Errors in levelling and it's solutions

Errors in Levelling

• Errors  in  Levelling  will always occur,  and may be  minor  to  the  point  of  being insignificant or  significant enough  to  warrant adjustment.  Errors in levelling come  from three main  sources: 

1. Instrumental  Errors

2. Personal  Errors

3.  Environmental Errors

 Common errors are listed below, under these headings. 

 Instrumental Errors

• Instrument not adjusted Staff not vertical Staff not standardised - worn at the base  or at the joins 

Personal Errors

• Incorrect readings

•  Incorrect  bookings
•  Incorrect addition - the three checks not applied 
• Bubble not centred before  each reading Parallax not eliminated 
• Staff not vertical 
• Staff not fully  extended
•  Poor change  point - staff settles into ground
•  Poor instrument station - tripod settles into the ground 

Environmental Errors 

• Wind  -  strong wind  causes the instrument to vibrate  and  makes the staff unsteady.
• Temperature  - heat may  cause a  shimmering  effect at ground level near base  of staff and make accurate sightings  difficult. These types  of errors may  be categorised under the  headings ‘gross,  systematic or random’. 

Gross Errors 

These are  often caused  by the observer or  staff-person and are due  to carelessness,  inexperience  or fatigue.   They are  shown listed  under the  heading `Personal Errors'.

Systematic Errors

 These are often due to the  instrumental  defects  listed  under `Instrumental Errors'.   The most important of these is the  collimation error where the  whole error for a single shot (intermediate) is carried  over  into the staff reading.    As  previously noted, equalising the lengths  of backsights  and foresights  eliminates the error.   In situations  where  a large number  of  intermediate  sights  are made, for example building  sites,  then the two peg test should be regularly carried out.
The  non-verticality of the  staff will  also  cause incorrect  readings.   Any  circular bubble used in conjunction  with the  staff should be checked  periodically against a plumbline and adjusted if necessary.

 Random Errors 

These  are  due mainly to  environmental  conditions with  resulting small  errors which tend to  be compensatory.  Extreme wind or  temperature can cause errors.   In windy weather  shelter the instrument,  if  possible, and  keep  sights and the  staff short.  In hot sun reduce  the length of  the sights, keeping  them  at least 0.5  m above ground  level to minimise the effects of refraction.

Sources of Error 

1. The main source of error  is the residual collimation  error of the  instrument.   From  the  two  peg  test  it should  be apparent that  this  error would  be  eliminated by  equalising the lengths  of the  backsight and foresight distances.
2. The staff not  held  vertical.  Fitting  a staff  bubble  to  the  staff, or  by  swaying  the staff backwards and  forwards in  the  direction of  the  level until a  minimum reading  is obtained  eliminates this  error.
3.  An error  in  reading  the staff. This  is minimised by  reducing  the  length of sight so that  the  readings are  easily defined.
4.  The staff moving off  the position at  a change point  when it is  turned  to face  the new instrument setting. Using a change  plate on soft  ground or clearly marking the  change  point on  hard  ground  eliminates  this  error.
5. The instrument settling in soft ground. Setting up on  firm ground,  pushing  the tripod  legs  well into  the  ground and avoiding excessive movement  about  the instrument, eliminates  this  error. 
6. Errors due  to refraction  from warm layers  of air at  ground  level. This  is minimised  by  keeping  readings at  least 1m  above  the  ground.
7.  Errors due  to  the  staff not being  fully extended.  This may be  due either  to carelessness on the  staff man’s  part  or  to  wear  on  the  joints or  retaining  spring. 
8. Finally,  it is  important  to  eliminate  parallax in  the  instrument  by  bringing  the cross hairs  into sharp  focus  using  the eye-piece  focusing  screw.  The  procedure should  always  be  carried  out before  commencing  any reading  to  a  staff. 

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