How tall is that tree? Walk 10 m back. Measure the angle from your eye to the top of the tree. Tangent does the rest.
Tree height
h = d × tan(θ) + eye-height
d = horizontal distance, θ = angle from horizontal
Try it
10 m back, angle 35°, eye 1.6 m up
h = 10 × tan(35°) + 1.6 ≈ 10 × 0.700 + 1.6 = 8.6 m.
A protractor + a drinking straw + a weight on a string = a working clinometer. Schools have built these for a hundred years.