[Talk-us] What to do with unnamed NHD streams

Paul Norman penorman at mac.com
Mon Oct 29 03:51:07 GMT 2012


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?




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