$21.$ A tilting level is used for differential leveling. You are told that it has a collimation error of $0\mathrm{.}002.$ You set up the device in the middle of points A and B that are $156$ m apart and measure the difference in elevation. You establish that the elevation of point A is $202\mathrm{.}3$ m and the elevation of B is $103\mathrm{.}9$ m $.$ Therefore, the difference in elevation is $1\mathrm{.}6$ m $.$ What is the actual difference in elevation between points A and B $?[2\mathrm{.}5$ pt$]$