag as "river" anything that has
more than two nodes of its flowline as "Artificial Path", and "stream"
anything that's smaller.  In my area (eastern New York), for instance,
that would mark out the Schoharie Creek, the Mohawk, the Hudson, and
the lower reaches of the Catskill, the Kaaterskill, the Esopus, and
maybe a few others. Is this a reasonable approach?

(3) Is it necessary to tag shorelines with name=? It would be something
of an effort to identify what flowline belongs to a shoreline, and it's
somewhat ambiguous near confluences. I'm inclined not to do anything
about this issue unless there's an overwhelming consensus that it
needs to be addressed.

(4) What about cadastral lines (administrative boundaries, and land
use, leisure, etc.) that appear to follow flowlines? My inclination is
not to touch these at all, even if the flowline is being refined.
Oftentimes, when a stream changes course, the cadastre remains
unchanged. Only the recorder's office in the jurisdiction in question
would actually know the situation. I think I'd rather see slightly
inconsistent boundaries than mess up something like that.

(5) In the area I have in mind, there are very few ways that actually
would need to be conflated (a few major rivers). Is it likely to be
called vandalism if I confine myself to copying the OSM-specific tags
from the OSM ways onto the NHD ones? I trust a land survey rather
more than I do someone tracing a shoreline from Bing imagery: in
well-graded streams, the shorelines can be variable and ambiguous,
and NHD has pretty sound information about high water marks.

(6) When I try a limited import, I get a lot of JOSM warnings about
waterways crossing highways. Do people think that all of these have
to be fixed before importing? In that case, I'll have to confine myself
to the very small area that has highway-crossing-stream that I can
visit myself (to try to settle what the boundaries of the bridge,
dam or culvert are, and what type of waterwork it is). Is it considered
acceptable simply to leave the crossings unmarked? (They still
render correctly in Mapnik, for what it's worth.)

(7) In the event that I find a node tagged with something other than
a waterway (or landuse=reservoir, man_made=dam, etc.) that collides
with an NHD node, is the correct approach to conflate the nodes,
introduce a duplicate, or offset the new node? Does this question
even have a correct answer?

I'm trying seriously to "do no harm", and getting paralyzed by the
fact that there seem to be no firm guidelines on what is acceptable.
I'm trying to start with areas where I have firm local knowledge,
although clearly I cannot survey streams on private lands, so I
have to depend on NHD there. But the result is not going to be of
much use to me unless I can generalize what I learn to do better
NHD imports in areas that I know less well.
73 de ke9tv/2, Kevin

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