CHAPTER 7 INDIRECT LEVELING/LEVEL AND TRAVERSE COMPUTATIONS As you know, leveling is the surveying operation that determines the difference in elevation between points on the earth’s 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 earth’s 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   by fluctuations from hour to hour in 1 observations  are,  therefore,  usually station  during  the  same  period  that day.  Barometric taken at a fixed observations  are made on a second barometer that is carried from point to point in the field. The use of two barometers enables you to correct for atmospheric disturbances that could not be readily detected if only one barometer were used. This  method  is  not  normally  used  in  construction surveying, except when a construction surveyor may need to run his own preliminary topographic control. Barometric or altimeter surveys are run by one of three methods: the  single-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 7-1