r.bilinear(1)	       Grass User's Manual	    r.bilinear(1)



NNAAMMEE
       rr..bbiilliinneeaarr - bilinear interpolation utility for raster map
       layers.

SSYYNNOOPPSSIISS
       rr..bbiilliinneeaarr  [--qq]  iinnppuutt==_n_a_m_e   oouuttppuutt==_n_a_m_e   [nnoorrtthh==_v_a_l_u_e]
       [ssoouutthh==_v_a_l_u_e]

DDEESSCCRRIIPPTTIIOONN
       _r_._b_i_l_i_n_e_a_r fills a grid cell (raster) matrix with interpo-
       lated values generated from a  set  of  input  layer  data
       points.	It uses the bilinear interpolation method, a sim-
       ple algorithm usually applied only to  completely  defined
       raster  areas (input data void of null data values).  Here
       the values of 4 input cells are used to define an interpo-
       lation function of constant gradient within each rectangu-
       lar area defined by the	cell  centers.	 User  should  be
       aware  that  the  gradient  of the interpolation functions
       changes discontinuously across lines intersecting the cell
       centers of the input raster.


       If there is a current working mask, it applies to the out-
       put raster file.  Only those cells falling within the mask
       will  be  assigned interpolated values.	The procedure for
       selection of input data will consider all input data rele-
       vant  to  interpolating	values at the cell centers of the
       current geographic region, ignoring the curent mask.  Note
       that  cells of the output raster that cannot be bounded by
       4 input cell centers are set to null.


       The command line input is as follows:



   FFllaaggss::
       --qq     specifies that _r_._b_i_l_i_n_e_a_r run  quietly  (supressing
	      the  printing  of program messages to standard out-
	      put)




   PPaarraammeetteerrss::
       iinnppuutt==_n_a_m_e
	      Name of an input raster map layer  containing  data
	      values to apply in the interpolation.

       oouuttppuutt==_n_a_m_e
	      Name  to	be assigned to new output raster map that
	      represents the surface generated from the data val-
	      ues in the input layer.

       nnoorrtthh==_v_a_l_u_e
	      Input raster value for the north pole (90N).  Valid
	      for longitude-latitude grids only.

       ssoouutthh==_v_a_l_u_e
	      Input raster value for the south pole (90S).  Valid
	      for longitude-latitude grids only.


NNOOTTEESS
       The nnoorrtthh and ssoouutthh parameters have been included to allow
       for specific input values to  be  assigned  to  the  north
       and/or  south  poles  for longitude-latitude grids.  These
       data, if included, are  used  to  interpolate  values  for
       cells  that  are north or south of a line intersecting the
       cell centers of the first or last row  of  input,  respec-
       tively.	 When  utilized, the interpolation procedure will
       be continuous from the north and/or south boundary of  the
       current	geographic  region.   This  option  is necessary,
       since the data structure defining a raster will not  allow
       for  data to be assigned to a cell centered at 90N or 90S.
       By including the option, the user can create  output  sur-
       faces that are continuous between the poles.  The interpo-
       lation will be "wrap-around" from  west	to  east  (across
       latitude)  only if the input raster has an east edge iden-
       tical to its west edge.


       For longitude-latitude databases, the interpolation  algo-
       rithm  is  based  on degree fractions, not on the absolute
       distances between cell centers.	Any attempt to	implement
       the  latter  would violate the integrity of the interpola-
       tion method.


       _r_._b_i_l_i_n_e_a_r may be used in some instances as an alternative
       to  the	nearest neighbor approach inherent to _r_._r_e_s_a_m_p_l_e.
       Note, however, that the extent of non-null  data  area  of
       the  output  raster  must  be  less than that of the input
       raster.	The only exception to this  occurs  in	the  case
       where the nnoorrtthh and ssoouutthh parameters are utilized for lon-
       gitude-latitude rasters.



SSEEEE AALLSSOO
       _r_._s_u_r_f_._i_d_w

       _r_._s_u_r_f_._i_d_w_2

       _g_._r_e_g_i_o_n

       _r_._r_e_s_a_m_p_l_e

AAUUTTHHOORR
       Greg Koerper

       ManTech Environmental Technology, Inc.

       Global Climate Research Project

       U.S. EPA Environmental Research Laboratory



       _L_a_s_t _c_h_a_n_g_e_d_: _$_D_a_t_e_: _2_0_0_2_/_0_2_/_2_5 _1_3_:_0_3_:_5_4 _$



GRASS 5.0					    r.bilinear(1)
