Image Rectification

If you know the positions of four or more points on a flat surface you can transform the image so as to obtain a perfect plan view of the surface. Measurements can then be taken from this image.

Image with 5m calibration square.
The positions of the four corners are used to compute the transformation required.

Rectified image.
Note that objects that are not in the plane of the road surface, such as the dunes appearing at the top, will be distorted. Measurements can only be applied to the road surface.