CHAPTER 7INDIRECT LEVELING/LEVEL ANDTRAVERSE COMPUTATIONSAs you know, leveling is the surveying operationthat determines the difference in elevation betweenpoints on the earth’s surface. This operation is dividedinto two major categories: direct leveling and indirectleveling. From your study of the EA3 TRAMAN, youshould, by now, be familiar with the methods andprocedures used in direct leveling. In this chapter youwill be introduced to the theory and basic proceduresused in indirect leveling.You also learned in the EA3 TRAMAN that perfectclosure in level nets and traverses is seldom, if ever,obtained. There is nearly always a certain amount oflinear or angular error. When this error exceeds aprescribed amount, then the level net or traverse mustbe rerun. However, when the error is within the specifiedallowable limits, then certain adjustments can be made.In this chapter you will study those adjustments and thecalculations needed to make the adjustments.Also discussed in this chapter are various methodsthat you can use to determine the area of traverses.INDIRECT LEVELINGIndirect methods of leveling include barometriclevelingand trigonometric leveling. A discussion ofthese methods is discussed in the following paragraphs.BAROMETRIC LEVELINGBarometric leveling makes use of the fact thatdifferences in elevation are proportional to differencesin the atmospheric pressure. Therefore, when you readthe atmospheric pressure with a barometer at variouspoints on the earth’s surface, you have a measurementof the relative elevation of these points. A mercurialbarometer, aneroid barometer, or sensitive altimetermay be used for this purpose. However, the mercurialbarometer is too cumbersome to take out into the field.Barometric leveling is used mostly in reconnaissancesurveys where differences in elevations are large; forexample, in mountainous regions. Elevationsdetermined by barometric leveling probably are severalfeet in error even after they are corrected for the effectsof temperature and humidity. These errors are caused bythe day-to-day pressure fluctuations, even byfluctuations from hour to hour in 1observations are, therefore, usuallystation during the same period thatday. Barometrictaken at a fixedobservations aremade on a second barometer that is carried from pointto point in the field. The use of two barometers enablesyou to correct for atmospheric disturbances that couldnot be readily detected if only one barometer were used.This method is not normally used in constructionsurveying, except when a construction surveyor mayneed to run his own preliminary topographic control.Barometric or altimeter surveys are run by one ofthree methods: the single-base, the two-base, and theleapfrog. The single-base method requires a minimumnumber of observers and less equipment. However, themethod needs a series of corrections and is neither aspractical nor as accurate as the other two. The two-basemethod is generally accepted as the standard method foraccuracy and is the one most widely used. It requiresfewer corrections than the single-base method. Theleapfrog method uses the same type of corrections as thesingle-base, but the altimeters are always in closerelationship to each other and are operating underreasonably similar atmospheric conditions. The resultsof the leapfrog method are more accurate than thesingle-base method and compare favorably with thetwo-base method.The two-base method will be described here only togive you an idea of how this system works. There areseveral factors and limitations that must be observed inbarometric leveling, which are beyond the scope of thistraining manual. For actual barometric leveling, youshould consult the instruction manual that goes with theinstrument. The theory of two-base barometric levelingis explained below.In the two-base method, you need at least threealtimeters, one at each lower and upper base whereelevations are known initially and one or morealtimeters roving where elevations are needed betweenthe upper and lower base elevations. Obviously, for thisoperation, points of unknown elevations to bedetermined must lie in heights within the range of theelevations of the lower and upper base stations. Thereadings of the altimeters at the unknown elevations aretaken at the same instant that both the upper and thelower base altimeters are read. When there is no radio7-1
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