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.