TRIGONOMETRIC LEVELING

Figure 7-1.—Diagram of a two-base altimeter survey. communication, a timepiece is needed for each altimeter. These timepieces are synchronized, and the altimeter readings are taken at prearranged intervals. Figure 7-1 shows a diagram of the two-base method when three altimeters are used. This figure shows the known elevations of the lower (Sta. A) and upper (Sta. B) base stations. Altimeter readings at each of the base stations and at field station C are also shown. The difference in elevation is computed by direct proportion, using either the lower base or the upper base as reference. For example, to find the differences in elevation between Sta. A and Sta. C, we proceed as follows: Then this result is added to the elevation of Sta. A, as shown in solution No. 1, figure 7-1. If we use the upper base as a reference, you compute the difference in elevation by using the same method; but to compute from Sta. B, subtract the result, as shown in solution No. 2, figure 7-1. For a more accurate result, altimeter surveys should be made on days when there is not much variation in barometric pressure. Windy days when detached clouds are traveling rapidly should be avoided because alternating sunlight and shade over the survey area can cause fluctuations in the altimeter reading. Steady barometric pressures generally occur on days with gentle winds and an overcast sky. The recommended time for observations is 2 to 4 hours after sunrise and 2 to 4 hours before sunset. Midday observation must be avoided if possible. Remember, you must shade the instrument at all times, and you must avoid jarring the instrument suddenly during its transfer from one station to another. TRIGONOMETRIC LEVELING When you know the vertical angle and either the horizontal or slope distance between two points, you can apply the fundamentals of trigonometry to calculate the difference in elevation between the points. That is the basic principle of trigonometric leveling. This method of indirect leveling is particularly adaptable to rough, uneven terrain where direct leveling methods are impracticable or too time consuming. As in any survey, the equipment that you will use in trigonometric leveling depends on the precision required. For most trigonometric-leveling surveys of ordinary precision, angles are measured with a transit, or alidade, and distances are measured either with a tape or by stadia, which you will study in chapter 8. On reconnaissance surveys the vertical angles may be measured with a clinometer, and distances maybe obtained by pacing. The method used in trigonometric leveling is described in the following paragraphs: 7-2