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intermediate  TPs.  In  each  case,  a  “higher”  TP (as   TP1)  and  a  “lower”  TP  (as   TP~~)  was  used, resulting in two different HIs for each. Computed by way of the higher HIs, the elevation of  BM2 came to 851.98 ft. Computed by way of the lower HIs, it came to 852.00 ft. The mean (average) of 851.99  ft  was  taken  as  the  correct  elevation. INDIRECT  LEVELING Indirect   methods   of   leveling   encompass both   trigonometric   and   barometric   leveling. TRIGONOMETRIC  LEVELING  uses  vertical angles   and   a   horizontal   distance   to   compute the  difference  in  elevation,  BAROMETRIC LEVELING  uses  the  difference  in  atmospheric pressures that are observed by a barometer or an altimeter to determine the elevation differences. Indirect methods of leveling will be discussed at the EA2 level. PRECISION  IN  LEVELING; MISTAKES  AND  ERRORS IN  LEVELING Leveling, like any other surveying operation, is carried out by following a prescribed ORDER OF PRECISION—meaning that the instruments you  use  and  the  methods  you  follow  have  to  be those that can give you the specified standard of accuracy. PRECISION  IN  LEVELING FIRST-ORDER  leveling  is  used  to  establish the  main  level  network  for  an  area  and  to  provide basic  vertical  control  for  the  extension  of  level networks   of   the   same,   or   lower,   accuracy   in support of mapping projects, cadastral (recording property boundaries, subdivision lines, buildings, etc.), and local surveys. Level lines must start and end on proven, existing BMs of the same order. New levels must be run between the starting BM being used and at least one other existing BM and must  show  there  is  no  change  in  their  relative elevations. SECOND-ORDER  leveling  is  used  to  sub- divide nets of first-order leveling and to provide basic  control  for  the  extension  of  levels  of  the same,  or  lower,  accuracy  in  support  of  mapping projects  and  local  surveys.  Second-order  levels  are divided  into  two  classes:  Class  I  and  Class  II. CLASS I is used in remote areas where the line must  be  longer  than  25  mi  because  routes  are unavailable for the development of additional or higher order networks and for spur lines. CLASS II levels are used for the development of nets in the more accessible areas. In Class I leveling, it is   required   that   all   lines   start   and   close   on previously  established  BMs  of  first  or  second order.  New  levels  have  to  be  run  between  the existing  BM  being  used  and  at  least  one  other existing BM to prove that they have not changed their relative elevations. The criteria for Class II are the same as for Class 1, except that Class II lines  are  run  in  one  direction  only. THIRD-ORDER leveling is used to subdivide an  area  surrounded  by  first-  and  second-order leveling  and  to  provide  elevations  for  the immediate control of cadastral, topographic, and construction  surveys  for  permanent  structures. The  following  criteria  should  be  observed  in  third- order  leveling: 1. All lines have to start from, and close on, two  previously  established  BMs  of  third,  or higher,  order  of  accuracy  if  the  new  leveling indicates they have not changed in their relative elevations. 2.   In   the   United   States,   third-order   lines should  not  be  extended  more  than  30  mi  from BMs of first or second order. In foreign or remote areas, the distance may be extended according to the  evaluation  of  the  existing  control  and  the situation. They may be single-run (one direction) lines but should always be loops or circuits that close  upon  BMs  of  an  equal  or  a  higher  order. 3.  When  a  line  from  previously  established third-order   marks   is   extended,   the   maximum length  of  the  new  line  is  greatly  reduced.  The distance and allowable error are to be carried back through the existing line to the nearest tie BM of the  second  or  higher  order. 4. Balanced sights should not be greater than 300 ft. BS and FS distances maybe measured by pacing  and  approximately  balanced  between  BMs, Rod  readings  are  read  to  thousandths  and  the  rod waved for extended rod readings. The bubble is checked to make sure it is exactly centered before each sighting and reading. Turning point pins or plates or well-defined points on solid objects are used  for  TPs. FOURTH-ORDER  leveling  is  used  to  sub- divide an area within a third-order network. This is  the  method  of  leveling  used  in  connection  “with the   location   and   construction   of   highways, railroads, and most other engineering works that concern the SEABEEs in advanced base projects. 14-21

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