an automatic steering of an agricultural tractor on a surface of a field to be processed is accomplished by equipping the tractor with a camera for taking a picture of a region of the field , which is in front of the tractor , including the surface that is to be processed , generating a pixel data file from a picture signal of the camera , generating texture information with respect to the texture of the surroundings of the pixels contained in the data file , classifying the pixels of the pixel data file while considering the texture information , in order to generate a binary information as to whether or not a particular pixel is to be associated with the surface that is to be processed , generating a steering signal based on the results of the classification , and repositioning the steering device of the harvesting machine according to the steering signal so that the machine is steered automatically along the surface that is to be processed .