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negative values in weight raster for flow a...   Kyle Thomas Jan 11, 2008
Re: negative values in weight raster for fl...   William Huber Jan 11, 2008
Re: negative values in weight raster for fl...   Kyle Thomas Jan 11, 2008
Re: negative values in weight raster for fl...   William Huber Jan 11, 2008
Re: negative values in weight raster for fl...   Kyle Thomas Jan 14, 2008
Re: negative values in weight raster for fl...   William Huber Jan 14, 2008
Re: negative values in weight raster for fl...   Kyle Thomas Jan 14, 2008
Re: negative values in weight raster for fl...   Dan Patterson Jan 14, 2008
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Subject negative values in weight raster for flow accumulation 
Author Kyle Thomas 
Date Jan 11, 2008 
Message We are doing some hydrologic modeling and using the flow direction/flow accumulation tools in the hydrology toolset. Does the weighting function allow negative values in flow accumulation? I am entering negative values, but the fa routine doesn't seem to be responding as expected with the negative values entered. The help menu doesn't shed any insight. Thank you!

Kyle E. Thomas, P.E.
Principal
Natural Systems Engineering 
   
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Subject Re: negative values in weight raster for flow accumulation 
Author William Huber 
Date Jan 11, 2008 
Message Kyle,

Many accumulation-like procedures on weighted graphs do not work correctly with negative values. If you find that's the case, you can still get the desired result by computing two flow accumulations involving positive values only. Let -w be equal to or less than the most negative weight in the grid (w is positive). For any weight raster [G] let f([G]) stand for the flow accumulation grid. Note that both [G] + w and w (a grid of constant values) have non-negative weights, so that you will be able to compute f1 = f([G]+w) and f0 = f(w). The grid f1 - f0 does the trick, because the flow accumulation procedure is a linear function of the weights. 
  --Bill Huber
Quantitative Decisions (http://www.quantdec.com )
More GIS Q&A at http://gis.stackexchange.com/q/3083/664 
   
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Subject Re: negative values in weight raster for flow accumulation 
Author Kyle Thomas 
Date Jan 11, 2008 
Message Bill,

Thank you. One thing I am not clear on. I have a weight raster that contains both positive and neagtive values. I am clear that I create a raster, w, of positive values equal to the absolute value of the lowest value in the weight raster. In your example what is [G]? How does my weight raster of both positive and negative values relate to your weight grid [G] of all positive values? Thanks again. 
   
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Subject Re: negative values in weight raster for flow accumulation 
Author William Huber 
Date Jan 11, 2008 
Message Kyle,

I apologize for being vague. [G] is the weight raster and -w is any number greater than or equal to the minimum value of [G] (equivalently, w is greater than or equal to the maximum value of -[G]). Thus [G] does not have all positive values. However, the values in [G] + w are non-negative (by definition of w) and the values of the constant grid w are also non-negative. 
  --Bill Huber
Quantitative Decisions (http://www.quantdec.com )
More GIS Q&A at http://gis.stackexchange.com/q/3083/664 
   
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Subject Re: negative values in weight raster for flow accumulation 
Author Kyle Thomas 
Date Jan 14, 2008 
Message Seems obvious now. Thanks! 
   
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Subject Re: negative values in weight raster for flow accumulation 
Author William Huber 
Date Jan 14, 2008 
Message Hi Kyle,

Please let us know whether this solution works for you, because in the future others with similar questions may be interested in this thread. 
  --Bill Huber
Quantitative Decisions (http://www.quantdec.com )
More GIS Q&A at http://gis.stackexchange.com/q/3083/664 
   
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Subject Re: negative values in weight raster for flow accumulation 
Author Kyle Thomas 
Date Jan 14, 2008 
Message Yes, it appears to have worked. Thanks again! 
   
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Subject Re: negative values in weight raster for flow accumulation 
Author Dan Patterson 
Date Jan 14, 2008 
Message then please logon and indicate which thread solved your problem, people sometimes only tracked threads and/or read answers which are flagged as such. also, see
http://forums.esri.com/help/help.asp?c=93#mvp
for more information 
  Geomatics, Carleton University, Ottawa, Canada