Method and apparatus for calibrating a positioning system including a control computer (1405) for producing command value signals to a positioning apparatus which positions an object against a work surface. Sensors (1406, 1407) detect the location of the positioning system and convert it to an actual value signal which can be fed back to the control computer. A calibration plane (601) placed on the work surface contains a number of calibration points (602, 603), the command value positions of which are entered into the control computer. The positions of at least some calibration points are sensed by a calibration probe (605) corresponding to the object and the actual values for the positions of said points are compared with the command values for the positions of these points on the work surface entered into the control computer. The differences obtained between the actual values and the command values for the positions of the calibration points are stored in the control computer and are used for compensation of the errors of the positioning system by interpolation. The calibration plane preferably consists of a calibration board of electrically insulating material and the calibration probe is an electrically conducting component, the centre of which is placed on the calibration board straight above a point, the position of which corresponds to the command value for a calibration point entered into the control unit. The position of the centre of the component in one coordinate direction in relation to the calibration point is determined by placing the component in two extreme positions located in the coordinate directions on either side of the calibration point, in which positions the component electrically connects test areas on the calibration board, said areas being connected to detector means and being eletrically conducting, whereafter the detected actual value of the position of the calibration point is compared with the command value, entered into the control unit, and any difference that may occur is entered into the control unit as a correction.