http://blogs.mathworks.com/steve/2007/06/08/upslope-area-part-5/ Steve Eddins manages the Image & Geospatial development team at <a href="http://www.mathworks.com/">The MathWorks</a> and coauthored <a href="http://www.mathworks.com/support/books/book5291.html?category=-1&language=-1">Digital Image Processing Using MATLAB</a>. He writes here about image processing concepts, algorithm implementations, and MATLAB.<br><br><img> Mon, 23 Nov 2009 01:32:22 +0000 http://wordpress.org/?v=2.3.1 http://blogs.mathworks.com/steve/2007/06/08/upslope-area-part-5/#comment-20896 Steve Mon, 21 Jul 2008 13:21:03 +0000 http://blogs.mathworks.com/steve/2007/06/08/upslope-area-part-5/#comment-20896 Felix—I'm glad to hear the code is working for you. I was also glad to get your comment, because it reminded me to put a note at the top to caution readers that I eventually implemented a different method. Felix—I’m glad to hear the code is working for you. I was also glad to get your comment, because it reminded me to put a note at the top to caution readers that I eventually implemented a different method.

]]> http://blogs.mathworks.com/steve/2007/06/08/upslope-area-part-5/#comment-20893 Felix Mon, 21 Jul 2008 09:24:10 +0000 http://blogs.mathworks.com/steve/2007/06/08/upslope-area-part-5/#comment-20893 Yes, sorry, I didnt find the rest of the blog entries before I read this... that's what you get for smart assing around ;-) Found your tools now and your flow direction script works a treat! Thanks! Yes, sorry, I didnt find the rest of the blog entries before I read this… that’s what you get for smart assing around ;-)
Found your tools now and your flow direction script works a treat! Thanks!

]]> http://blogs.mathworks.com/steve/2007/06/08/upslope-area-part-5/#comment-20889 Steve Fri, 18 Jul 2008 15:40:36 +0000 http://blogs.mathworks.com/steve/2007/06/08/upslope-area-part-5/#comment-20889 Felix—P.S. I added a note to the top of this post to refer readers to the later one. Felix—P.S. I added a note to the top of this post to refer readers to the later one.

]]> http://blogs.mathworks.com/steve/2007/06/08/upslope-area-part-5/#comment-20888 Steve Fri, 18 Jul 2008 15:37:32 +0000 http://blogs.mathworks.com/steve/2007/06/08/upslope-area-part-5/#comment-20888 Felix—I discovered this problem after I originally wrote the post above, and I revisited plateau handling later. See <a href="http://blogs.mathworks.com/steve/2007/08/23/upslope-area-handling-plateaus-revisited/" rel="nofollow">this post</a> for details. Felix—I discovered this problem after I originally wrote the post above, and I revisited plateau handling later. See this post for details.

]]> http://blogs.mathworks.com/steve/2007/06/08/upslope-area-part-5/#comment-20887 Felix Fri, 18 Jul 2008 15:10:31 +0000 http://blogs.mathworks.com/steve/2007/06/08/upslope-area-part-5/#comment-20887 The example a = 120 118 123 120 118 125 119 119 126 being filled using roifill to give b = 120.0000 118.0000 123.0000 120.0000 120.5000 125.0000 119.0000 119.0000 126.0000 is hydrologically not sensible, as it introduced a local watershed at the center pixel. While in a, the water would be most likely to leave the 3x3 area through the 1,2 pixel, in b there are two pour points 1,2 and 3,2, so you wouldn't want that. Instead you would want to assign a flow direction of the nearest pixel with the same elevation. For sinks (local minima) you would fill the sink to the minimum neighbouring elevation and assign it its flow direction. This way you make sure you dont change any hydrological properties. The example
a =
120 118 123
120 118 125
119 119 126
being filled using roifill to give
b =
120.0000 118.0000 123.0000
120.0000 120.5000 125.0000
119.0000 119.0000 126.0000
is hydrologically not sensible, as it introduced a local watershed at the center pixel. While in a, the water would be most likely to leave the 3×3 area through the 1,2 pixel, in b there are two pour points 1,2 and 3,2, so you wouldn’t want that. Instead you would want to assign a flow direction of the nearest pixel with the same elevation.
For sinks (local minima) you would fill the sink to the minimum neighbouring elevation and assign it its flow direction. This way you make sure you dont change any hydrological properties.

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