Ammonia is lighter than air and collects at the ceiling (or the highest point of the room). Therefore the gas detector/sensor should be mounted on or at the ceiling. If you were to mount the detector at breathing height (1.2 - 1.8 m / 4 - 6 ft from the floor), the concentration of ammonia would start at the ceiling and move down towards the breathing height as more and more leaked and collected. By the time the ammonia level was at breathing height, the room would be so full of ammonia, not only would it be a breathing hazard, but also an explosion/fire hazard, not to mention the high cost of losing all that ammonia. The goal is to be alerted of an ammonia leak as soon as possible so safety measures can be taken to stop and repair it.
NOTE: In the presence of moisture (such as high relative humidity), ammonia gas forms vapour that is heavier than air and may spread along areas with poor air flow where people may be exposed.