def build_lower_tube_envelope(self, arc_lengths, land_elevations):
land_elevation_troughs_indices_tuple = scipy.signal.argrelmin(
land_elevations, order=10)
land_elevation_troughs_indices = \
land_elevation_troughs_indices_tuple[0].tolist()
lower_tube_envelope = \
smoothing_interpolate.bounded_curvature_extrema_interpolate(
arc_lengths, land_elevations, land_elevation_troughs_indices,
parameters.MAX_VERTICAL_CURVATURE)
return lower_tube_envelope