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Figure  13-12.-Sample  field  notes  from  a  deflection  angle  transit-tape  survey.
Measuring Angles by Repetition

Engineering Aid 3 - Beginning Structural engineering guide book
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Figure 13-13.-Sample field notes for closing the horizon. The instrumentman used the same method at each traverse station, working clockwise around the traverse to station E. Note that the algebraic sum of the measured deflection angle (angles to the right considered as plus, to the left as minus) is 350°59´. For a closed traverse, the algebraic sum of  the  deflection  angles  from  the  standpoint  of pure geometry is 360°00´. Therefore, there is an ANGULAR  ERROR  OF  CLOSURE  here  of 0°01´.    This   small   error   would   probably   be considered  a  normal  error.  A  large  variance  would indicate a larger mistake made in the measure- ments. In  the  example  just  presented,  the  general accuracy  of  all  the  angular  measurements was   checked   by   comparing   the   sum   of   the deflection   angles   with   the   theoretical   sum. The   accuracy   of   single   angular   measurement can   be   checked   by   the,   procedure   CLOSING THE   HORIZON.   The   method   is   based   on the  fact  that  the  theoretical  sum  of  all  the angles  around  a  point  is  360°00´. The   field   notes   in   figure   13-13   show   the procedure  for  closing  the  horizon.  The  transit  was set up at station A, and angle BAC was turned, measuring   51°15´.   Then   the   angle   from   AC clockwise  around  to  AB  was  turned,  measuring 308°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 Angles The vertical circle and the vertical vernier of a  transit  were  discussed  in  chapter  11  of  this training  manual.  They  are  used  for  measuring vertical angles. A vertical angle is the angle measured vertically from  a  horizontal  plane  of  reference.  (See  fig. 13-14, view A.) When the telescope is pointed in the  horizontal  plane  (level),  the  value  of  the vertical  angle  is  zero.  When  the  telescope  is pointed  up  at  a  higher  feature  (elevated),  the vertical angle increases from zero and is a PLUS VERTICAL  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   that it  is  below  the  horizontal  plane,  is  a  MINUS VERTICAL  ANGLE  or  ANGLE  OF  DEPRES- SION. These numerical values increase from 00 to  –90°  when  the  telescope  is  pointed  straight down. To measure vertical angles, you must set the transit upon a definite point and level it. The plate bubbles must be centered carefully, especially for transits that have a fixed vertical vernier. The line of sight is turned approximately at the point; the horizontal axis is clamped. Then, the horizontal cross hair is brought exactly to the point by means of the telescope tangent screw. The angle is read 13-12







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