[Talk-us] Low-quality NHD imports

Ben Discoe bdiscoe at gmail.com
Tue Oct 17 20:38:00 UTC 2017

I've probably done the most NHD cleanup so far (at least some degree
of fixing on the entire state of NC, most of IL, northern MI, parts of
OK/TX/UT/CO, and a lot of CA), many hundreds of hours of manual work.

Just to chime in with agreement on what everyone has said, yes to:
1. NHD has lots of issues
2. a lot of it could have been imported better
3. there are some NHD tags (like 'nhd:com_id') which are of little use
and just get in the way
4. a lot is badly out of date
5. a lot is badly overnoded (like a perfectly straight ditch using 200
noisy nodes)
6. some is badly tagged (like waterway=canal for ditches in the western USA)
7. it's still very valuable and in some places, of surprising quality
and completeness

I never just remove useless tags for the sake of removing.  They only
get touched as part of manual cleanup, which goes through a lot of

I really should write a Diary post on the steps I go through to clean
up NHD (part of these steps I already covered in

1. Select all the waterways ("type:way AND (natural=water OR
natural=wetland OR waterway OR (child natural=water) OR (child
natural=wetland) OR (child waterway)) AND allindownloadedarea ")
2. Use JOSM validator fix-it, which will solve topology problems like
dupe vertices.
3. If the region is badly overnoded, simplify to an appropriate value
(like 80cm), then follow up with a manual cleanup/alignment.
4. Check the "waterway ends without a connection" warnings; some can
be manually fixed
5. For at least major crossings, add the bridges and culverts (I just
JOSM scripts I wrote to make this faster, but it's still very manual,
one at a time)
6. For major features that are out of date (like streams or wetlands
that were destroyed and are now shopping malls), delete or re-align
7. Look over the data and fix anything else that looks very wrong

So, to answer Frederik's original question, "Is there any systematic
(or even sporadic) effort of cleaning up these old imports?"

Answer: Yes.  In addition to all the people doing great work in their
local areas, there is me.

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