Figure 8-9.-Difference in elevation.you can see, when the center cross hair is sighted on arod graduation that is equal to the h.i. before reading thevertical angle, then calculating the difference inelevation is greatly simplified. Obviously, though, if theline of sight is obstructed and you cannot sight on a rodgraduation that is equal to the hi., then you must sighton some other graduation.Another, although less frequent, occurrence intopographic work using stadia is shown in figure 8-9,view B. In this situation it is desired to determine thedifference in elevation between two points on theground (P_{1 }and P_{2}) from an instrument station (E) thatis located between the two points.For this discussion, let us assume that a backsight istaken on a rod held at P_{1 }and then a foresight is taken toP_{2}. Now the difference in elevation (DE) between thetwo points can be written as follows:In reverse, if a backsight was taken to P_{2 }with a foresightto P_{1}, then the expression for DE can be written asfollows:8-9