Lovely hydro, pretty hydro...
Posted 28 January 2010 - 01:15 PM
I love the artwork from the Digital Line Graphs (DLGs). It's smooth, it doesn't generalize away narrow bits in waterways and make them into streams. It's pretty.
Trouble is, pretty much every statewide coverage I can find, has been post-processed. Not awfully, not so's you'd probably even notice if you weren't paying close attention. Which I am. The project involves mapping water for recreational purposes.
Even the National Hydrography Dataset, which claims to incorporate and so supersede the DLG data, has done some of this generalization. Why? It was such nice data to start with.
Grumble, grumble. Any thoughts? Am I stuck going sheet-by-sheet with the DLGs and then joining polygons when they touch at sheet-edges?
Posted 28 January 2010 - 04:46 PM
Posted 29 January 2010 - 04:03 AM
Just a thought, but I believe that there is VMAP Level 1 data available for this area. I've no idea whether or not it will be good enough.
I'm working on a project where I want the best hydro coverage I can get for the Minneapolis-St Paul area. You'd think this wouldn't be a problem, this being the land of 10,000 lakes and a whole lot of mosquitos...
Posted 29 January 2010 - 12:00 PM
No, classification isn't the issue. See this PDF. You can click on and off layers in Acrobat:
In most cases NHD is far superior than DLGs (primarily in terms of accuracy - seems to much for your needs). What generalizations do you mean? Are you having trouble classifying the data for your recreation map?
The top layer (orange) is the MN DNR wetlands, which is basically adapted from the NHD (second layer shows the lakes classification in green). Now compare that to the DLG layer (third layer, red). Note how it shows continuous areas between the three big lakes. The NHD and derived bases show these connections as single-line streams. At 1:24,000, I want the area shape where I can get it, and clearly from the DLGs it can be gotten.
Just for jollies, I've thrown on the MNDOT-based lakes layer at the bottom in black, which, interestingly, is the main one used by the DNR for its hydrologic analysis. Extra crispy...
Posted 29 January 2010 - 01:37 PM
Except these particular lakes have been essentially the same shape since early last century (Well, OK, they re-dredged the one in the upper right 2-3 years ago and may have altered the shoreline a little as a result). And my concern is less with variation in shape than with showing 2-dimensional shapes instead of 1-dimesional lines where possible.
Very interesting study, Nat! Have you compared any of this with recent aerial photography? Some of those 24K bases are pretty old, and watercourses do change. Maybe the DLG really is out of date....
I'm thinking the thing to do is use DLGs for hydro areas and NHD for stream lines, lying under the areas layer.
Now if I could just get the EROS Data Center FTP to open for me: it's been hanging me up for 2 days now.
Grumble, grumble. I'm too young to be a cranky old cartographer.
Posted 04 February 2010 - 11:54 AM
In your PDF, is the NHD layer only the NHDWaterbody polygons? Sometimes those missing stream channels are in the NHDArea layer, classified as StreamRiver. Then again, sometimes they aren't...
Thanks, we caught that between our last post and yours. An important piece...
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