INTERVISIBLE FEATURES
G. T. M. · Survey Review · 1936
It is frequently required to find whether a feature A of height h 0 will interrupt the view between two other features A1 and A2, of heights h 1 and h 2 respectively. Suppose that the right line from A1 to A2, whose zenith distance is ζ at A1, has a height h at A; it is then obvious that no more is necessary than to compute h and compare it with the known height h 0 of the feature A.