1. A method of estimating an orientation of a camera relative to a surface, wherein a camera coordinate system is defined and a normal vector normal to the surface is represented by three components in the camera coordinate system, the method comprising: providing a first image and a subsequent second image captured by the camera; selecting a first point from the first image and a second point from the second image, wherein the first and second points represent the same object; defining a first optical flow vector connecting the first point and the second point; carrying out a first estimation step comprising estimating two components of the normal vector in the camera coordinate system by using the first optical flow vector and restricting parameter space to only the two components of the normal vector, wherein a linear equation system derived from a homography matrix that represents a projective transformation between the first image and the second image is provided and the two components of the normal vector in the camera coordinate system are estimated by solving the linear equation system; and determining the orientation of the camera relative to the surface using the results of the first estimation step.