Let an object be above the line of sight of an observer.
If the observer is present on a horizontal plane, the angle between this horizontal plane and the line joining the observer to the object is called the angle of elevation.
Let an object be below the line of sight of an observer.
If the object is present on a horizontal plane, the angle between this horizontal plane and the line joining the observer to the object is called the angle of depression.
A geometric right angeled triangle can be formed using the positions of the object and observer.
Therefore, trigonometric ratios can be used to calculate these angles.