How will you measure the height of an inaccessible object using triangulation method?

Explanation

An object whose distance from the point of observation can not be measured is called inaccessible object e.g. a tree or building on the other side of a river .
Let AB be an inaccessible tower of height h. Let `alpha` and `beta` be the angles of elevation subtended at observation points C and D respectively. The distance `DC=d` can be measured directly with the help of metre rod or measuring tape
From `DeltaACB, cot alpha=(l)/(h)`.............(i)
From `DeltaADB cot beta =(l+d)/(h)` ............(ii)
Subtracting (i) from (ii)
`(cot beta-cot alpha)=(l+d)/(h)-(l)/(h)=(d)/(h)` or `h=(d)/((cotbeta-cotalpha))`
Now by knowing the values of `alpha, beta` the height h of inaccessible object can be calculated