The adjusted elevation of TBM 16 is134.851-0.003 = 134.848 ft.The adjustments for intermediate TBMs 17 and 18 aremade in a similar manner.Reciprocal LevelingThis procedure is used for either differential ortrigonometric leveling when along sight across a wideriver, ravine, or similar obstacle must be made. Thislong sight will be affected by curvature and refractionand by any small error in aligning the line of sight withthe bubble axis. The alignment error can be minimizedby balancing the long sight and computing thecurvature. The atmospheric conditions will vary somuch over an open expanse that the refractioncorrection will be quite erratic. Reciprocal leveling isdesired to minimize the effect of the atmosphere as wellas the line of sight and curvature corrections. To do this,take the following actions:1. In reciprocal leveling, balance the BSs and FSsas carefully as possible before you reach the obstacle.In figure 14-15, a TP, N, is selected close to the edge ofthe obstruction so that it is visible from a proposedinstrument location, B, on the other side. A second rodis held on the other side of the obstruction at F. Point Fshould be selected so that the equivalent distances, ANand FB, and AF and NB, are almost equal. Theinstrument is setup at point A and leveled carefully. ABS reading is taken on the N rod and an FS on the F rod.These readings are repeated several times. Theinstrument is moved to point B, set up, and carefullyleveled. The rods remain at their stations. Again, a BSis taken on the N rod and an FS on the F rod, andrepeated several times. Since instrument leveling isespecially critical on reciprocal leveling, you need tocheck the bubble before each reading and center itcarefully. If it is off-center a slight amount, theprocedure must be repeated. The difference inelevation between N and F is computed from thereadings at A setup and from the readings at B setupseparately. Because of the errors in the long sight, thetwo results will have slightly different values. Note,however, that the long sight is an FS from A and a BSfrom B. The true difference in elevation is the averageof both values, since the errors have opposite signs andwill cancel each other.Figure 14-15.-Reciprocal leveling.2. For more accuracy, make several long sightreadings for each short sight and average them. Youshould use a target on the rod and reset it for eachreading. Average each series of long sights andcombine this average with corresponding short sightsfor the computations.3. Changes in atmospheric density andtemperature affect the refraction of a line of sight. Thelonger the time interval is between reciprocal longsights, the greater the chance of an atmospheric changeand a variation in the refraction value. For this reason,you should keep the time lapse between the long sightsas short as possible.4. An excellent method of avoiding the time lapseproblem is simultaneous-reciprocal observation. Theobject is to read both long sight values at the same time.This requires two instruments and two observers andtwo rods and two rodmen. Some method ofcommunication or sequence of operations must beagreed upon.5. The note keeping for reciprocal leveling isidentical to differential leveling. Take a series of eitherBS or FS readings on the far rod from one setup andtake only one sighting on the rear rod. Average theseries of readings, and use a single value to make theelevation computations.Profile LevelingIn surveying, a PROFILE is a vertical section ofthe earth measured along a predetermined or fixedline. In practice, profiles are a series of groundelevations determined by differential leveling orother methods that, when plotted along14-15
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