# D8 Extreme Upslope Value

(c) 2010 by David G. Tarboton
Evaluates the extreme (either maximum or minimum) upslope value from an input grid based on the D8 flow model. This is intended initially for use in stream raster generation to identify a threshold of the slope times area product that results in an optimum (according to drop analysis) stream network.
If the optional outlet point shapefile is used, only the outlet cells and the cells upslope (by the D8 flow model) of them are in the domain to be evaluated.
By default, the tool checks for edge contamination. This is defined as the possibility that a result may be underestimated due to grid cells outside of the domain not being counted. This occurs when drainage is inwards from the boundaries or areas with no data values for elevation. The algorithm recognizes this and reports "no data" for the result for these grid cells. It is common to see streaks of "no data" values extending inwards from boundaries along flow paths that enter the domain at a boundary. This is the desired effect and indicates that the result for these grid cells is unknown due to it being dependent on terrain outside of the domain of data available. Edge contamination checking may be turned off in cases where you know this is not an issue or want to ignore these problems, if for example, the DEM has been clipped along a watershed outline.

## Parameters

Number of Processes
Integer
  The number of stripes that the domain will be divided into and the number of MPI parallel processes that will be spawned to evaluate each of the stripes.
D8 Flow Direction Grid
Raster Grid
  A grid of D8 flow directions which are defined, for each cell, as the direction of the one of its eight adjacent or diagonal neighbors with the steepest downward slope. This grid can be obtained as the output of the "D8 Flow Directions" tool.
Upslope Values Grid
Raster Grid
  This is the grid of values of which the maximum or minimum upslope value is selected. The values most commonly used are the slope times area product needed when generating stream rasters according to drop analysis.
Outlets Shapefile
Point Shapefile (optional)
  A point shape file defining outlets of interest. If this input file is used, only the area upslope of these outlets will be evaluated by the tool.
Use maximum upslope value
Boolean
  A flag to indicate whether the maximum or minimum upslope value is to be calculated. Default is True.
Check for edge contamination
Boolean
  A flag that indicates whether the tool should check for edge contamination. Default is True.

## Outputs

Extreme Value Grid
Raster Grid
  A grid of the maximum/minimum upslope values.