For long profiles, it requires a correction for the curvature
of the Earth’s surface (the horizon): when the correction is
applied, the true slope is steeper than the measured apparent
slope.
● When the horizon is not visible (e.g., in a lake, behind a
tall dune, or on a foggy day), the approximate distance to
a reference point must be known.
● Errors accumulate because elevation is obtained from the
sum of differences of pairs of readings.