Elevation angle is the angle above the horizontal line, and depression angle is the angle below the horizontal line. In other words, the elevation angle is looking up and the depression angle is looking down.

Elevation is online viewing: when an observer looks up at an object, the angle between his line of sight and the horizontal line is called elevation.

Depression angle is overlooking: when an observer looks down at an object, the angle formed by his line of sight and the horizontal line is called depression angle.

Geometry is the study of spatial structure and properties. It is one of the most basic research contents in mathematics, which has the same important position as analysis and algebra, and has a very close relationship. Geometry has a long history and rich contents. It is closely related to algebra, analysis and number theory.

Geometric thought is the most important thought in mathematics. The temporary development of each branch of mathematics tends to geometry, that is, to explore various mathematical theories with geometric viewpoints and thinking methods. Common theorems are Pythagorean Theorem, euler theorem Theorem and Stewart Theorem.