Surveyors gave us two tidy phrases. The angle of elevation opens UP from level ground to something high — a treetop, a tower’s tip. The angle of depression tips DOWN from level eyesight to something low — a swimmer, seen from a cliff top. Both are measured from the HORIZONTAL, never from a wall or the cliff face.

And here’s the lovely secret: they match. The depression angle from cliff down to boat EQUALS the elevation angle from boat up to cliff — one slanted sightline, two ends, and the two level lines never disagree. So both problems collapse into the same right triangle, whose two legs — height and level distance — belong to tangent. Estimate before you compute: below $45^\circ$ the height is SMALLER than the level run; above $45^\circ$, taller. Five seconds of that catches the classic upside-down slip.