motion of the rod permits the instrumentman to read the rod when it reaches a vertical position at the top of the arc and when the lowest reading appears on the rod. Before or after the rod is in this vertical position, the rod reading is greater. 5.  Set  the  turning  pin  or  pedestal  firmly  in contact with the ground when setting a TP. Any unfirm  footing  can  sag  under  the  weight  of  the rod and result in incorrect readings between the FS  and  BS.  During  freezing  and  thawing  weather conditions,  the  ground  surface  can  heave  in  a comparatively short time. Pins and pedestals can be affected by the heave between the FS and the following   BS.   For   higher   order   of   accuracy surveys,  you  should  be  aware  of  this  possibility and  select  firm  locations. 6.  Extend  the  leveling  rod  to  its  maximum length when the instrumentman calls for extending the rod. The standard Philadelphia leveling rod can  be  read  to  7.100  ft  or  2.164  meters  when collapsed  and  13.000  ft  or  3.962  meters  when extended.  An  extended  leveling  rod  is  called  a LONG   ROD. A leveling rod is a precision instrument and has to be treated with care. Most rods are made of  carefully  selected,  kiln-dried,  well-seasoned hardwood  and  have  metal  scale  faces  on  which the  scale  graduations  are  painted.  Unless  a  rod is always handled with great care, the painted face will  become  scratched,  dented,  or  damaged  in other  ways.  Accurate  readings  on  a  rod  that  is damaged  are  difficult. Letting  an  extended  rod  close  “on  the  run” by  allowing  the  extended  upper  section  to  drop tends to damage both sections of the rod and to displace  the  vernier.  Always  close  an  extended  rod by  easing  down  the  upper  section. A rod will read accurately only if it is perfectly straight, so you must avoid anything that might bend or warp the rod. Do not lay a rod down flat unless  it  is  supported  throughout  on  a  flat surface.  Do  not  use  a  rod  as  a  support  or  as  a lever.  Store  the  rod  in  a  dry  place  to  avoid  any possible  warping  and  swelling  from  dampness, and always wipe a wet rod dry before stowing it away. If  there  is  mud  on  the  rod,  rinse  it  off,  but do not scrub it. If you have to use a soap solution to remove grease, use a mild solution. Repeated washings with strong soap solutions will eventually cause  the  painted  graduations  to  fade. FIELD PROCEDURES FOR DIFFERENTIAL  LEVELING Leveling operations require the teamwork of both  the  instrumentman  and  the  rodman  to achieve  consistent  results.  The  accuracy  of  the survey depends upon the refinement with which the  line  of  sight  can  be  made  horizontal  by  the instrumentman, the ability of the rodman to hold the  rod  vertically,  and  the  precision  with  which the  rod  reading  is  made.  Some  of  the  basic procedures and preparations applicable to direct leveling are presented below. Selecting Setup Points Terrain  and  atmospheric  conditions  are  the main   considerations   affecting   the   selection   of setup points. It is essential that you select a point from  which  you  can  best  observe  a  rod  reading on  the  BS  and  FS  points.  In  the  interest  of balanced  shots,  a  setup  point  should  be  about equidistant  from  both  BS  and  FS.  In  addition, shorter  setup  distances  will  result  in  smaller instrument   errors   caused   by   the   atmospheric refraction  and  curvature  of  the  earth. The average instrument height at any setup is about 5 ft (1.5 m). On even downhill slopes, the ground where the instrument is set up may not be  more  than  3  to  5  ft  below  the  TP  for  a  level BS. On the FS, the extended rod can be held on the   ground   about   8   ft   (2.5   m)   below   the instrument ground level and still permit a reading to be taken. This means that the tendency will be to make FS distances longer going downhill and to  make  BSs  longer  going  uphill. Therefore,  it  is  necessary  to  conduct  a reconnaissance  of  the  terrain  before  you  start leveling.  You  should  note  probable  locations of   instrument   setup   and   TPs.   During   the reconnaissance,  you  should  estimate  the  line  of sight  by  sighting  through  a  hand  level. Setting Up a Level In  setting  up  the  tripod,  you  first  hold  two tripod legs with both hands and spread the tips of  these  legs  a  convenient  distance  apart.  Then bring the third leg to a position that approximately levels  the  top  of  the  protector  cap  when  the tripod stands on all three legs. Then unscrew the protector   cap. Next,  you  lift  the  instrument  out  of  the carrying case by the footplate or level bar, NOT by  the  telescope,  and  set  it  gently  and  squarely 14-7