&args cover boundgrid outgrid
/* This grids a line nicely
/* It oversamples by a factor of 5, then reselects those cells that
/* have contain at least 5 subcells.
/* This gives us a degree of thinning, and does it better than anyone else.
/* It contains code pulled out of gridriv

/* cover is a river coverage to be converted to a grid
/* boundgrid is any grid used to establish window and cellsize
/* This version captures intersections better.
&type boundriv.aml Version 0.0 7/9/99
&type Harvey Greenberg, University of Washington hgreen@u.washington.edu
&if [show program] ne 'GRID' &then
  &RETURN ERROR in gridnice.aml: This must be started from GRID
&if ^ [exists %boundgrid% -grid] &then
  &RETURN ERROR in gridnice.aml: The grid %boundgrid% (argument 2) does not exist.
&if  [exists %outgrid% -grid] &then
  &RETURN ERROR in gridnice.aml: The grid %outgrid% (argument 3) already exists.

&s tag [after [date -ftag]  '.']

&describe %boundgrid%    /* so we can determine the cell size
setwindow %boundgrid%
setcell [calc %grd$dx% / 5] /* temporarily work on a finer grid
tmpgrd%tag%  = ^ isnull(LINEGRID(%cover%))
tmpcnt%tag% =  aggregate(tmpgrd%tag% ,5,sum,#,data)
setcell maxof
%outgrid% = con(tmpcnt%tag% ge 3,1)
/* kill tmpgrd%tag%
/* kill tmpcnt%tag%

&type You have a thinnish river mask name %outgrid%
&type Your grid window is [show setwindow]