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 interpolated values
       generated from a set of input layer data points.  It uses the  bilinear
       interpolation  method,  a simple algorithm usually applied only to com-
       pletely defined raster areas (input data void  of  null	data  values).
       Here  the  values  of 4 input cells are used to define an interpolation
       function of constant gradient within each rectangular area  defined  by
       the cell centers.  User should be aware that the gradient of the inter-
       polation functions changes discontinuously  across  lines  intersecting
       the cell centers of the input raster.


       If  there  is  a  current working mask, it applies to the output raster
       file.  Only those cells falling within the mask will be assigned inter-
       polated	values.   The  procedure for selection of input data will con-
       sider all input data relevant to interpolating values at the cell  cen-
       ters  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 output)




   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 values in the input layer.

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

       ssoouutthh==_v_a_l_u_e
	      Input  raster  value for the south pole (90S).  Valid for longi-
	      tude-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 longi-
       tude-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,  respectively.   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 surfaces that are continuous
       between the poles.  The interpolation will be "wrap-around"  from  west
       to  east  (across  latitude)  only if the input raster has an east edge
       identical to its west edge.


       For longitude-latitude databases, the interpolation algorithm is  based
       on  degree  fractions,  not on the absolute distances between cell cen-
       ters.  Any attempt to implement the latter would violate the  integrity
       of the interpolation method.


       _r_._b_i_l_i_n_e_a_r may be used in some instances as an alternative to the near-
       est 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 longitude-
       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)
