PROBLEM: Compute fetch distance

[Harvey Greenberg]

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>From UW-GIS-L-owner at u.washington.edu Fri Mar 13 16:34 PST 1998
Date: Fri, 13 Mar 1998 16:34:18 -0800
Reply-To: uw-gis-l at u.washington.edu
Sender: UW-GIS-L-owner at u.washington.edu
From: Blake Feist <bfeist at u.washington.edu>
To: "UW - GIS Discussion & Support" <uw-gis-l at u.washington.edu>
Subject: PROBLEM: Compute fetch distance

I want to calculate theoretical maximum wave height for any given point (or
predetermined linear waterfront length) on the shoreline of Willapa Bay,
Washington.  In order to do this, I need to calculate the longest linear
distance from any given point on the shoreline to another (fetch length).
I would also need to know the angle of this imaginary line relative to true
north.  To confound things, there is a large island in the middle of
Willapa Bay (Long Island), that creates a "donut" type shape.  I have USGS
1:24,000 ArcInfo coverages of Willapa Bay to work with.

Any suggestions would be greatly appreciated.

Sincerely,

Blake

......... , ................................
      .:/                    Blake E. Feist
  ,::::::::,     , University of Washington
 o::\::::::::..,/    Wetland Ecosystem Team
>:::/::::::::,``\                Box 357980
  `::::::::'     `  Seattle, WA  98195-7980
      '\                 PHONE 206.685.3505
                           FAX 206.685.7471
http://www.fish.washington.edu/people/asif/




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I would try brute force, something like this:
1) APPEND all shoreline (I presume that you like this version of the shoreline)
   arcs into one coverage.
2) GENERALIZE it, if desired
3) UNSPLIT arcs, as much as permitted by the vertex limit
4) DENSIFYARC with the ARC option 
5) BUILD cover NODE
6) ADDXY cover NODE (or did you just want to click on a point to test?)
7) For ech point
      calculate the coordinates of the endpoints of a large compass rose.
      GENERATE a cover from this rose.
      APPEND the shore cover to these arcs
      CLEAN
      For each direction{
         select the radial line by its cover-ID.  The line is broken into segments
         reselect the segment NEAR the point on the shore.
         test if this segment lies in a land polygon or water polygon.
         If it's on land, then
           the fetch length is zero
         else
           the fetch length is the length of the line segment.
      }
      discard the rose coverage
    }
      
I would not attempt to solve directly for the fetch length of a shoreline segment;
I would work only with points.

harvey

Harvey Greeenberg	hgreen at u.washington.edu  162 Johnson
206-685-7981		http://duff.geology.washington.edu/
	University of Washington
	Geological Sciences
	Box 351310
	Seattle WA 98195-1310