COMPASS RICH-1 is a large size gaseous Cherenkov detector. Two reflecting spherical surfaces, covering a total area of about 21 m 2 hosted in the radiator vessel provide image focusing. The segmented surfaces are formed by 116 mirrors, with individual degrees of freedom for angular adjustment. An original method has been developed and applied for the on-line monitoring of the mirror alignment. Relative tilts between neighbouring mirror elements create discontinuities in the image of continuous lines reflected by the mirror wall. The measurement of the discontinuities can also provide information about the absolute tilt with a resolution of 0.1 mrad. An independent check and calibration of this method has been performed in laboratory using, as reference, a laser interferometer. We present the technical solutions adopted to implement the dedicated setup, the results of the method calibration by laser interferometer, and the preliminary results obtained measuring the alignment of the RICH-1 mirror wall.