1. A method for monitoring a spatial region comprising a number of movable objects and a structure having a plurality of relatively closely spaced substantially parallel edges, 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 each of the first and second image recording units, determining a position of said structure on the basis of the first and second images, wherein the position of said structure is defined by a spatial distance of said structure relative to each of the image recording units, and generating a switching signal depending on the object positions, wherein a number of defined reference marks are placed in the spatial region proximate to said structure, wherein a number of reference distances between the first and second image recording units and the reference marks is determined, and wherein the spatial distance of said structure relative to each of said image recording units is determined on the basis of the reference distances.