[Talk-us] NHD data import question

Ben Supnik bsupnik at xsquawkbox.net
Thu Apr 28 23:04:48 BST 2011


Hi Y'all,

I have a command line tool that copies subsets of shapefiles based on a 
polygonal region of another shapefile...you feed it the key/value pair 
of a HUC_8 from the master sub-basin file and one or more* NHD 
shapefiles and it produces an extract.

I hit one tricky case while looking at flow-lines.  In Boston, the head 
of the mystric river and Ft. Point channel are actually _excluded_ from 
the sub-basin's spatial extent.  I think this is because they directly 
contact the harbor/ocean.  (How the USGS decided where to 'cut' from 
ocean to river I don't know...I'm sure there are reasons I cannot 
fathom. :-)

So...the flow lines inside the mystic river get dropped by my check, 
since technically they are not 'inside' the sub-basin.

I added a second check: if an entity has a reach code which matches the 
HUC_8, I take it...this is also nice because it speeds up extraction - 
quicker to check the reach code than do a gajillion geometry tests.

Anyway, there may be data sets that would be difficult to properly 
extract.  If an entity is 'on the line' (or in the ocean) and has no 
reach code, my extractor may miss it.

I hope to have a test sub-basin in .shp form posted later tonight.  Does 
anyone have a particular HUC_8 that you want to inspect?

cheers
Ben


* In some cases the NHD data spans several shapefiles due to shapefile 
size limits.  The data isn't in any particular order - I think it's 
sorted by something like the internal Arc object ID or whatever...



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