1. Apparatus for monitoring a door with a 3D sensor which is able to detect distances to an object in the monitoring area as the third dimension, the 3D sensor being arranged in a housing, and immovably arranged transmission means for transmitting a measurement beam and receiver means for receiving a reflected measurement beam being provided in the housing, said apparatus further comprising a control unit which is designed to obtain an item of information relating to the position of the apparatus relative to a horizontal basic mounting position, and wherein the control unit evaluates the measurement beam on the basis of an item of position information, wherein the control unit is designed to determine a mounting height of the apparatus after mounting by evaluating the measurement beam on the basis of the item of position information, and to use a height of the mounting location as a parameter for defining, a monitoring area, wherein the sensor is adjusted to a defined monitoring volume.