Figure 8-5.-Stadia Interval

Figure 8-4.-(A) Angle of elevation and (B) angle of depression. The instrument constant is the same for all readings. Suppose that you are using an externally focusing instrument with an instrument constant of 1.0. If the stadia interval is 1 foot, then the horizontal distance is as follows: h = (100)(1) + 1 = 101 feet. If the stadia interval is 2 feet, the horizontal distance is as follows: h = (100) (2) + 1 = 201 feet. Now suppose that you are using an internally focusing instrument. In this case, the instrument constant is zero and can be disregarded. This is the advantage of an internally focusing telescope. So, if the stadia interval is 1 foot, the horizontal distance is simply the stadia distance which is 100 feet. For a stadia reading of 2 feet, the horizontal distance is 200 feet. Horizontal distance usually is stated to the nearest foot. Occasionally on short distances (under 300 feet), it maybe specified that tenths of a foot be used. Stadia Formulas for Inclined Sights.— -Most often the sights needed in stadia work are not horizontal. It may be necessary to incline the telescope upward or downward at a vertical angle. This vertical angle (a) may be either an angle of elevation or an angle of depression, as shown in figure 8-4. If the line of sight is elevated above the horizontal, you speak of it as an angle of elevation. If the line of sight is depressed below the horizontal, the vertical angle is an angle of depression. In either case, you find the horizontal and vertical distances by using the following formulas: These two expressions are called the stadia formulas for inclined sights in which h⁼ v⁼ h = a = f + c = horizontal distance vertical distance stadia distance vertical angle instrument constant Refer to figure 8-5 for clarification of the terms in the stadia formulas for inclined sights. Figure 8-5.-Stadia Interval—inclined sight. 8-5