Figure 13-13.-Sample field notes for closing the horizon.The instrumentman used the same method ateach traverse station, working clockwise aroundthe traverse to station E. Note that the algebraicsum of the measured deflection angle (angles tothe right considered as plus, to the left as minus)is 350°59´. For a closed traverse, the algebraic sumof the deflection angles from the standpoint ofpure geometry is 360°00´. Therefore, there is anANGULAR ERROR OF CLOSURE here of0°01´. This small error would probably beconsidered a normal error. A large variance wouldindicate a larger mistake made in the measure-ments.In the example just presented, the generalaccuracy of all the angular measurementswas checked by comparing the sum of thedeflection angles with the theoretical sum.The accuracy of single angular measurementcan be checked by the, procedure CLOSINGTHE HORIZON. The method is based onthe fact that the theoretical sum of all theangles around a point is 360°00´.The field notes in figure 13-13 show theprocedure for closing the horizon. The transit wasset up at station A, and angle BAC was turned,measuring 51°15´. Then the angle from ACclockwise around to AB was turned, measuring308°45´. The sum of the two angles is 360°00´.The angular error of closure is therefore 0°00´,meaning that perfect closure is obtained.Measuring Vertical AnglesThe vertical circle and the vertical vernier ofa transit were discussed in chapter 11 of thistraining manual. They are used for measuringvertical angles.A vertical angle is the angle measured verticallyfrom a horizontal plane of reference. (See fig.13-14, view A.) When the telescope is pointed inthe horizontal plane (level), the value of thevertical angle is zero. When the telescope ispointed up at a higher feature (elevated), thevertical angle increases from zero and is a PLUSVERTICAL ANGLE or ANGLE OF ELEVA-TION. These values increase from 0° to +90°when the telescope is pointed straight up.As the telescope is depressed (pointed down),the angle also increases in numerical value,A depressed telescope reading, showing thatit is below the horizontal plane, is a MINUSVERTICAL ANGLE or ANGLE OF DEPRES-SION. These numerical values increase from 00to –90° when the telescope is pointed straightdown.To measure vertical angles, you must set thetransit upon a definite point and level it. The platebubbles must be centered carefully, especially fortransits that have a fixed vertical vernier. The lineof sight is turned approximately at the point; thehorizontal axis is clamped. Then, the horizontalcross hair is brought exactly to the point by meansof the telescope tangent screw. The angle is read13-12
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