[Imports] What to do with unnamed NHD streams

[Drew Moffitt]

The major issue I see with blindly importing NHD data is it's attributes CAN
NOT be relied upon.  Your import might end up extremely fragmented.  I work
with a lot with streams that in one segment have a name and the Perennial
tag, the next segment no name and the artificial channel tag.  I really want
NHD imported as much as you do, but I think a discontinuous import is worse
than none.  

What about regions, sub regions, etc that have been 'upgraded' to NHD plus?
These areas might have 'better' data. I am not sure.



-----Original Message-----
From: Paul Norman [mailto:penorman at mac.com] 
Sent: Sunday, October 28, 2012 10:51 PM
To: OpenStreetMap US Talk
Cc: Russ Nelson; imports at openstreetmap.org
Subject: [Imports] What to do with unnamed NHD streams

Background: I'm working on converting NHD to .osm format

NHD is an extremely large data set. It's about 25G of zipfiles and all of
this converted to .osm would total about 3 TB. This is about 10x-15x times
the size of planet.osm.

There are three factors that lead to this large size. The third is what this
email is about

1. The NHD covers a massive area. 

2. Some ways are very over-noded. The NHD accuracy standard is <12m error
90% of the time. Running a 1m simplify in JOSM reduces the number of nodes
to 25%-50% of what it was before. Like everything with the NHD, this varies
from region to region. I'm thinking a 2.5m simplification would be best -
it's 1/5th of the accuracy standard. Of course, running a simplification on
a dataset this large is a challenge in itself.

3. A lot of NHD is very minor streams "only of use to hydrologists." There
are streams that you would be hard pressed to locate if you were there in
person and in some cases they do not exist anymore.

A sensible solution in any NHD translation may be to drop any FCode 46003
(intermittent) streams without a name. It may also be worth dropping FCode
46006 (perennial) streams without a name.

I've looked at a couple of regions with this adjustment made and they seem a
lot more reasonable. The data is a lot more manageable and of more relevance
to a general purpose map like OSM. There are also a lot fewer cases of
streams that no longer exist.

Does anyone have any thoughts on what should be done in a NHD translation
with these streams?


_______________________________________________
Imports mailing list
Imports at openstreetmap.org
http://lists.openstreetmap.org/listinfo/imports