As shown in the diagram, vertical leveling staffs are set up at points A and B. The level instrument is placed at point M. The reading on the A staff at the center crosshair is $a'_{M}=1.355m$, and the reading on the B staff at the center crosshair is $b'_{M}=1.455m$. When the level instrument is moved to point N, the reading on the A staff is $a'_{N}=1.585m$, and the reading on the B staff is $b'_{N}=1.485m$. What is the correct height difference $h_{AB}$ between points A and B?<image 1>
['-0.01m', '+0.02m', '0', '+0.01m']