ERRORS AND REDUCTION IN COMPASS SURVEY

SOURCES OF ERRORS IN COMPASS SURVEY:
Errors in compass surveying arise from both angular and linear measurements. Here we are only considering angular errors. The following are some of the sources of angular errors in compass surveying:

• Instrumental Error:

         Instrumental errors are errors inherent in the instrument itself. They arise chiefly from improper graduation of the compass ring, wrong material of the pivot, excessive weight of the needle etc. In practice, no correction is applied to counter this error, but they are minimized by reading both the forward and back bearings of a line.

• Observational Error:

         The best of the prismatic compasses are graduated to read to the nearest half of a degree. More precisional  measurement can be made with them only by estimation. Great care should be exercised in estimating readings as they can lead to observational errors. Observational errors can also arise due to poor alignment of the two value ( i.e. forward bearing and back bearings) during observation. Small observational errors are not normally corrected for, but gross observational errors render the observed quantity unreliable and therefore the same must be rejected.

• Local Attraction Errors:

          Local attraction error is the main source of error in compass surveying. Local attraction errors are due to magnetic objects in the environment of the observation; which tend to pull the magnetic needle away from its correct position. They are manifested by significant differences between the forward and backward readings of a line. The errors caused by local attraction are normally corrected for in compass surveying. 

• Magnetic Variations:

          Magnetic variation affects the values given by compasses. However, no attempt is made to correct for this type of error. The date of observation is important in this regards, as some of these variations depends on time.

• Parallax Effects:

         In relation to compass surveying, parallax effect is the apparent motion of a target in relation to the sighting wire at the time of observation. Care should be taken to avoid this effect as it can lead to errors in the field.

REDUCTION

To reduce the observations, the columns of differences and errors are worked out and studied in order to determine if error due to local attractions are present. this is possible since some errors in the observation may be due to imperfections on the part of the surveyor. Errors due to imperfections are not adjusted but the mean directions indicated by the forward and back bearings are used. However, if errors due to local attraction are present, the following procedure is used in adjusting the errors. Consider the observation given below:

  LINE     LENGTH    FORWARD     BACK    DIFF    ERROR     CORRECTED     CORRECTED  
                                      BEARING      BRG.                                       FORWARD        BACK BRG. 
    AB        125               086                  266        180        0                      086                     266
    BC         50                 170                  352        182        2                      170                     350
    CD         50                 197                  015         182       2                      195                     015
    DE         100               270                   090         180       0                      270                     090
    EA         80                  345                   165         180       0                     345                     165
     In which it is suspected that local attraction errors are present. To start with, a line in which the two bearings differ by exactly 180∘ is selected, and it is assumed that there is no local attraction anywhere on this line. Thus, bearings taken at either end of this line are accepted as recorded. Now, consider the above example, from the table, we noted that lines BC and CD have errors of 2∘ each. Suppose we start the adjustment from line EA, we observe that station E and A are also free from local attraction, so is station B.
      On line BC, an error of 2∘ is obtained which most probably comes from some local attraction at station C an line CD, the same error of 2∘ at station C manifests itself. thus, the corrected forward and back bearings are as given in the table above. It should be noted that this adjustment does not ensure the internal geometry of the figure observed. That is to  say, the figure ABCDEA may not have internal angle equal to (2n - 4)90 or external angles equal to (2n + 4)90.
NOTE:

• The same correction should be applied to all bearings observed from one station.
• After adjustment, the forward and back bearing of a line should differ by exactly 180∘
• If in all the observations, no two bearing of a line differ by exactly 180, the correction should be made starting from a line whose bearing have the least discrepancy between the back and forward bearing readings. 
• The said corrections refer to local attraction errors and not to observation errors arising from imperfections of the observer and instrument. If gross error of observation occur, the surveyor must go back to the field, to obtain a true sample of the quantity measured. If small errors occur due to imperfections, mean observed directions must be used.