This is what trigonometry was invented for: a height nobody can climb, measured from a distance anybody can walk. Stand back a measured distance, sight the top, read the angle — the angle of elevation, taken up from the horizontal. Looking DOWN from a height instead gives the angle of depression, taken down from the horizontal. The two are equal for the same pair of points, because horizontal lines are parallel and a slanted line crossing them cuts equal alternate angles. One sightline, two names, one triangle.

Both givens — the ground distance and the height — are LEGS, so both problems belong to $\tan\theta = \dfrac{o}{a}$. Which way the algebra runs depends only on where the unknown sits in the fraction: on top, multiply, $o = a\tan\theta$; underneath, divide, $a = \dfrac{o}{\tan\theta}$.

And once both legs are known, the sightline itself is free: $c = \sqrt{a^2 + b^2}$ closes the triangle with no angle needed at all. Push the units through every rearrangement you write — a bare ratio added to a length is dead on arrival. Units that cancel correctly never prove you right, but units that refuse to cancel prove you wrong, no appeal.