Indirect measurements are methods to calculate a measure when you cannot measure it directly. Normally it requieres the use of triangles properties (Pytagora's theorem, similarity of triangles, trigonometric functions, sine or cosine rules).
For example, if you measure the horizontal distance from certain point to the top of a tree, and you measure the angle of elevation from the horizontal line to the top of the tree, you can use the cosine function to determine the height of the tree.
