let landmark be at L
ZN = 100, NL = x (say)
tan 54 = (100+x)/H
100 + x = H tan 54 -------------- (1)
------------------------------
tan 61 = H/x ................ [alternate angle]
x = H cot 61 ------------------------- (2)
put x from (2) into (1)
H cot 61 + 100 = H tan 54
H = 100 / [tan 54 -cot 61]
H = 100 / [1.376 - 0.554]
H = 121.65 feet
