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photograph, and figure 11-3 shows the line map made  by  the  use  of  overlays. are all used for measuring unknown angles and distances and/or for laying off known angles and distances. BASIC  SURVEYING  INSTRUMENTS MAGNETIC   COMPASS Most  fieldwork  done  by  an  Engineering  Aid (especially at the third- and second-class levels) is likely to consist of field measurements and/or computations   that   involve   plane   surveying   of ordinary  precision.  This  section  describes  the  basic instruments, tools, and other equipment used for this  type  of  surveying.  Other  instruments  used  for more precise surveys will also be described briefly. Surveying  instruments  come  in  various  forms, yet their basic functions are similar; that is, they A  magnetic  compass  is  a  device  consisting principally  of  a  circular  compass  card,  usually graduated  in  degrees,  and  a  magnetic  needle, mounted and free to rotate on a pivot located at the  center  of  the  card.  The  needle,  when  free  from any local attraction (caused by metal), lines itself up with the local magnetic meridian as a result of  the  attraction  of  the  earth’s  magnetic  North Pole. Figure 11-3.-Line map made by overlays from the aerial photograph in figure 11-2. 11-12

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