When a tram gauge is used to measure the underhood dimensions of this Mercedes the diagonals are within 2mm and the lengths are within 1mm. 

Notice what happens when we measure in XYZ. The green data (Point #4) shows the sag (Height) is -17mm. The corner is low by 17mm! Tram gauges are 2 dimensional and cannot measure height. Only true XYZ (3-D) dimensions give the entire information.