Mathematics for IGCSE & O level - Pythagoras' Theorem And Trigonometry (Section 1 - No. 12)
The angle of elevation and angle of depression are equal because:
They are complementary angles
They are supplementary angles
They are alternate interior angles
They are corresponding angles
Uitleg
Alternate interior angles are equal when formed by parallel lines (the horizontal plane) cut by a transversal (the line of sight).