laser unit for directing a laser beam in a strictly vertical direction , comprising a laser proper having a substantially vertical axis , an afocal system , an optical device for automatically correcting the verticality of the laser beam , and a base structure provided with means for supporting the assembly of said laser , afocal system and optical device in such position that the laser axis be substantially vertical . this invention is applicable to all fields and techniques in which it is desired to determine with the maximum precision the vertical at a given locus where the laser is installed .