I would let $E$ be the elevation of the observer, $h$ be the height of the elevator above the ground, and $w$ be the horizontal distance of the observer from the elevator shaft.
From the diagram, we see we mat then state:
\(\displaystyle \tan(\theta)=\frac{h-E}{w}\)
Differentiating with respect to time $t$ (recognizing that $\theta$ and $h$ are the only variables that change with time), we find: