# Formula to compute echotop converting from meter to MF (1,000ft) :
#    echotop = (retop * 3.281) / 1000
# echotop unit : MF (1,000 Ft)
# sgt function is used to filter out grid points whose values are
# less than the tolerant value : 0.1 foot.
# E.g.:  sgt(retop * 3.281, 0.1)
tolerance="0.1"