[Talk-us] NHD data extract

James Umbanhowar jumbanho at gmail.com
Tue Apr 26 18:27:07 BST 2011


On Tuesday 26 April 2011 12:50:41 Ben Supnik wrote:
> Hi Y'all,
> 
>  From what I can tell:
> 
> - Every water body does get a reach ID.  I've seen nulls in this file
> but haven't yet figured out what they are...in local sample areas, all
> water bodies have reach IDs.
> 
> - If there is linkeage between areas and flow lines (which I _thought_
> there was when reading the docs this morning) I don't see it when
> browsing the real data.
> 

I don't know if there is a database linkage, but the flowlines that are inside 
an area have a different FCode to indicate that they are connectors. 

> - I can confirm that areas cross sub-basin boundaries.  (I downloaded
> the sub-basin shapefiles to check.)
> 
> I'm not sure where this leaves us...sub basins are small and manageable
> but don't partition the data particularly well (in that it looks like
> we'd need an expensive spatial check for rivers).  Users also don't
> necessarily know their sub-basin code unless they can find an online
> source to browse or view shapefiles.
> 
I actually think that the spatial check for rivers is not that expensive.  If 
someone is going to do a subbasin import well, they need to do a fairly 
extensive check anyway to look for previously mapped features such as rivers 
and waterbodies.  Double checking to make sure that the riverbanks have not 
been imported is not difficult.  There will typically be only one large 
riverbank relation per subbasin, maximum.  I just think we would need to agree 
on some process to dole these out to the subbasin files sanely.  

Per the subbasin code, I think the NHD has a boundary file that could be used 
to make a layer that would allow mappers to figure out their local code if they 
couldn't do it via the National Map web interface.

> Richard's idea of building an NHD tile map for tracing seems very
> do-able, but it wouldn't save a ton of time - every water feature would
> have to be hand-traced, even though we do have them in vector form
> already.  But I'm not sure that anything other than tile maps provide
> the level of user simplicity I was hoping for, e.g. being able to find
> just the part of the data you want to import in a form that's ready for
> import.
> 
> cheers
> Ben
> 



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