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 correc- tion 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. STADIA READING X STADIA CONSTANT 14