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       r.slope.aspect   -  Generates  raster  maps  of  slope,  aspect,  curvatures  and  partial
       derivatives from a elevation raster map.
       Aspect is calculated counterclockwise from east.


       raster, terrain


       r.slope.aspect help
       r.slope.aspect  [-qa]  elevation=name   [slope=name]     [aspect=name]     [format=string]
       [prec=string]     [pcurv=name]     [tcurv=name]     [dx=name]     [dy=name]     [dxx=name]
       [dyy=name]    [dxy=name]     [zfactor=float]     [min_slp_allowed=float]     [--overwrite]
       [--verbose]  [--quiet]


           Do not align the current region to the elevation layer

           Allow output files to overwrite existing files

           Verbose module output

           Quiet module output

           Name of elevation raster map

           Name for output slope raster map

           Name for output aspect raster map

           Format for reporting the slope
           Options: degrees,percent
           Default: degrees

           Type of output aspect and slope maps
           Options: default,double,float,int
           Default: float

           Name for output profile curvature raster map

           Name for output tangential curvature raster map

           Name for output first order partial derivative dx (E-W slope) raster map

           Name for output first order partial derivative dy (N-S slope) raster map

           Name for output second order partial derivative dxx raster map

           Name for output second order partial derivative dyy raster map

           Name for output second order partial derivative dxy raster map

           Multiplicative factor to convert elevation units to meters
           Default: 1.0

           Minimum slope val. (in percent) for which aspect is computed
           Default: 0.0


       r.slope.aspect  generates  raster  maps  of slope, aspect, curvatures and first and second
       order partial derivatives from a raster map  of  true  elevation  values.  The  user  must
       specify the input elevation file name and at least one output file name. The user can also
       specify the format  for  slope  (degrees,  percent;  default=degrees),  and  the  zfactor:
       multiplicative factor to convert elevation units to meters; (default 1.0).

       The  elevation  input raster map specified by the user must contain true elevation values,
       not rescaled or categorized data. If the elevation values are in feet or other units  than
       meters  (with  a conversion factor meters:, defined in PROJ_UNITS), they must be converted
       to meters using the parameter zfactor.

       The aspect output raster map indicates the direction that slopes are  facing.  The  aspect
       categories  represent  the number degrees of east. Category and color table files are also
       generated for the aspect map layer. The aspect categories represent the number degrees  of
       east  and they increase counterclockwise: 90deg is North, 180 is West, 270 is South 360 is
       East. The aspect value 0 is used to indicate undefined aspect in flat areas with slope=0.

       The slope output raster map contains slope values, stated in degrees of  inclination  from
       the  horizontal  if  format=degrees option (the default) is chosen, and in percent rise if
       format=percent option is chosen.  Category and color table files are generated.

       Profile and tangential curvatures are the curvatures in the direction  of  steepest  slope
       and  in the direction of the contour tangent respectively. The curvatures are expressed as
       1/metres, e.g. a curvature of 0.05 corresponds to a radius of  curvature  of  20m.  Convex
       form values are positive and concave form values are negative.


       Example DEM

       Slope (degree) from example DEM

       Aspect (degree) from example DEM

       Tangential curvature (m-1) from example DEM

       Profile curvature (m-1) from example DEM

       For  some  applications, the user will wish to use a reclassified raster map of slope that
       groups slope values into ranges of slope. This can be done using r.reclass. An example  of
       a useful reclassification is given below:
                            (in degrees)    (in percent)
                    1         0-  1             0-  2%
                    2         2-  3             3-  5%
                    3         4-  5             6- 10%
                    4         6-  8            11- 15%
                    5         9- 11            16- 20%
                    6        12- 14            21- 25%
                    7        15- 90            26% and higher
            The following color table works well with the above
                 category   red   green   blue
                    0       179    179     179
                    1         0    102       0
                    2         0    153       0
                    3       128    153       0
                    4       204    179       0
                    5       128     51      51
                    6       255      0       0
       7         0      0       0


       To  ensure  that  the  raster  elevation  map  layer is not inappropriately resampled, the
       settings for the current region are modified slightly (for the execution  of  the  program
       only): the resolution is set to match the resolution of the elevation map and the edges of
       the region (i.e. the north, south, east and west) are shifted, if necessary,  to  line  up
       along  edges  of  the  nearest  cells  in  the elevation map. If the user really wants the
       elevation map resampled to the current region resolution, the -a flag should be specified.

       The current mask is ignored.

       The algorithm used to determine slope and aspect uses a 3x3 neighborhood around each  cell
       in  the  elevation  file.  Thus,  it is not possible to determine slope and aspect for the
       cells adjacent to the edges in the elevation map layer. These cells are assigned  a  "zero
       slope" value (category 0) in both the slope and aspect raster map layers.

       Horn's formula is used to find the first order derivatives in x and y directions.

       Only  when  using  integer  elevation models, the aspect is biased in 0, 45, 90, 180, 225,
       270, 315, and 360 directions; i.e., the distribution of aspect categories is very  uneven,
       with  peaks  at  0,  45,...,  360  categories.  When working with floating point elevation
       models, no such aspect bias occurs.

       Because most cells with a very small slope end up having category 0, 45, ..., 360,  it  is
       sometimes  possible  to reduce the bias in these directions by filtering out the aspect in
       areas where the terrain is almost flat. A new option min_slp_allowed was added to  specify
       the  minimum  slope  for  which  aspect is computed. The aspect for all cells with slope <
       min_slp_allowed is set to null.


                      Horn, B. K. P. (1981). Hill Shading and the Reflectance Map, Proceedings of
                     the IEEE, 69(1):14-47.

                      Mitasova, H. (1985). Cartographic aspects of computer surface modeling. PhD
                     thesis.  Slovak Technical University , Bratislava

                      Hofierka, J., Mitasova, H., Neteler, M., 2009. Geomorphometry in GRASS GIS.
                     In:  Hengl,  T.  and Reuter, H.I. (Eds), Geomorphometry: Concepts, Software,
                     Applications.  Developments in Soil Science, vol. 33, Elsevier, 387-410  pp,


       r.mapcalc, r.neighbors, r.reclass, r.rescale


       Michael Shapiro, U.S.Army Construction Engineering Research Laboratory
       Olga Waupotitsch, U.S.Army Construction Engineering Research Laboratory

       Last changed: $Date: 2011-11-08 03:29:50 -0800 (Tue, 08 Nov 2011) $

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