[Talk-us] Low-quality NHD imports

Wolfgang Zenker wolfgang at lyxys.ka.sub.org
Fri Oct 13 07:18:09 UTC 2017


* Frederik Ramm <frederik at remote.org> [171013 08:06]:
>    there's a LOT of NHD:* (and nhd:*) tags on OSM objects, see

> https://taginfo.openstreetmap.org/search?q=NHD%3A

> - 1.9 million NHD:FCode, but also 188k "NHD:Permanent_" (note the
> underscore), 10k "NHD:WBAreaComI", or 1.5m "NHD:Resolution" just to grab
> a few.

> I haven't researched who added them and when, but they would certainly
> not clear the quality standards we have for imports today. Most of this
> information can be properly modelled in usual OSM tags, and where it
> cannot, it probably shouldn't be in OSM in the first place.

> Is there any systematic (or even sporadic) effort of cleaning up these
> old imports? Is there reason to believe that the neglect extends to more
> than just the tags - do geometry and topology usually work well on
> these, or are the funny tags a huge "this whole area hasn't had any love
> in a long time" sign?

the NHD imports that I have encountered so far have numerous problems:
The data is several decades old, the so-called "medium resolution" is
pretty bad, and the data was basically just dumped into the OSM database
without any conflation happening. And larger rivers where often imported
as monstrous riverbank polygons without the river itself as a flowline.

The worst junk like lakes covering motorways has been mostly cleaned up
by now, but it is still easy to see where NHD data has been imported by
looking a KeepRights display of broken highway/waterway crossings.

I clean up the imports in areas where I'm doing TIGER reviews, but I
have to admit that a few times I have decided to work on different areas
instead because the huge riverbank polygons where almost impossible to


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