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CHAPTER 7 INDIRECT LEVELING/LEVEL AND TRAVERSE COMPUTATIONSAs you know, leveling is the surveying operation that determines the difference in elevation between points on the earths surface. This operation is divided into two major categories: direct leveling and indirect leveling. From your study of the EA3 TRAMAN, you should, by now, be familiar with the methods and procedures used in direct leveling. In this chapter you will be introduced to the theory and basic procedures used in indirect leveling.You also learned in the EA3 TRAMAN that perfect closure in level nets and traverses is seldom, if ever, obtained. There is nearly always a certain amount of linear or angular error. When this error exceeds a prescribed amount, then the level net or traverse must be rerun. However, when the error is within the specified allowable limits, then certain adjustments can be made. In this chapter you will study those adjustments and the calculations needed to make the adjustments. Also discussed in this chapter are various methods that you can use to determine the area of traverses.INDIRECT LEVELING Indirect methods of leveling include barometric leveling and trigonometric leveling. A discussion of these methods is discussed in the following paragraphs.BAROMETRIC LEVELING Barometric leveling makes use of the fact that differences in elevation are proportional to differences in the atmospheric pressure. Therefore, when you read the atmospheric pressure with a barometer at various points on the earths surface, you have a measurement of the relative elevation of these points. A mercurial barometer, aneroid barometer, or sensitive altimeter may be used for this purpose. However, the mercurial barometer is too cumbersome to take out into the field. Barometric leveling is used mostly in reconnaissance surveys where differences in elevations are large; for example, in mountainous regions. Elevations determined by barometric leveling probably are several feet in error even after they are corrected for the effects of temperature and humidity. These errors are caused by the day-to-day pressure fluctuations, even bysingle-base, the two-base, and the leapfrog. The single-base method requires a minimum number of observers and less equipment. However, the method needs a series of corrections and is neither as practical nor as accurate as the other two. The two-base method is generally accepted as the standard method for accuracy and is the one most widely used. It requires fewer corrections than the single-base method. The leapfrog method uses the same type of corrections as the single-base, but the altimeters are always in close relationship to each other and are operating under reasonably similar atmospheric conditions. The results of the leapfrog method are more accurate than the single-base method and compare favorably with the two-base method.The two-base method will be described here only to give you an idea of how this system works. There are several factors and limitations that must be observed in barometric leveling, which are beyond the scope of this training manual. For actual barometric leveling, you should consult the instruction manual that goes with the instrument. The theory of two-base barometric leveling is explained below.In the two-base method, you need at least three altimeters, one at each lower and upper base where elevations are known initially and one or more altimeters roving where elevations are needed between the upper and lower base elevations. Obviously, for this operation, points of unknown elevations to be determined must lie in heights within the range of the elevations of the lower and upper base stations. The readings of the altimeters at the unknown elevations are taken at the same instant that both the upper and the lower base altimeters are read. When there is no radio
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 |
 
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