## DESCRIPTION

r.bilinear 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 completely 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 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 output 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 relevant 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.

## NOTES

The north and south 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, 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 centers. Any attempt to implement the latter would violate the integrity of the interpolation method.

r.bilinear may be used in some instances as an alternative to the nearest neighbor approach inherent to r.resample. 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 north and south parameters are utilized for longitude-latitude rasters.

## SEE ALSO

r.surf.idw, r.surf.idw2, g.region, r.resample, r.resample

## AUTHOR

Greg Koerper
ManTech Environmental Technology, Inc.
Global Climate Research Project
U.S. EPA Environmental Research Laboratory

Last changed: $Date: 2003/04/30 15:17:25 $