areas  of  mistakes.  Errors  cannot  be  completely eliminated, but they can be minimized so that their effect  on  the  survey  accuracy  will  be  small  and within the tolerances permitted. Identifying Leveling Mistakes The leveling mistakes discussed here are not intended  to  include  all  possibilities  but  will give  an  idea  of  the  more  common  ones.  The survey party personnel should be aware of these possibilities and should be careful to avoid these mistakes.  Some  of  the  common  mistakes  are  as follows: 1. Not setting the rod on the same point for an FS and the following BS. Using a turning pin, pedestal, stake, or marking the location with chalk on  hard  surfaces  will  help  you  to  recover  the identical  point. 2.  Neglecting  to  clamp  the  target  or  the  rod when extended. Any slippage can pass unnoticed and result in a wrong reading that may require an entire rerun of the line to discover the mistake. The  rodman  should  watch  the  rod  or  target  for any movement as the clamp is tightened. The rod extension  or  target  should  be  read  again  after  the clamp has been set. 3. Reading the wrong mark. This is a common mistake.  The  figures  on  a  rod  may  be  obscured by brush or may fall in a position in the field of view so that the instrumentman cannot see two consecutive numbers. Under these conditions, he may  read  the  wrong  mark  or  even  read  in  the wrong direction. This is a great possibility when an inverting eyepiece is being used. For example, if  the  figure  2  is  the  only  number  visible,  the instrumentman  might  read  “up”  the  rod—2.1, 2.2, 2.3 when actually he should be reading 1.9, 1.8,  1.7.  Another  possibility  is  miscounting  the number of divisions. There is no way to check or discover these mistakes except to be aware of their possibility  and  to  read  carefully. 4. Recording a reading in the wrong column. In  leveling,  readings  are  not  entered  into  the notebook  in  a  normal  sequence,  such  as  left  to right across the page. There is always a chance that one or more values may be recorded in the wrong  column.  The  recorder  must  be  alert  to avoid making this mistake. 5.  Reading  the  wrong  angle  sign  in  trigono- metric leveling. The instrumentman can accidentally call out a wrong sign in reading the angle. This type  of  mistake  can  be  eliminated  by  the  recorder watching the telescope as a pointing is made on the rod. If the wrong one is called out, both the recorder  and  the  instrumentman  can  resolve  it immediately. 6. Recording the wrong sign. The sign varies depending on whether the rod reading is a BS or an FS, and whether the angle is a depression or an  elevation.  Also,  the  difference  in  elevation computation requires a sign reversal if the angle is  read  for  the  BS,  but  not  for  the  FS.  These variations can be confusing; the recorder has to be  careful  to  avoid  mistakes.  This  can  be  done by recording the angle and rod reading signs as read.  The  sign  conversion,  if  needed,  shows  up when   you   compute   the   DE.   Examining   the computations  to  see  if  all  BS  DEs  have  a  sign opposite  to  the  angle  sign  is  simple. 7.  Subtracting  the  BS  or  adding  the  FS  in differential leveling. If the BS or FS is recorded properly (see Number 4 above), you can discover the mistake when you add the BS column and the FS  column  for  a  computation  check. 8.  Using  the  wrong  horizontal  cross  hairs. This occurs on an instrument provided with stadia hairs. Identifying Leveling Errors Generally,  errors  cannot  be  totally  eliminated, but   they   can   be   contained   within   acceptable tolerances.  This  requires  you  to  use  the  prescribed methods and instruments and apply corrections established  either  mathematically  or  by  experience. Some  of  the  conditions  that  produce  errors  are listed  below. 1. Instrument not properly adjusted. A small amount of residual error will always exist in any adjustment.  For  the  more  accurate  surveys,  the residual error can be minimized by using BS and FS  balancing  and,  in  trigonometric  leveling,  by taking  direct  and  reverse  (circle  left  and  circle right)  readings  for  the  angles. 2.  Instrument  not  leveled  properly.  Unlike  the residual  adjustment  error  that  will  affect  the readings one way consistently, this is a random or accidental error. It may affect the line of sight differently  at  each  setup.  This  error  can  be minimized only by careful leveling each time the instrument is set up and by recentering the bubble before  each  reading. 3.  Telescope  not  focused  properly.  Misfocusing and  parallax  in  the  eyepiece  create  accidental errors that cannot be corrected. The only way to avoid  or  minimize  this  error  is  to  take  care  to focus  properly  at  each  setup.  The  instrumentman 14-24