STADIA REDUCTION DIAGRAM Enter on the horizontal scale with the value of the stadia reading (the interval times the stadia constant) and run vertically upward to in- tersection with line representing the vertical angle. The location of this point with reference to the dotted lines marked "ONE" etc. gives the correction to be subtracted from the entering value, and to which "f + c" (usually about 1 foot) must be added to obtain the true horizontal distance. The reading on the vertical scale plus about 0.1 ft. for each 5° of vertical angle |(f + c) sin α| is the true vertical dis- tance. If the vertical angle is beyond the range of diagram, use H = R cos²α + 1 to find true horizon- tal distance and V = R × ½ sin 2α for true vertical distance, where R equals ob- served stadia distance and α equals vertical angle. 90 85 80 75 70 65 60 55 50 45 40 35 30 25 20 15 10 5 0 90 85 80 75 70 65 60 55 50 45 40 35 30 25 20 15 10 5 300 350 400 450 500 VERTICAL DISTANCE STADIA READING X STADIA CONSTANT 14