A surface (104) is detected repeatedly be a detector row (102) of at least one detector (100), the direction of the detector row (102) being the same as the surface's (104) primary direction of movement, and simultaneously a distance between the surface (104) and the detector (100) is detected to produce enlargement data and response rows. Successive response rows are arranged into a response matrix, and the direction of at least one curve in the matrix is determined. Translation of the surface (104) is determined in response matrices formed by means of the direction or directions of at least one curve or curve portion on the basis of enlargement data.