A person observes the top P of a vertical tower OP of height h from a station $S_1$ and finds that ${\beta }_1$ is the angle of elevation. He moves in a horizontal plane to second station S2 and finds that $\angle P{S}_2{S}_1$ is ${\gamma }_1$ and the angle subtended by $S_2{S}_1$ at P is ${\delta }_1$ and the angle of elevation is β2. He moves again to a third station $S_3$ such that $S_3{S}_2=S_2{S}_1,\mathit{ }\angle P{S}_3{S}_2={\gamma }_2$ and the angle subtended by $S_3{S}_2$ at P is ${\delta }_2$. then 

$\frac{\mathit{sin}{\gamma }_1\mathit{sin}{\beta }_1}{\mathit{sin}{\delta }_1}=\frac{\mathit{sin}{\gamma }_2\mathit{sin}{\beta }_2}{\mathit{sin}{\delta }_2}=\frac{h}{S_1{S}_2}$