flag = 0: grid (x,y) same units as width, Cartesian distance calculation.
flag = 1: grid (x,y) in degrees, width in kilometers, Cartesian distance calculation.
flag = 2: grid (x,y) in degrees, width in km, dx scaled by cosine(middle y), Cartesian distance calculation.
The above options are fastest because they allow weight matrix to be computed only once. The next two options are slower because they recompute weights for each East-West scan line.
flag = 3: grid (x,y) in degrees, width in km, dx scaled by cosine(y), Cartesian distance calculation.
flag = 4: grid (x,y) in degrees, width in km, Spherical distance calculation.
grdfilter north_pacific_dbdb5.grd -Gfiltered_pacific.grd -Fm600 -D4 -R150/250/10/40 -I0.5 -V