1. A method for monitoring a spatial region comprising a number of movable objects, the method comprising the steps of: providing a first and a second image recording unit, which are arranged at a defined distance from one another, recording a first image of the spatial region by means of the first image recording unit and recording a second image of the spatial region by means of the second image recording unit, determining a number of object positions on the basis of the first and second images, wherein each object position represents a spatial distance of an object relative to the first and second image recording units, and generating a switching signal depending on the object positions, wherein the spatial region comprises a structure having a plurality of at least substantially parallel edges, wherein a number of defined reference marks is arranged at the structure, wherein a number of reference distances between the first and second image recording units and the reference marks is determined, and wherein a structure position of the structure is determined on the basis of the reference distances.