An arrangement includes an installation platform arranged to be movable upwards and downwards in a first direction in the elevator shaft, and two laser transmitters arranged at a bottom of the elevator shaft. Each of the two laser transmitters produce a vertical upwards directed laser plane in the elevator shaft, the laser planes being perpendicular to each other. At least four primary light sensitive detectors are attached to the installation platform so that they are visible to the laser transmitters, each of the primary light sensitive detectors being positioned on a respective side of a rectangle. The position of the installation platform in relation to the elevator shaft can be measured from the hitting points of the laser planes on the primary light sensitive detectors.