2. Errors Summary

Although we could go on (and on) digging up more leveling errors from each source, we've identified and described the major ones having the greatest impact. Table D-3 categorizes each by Source and identifies their Type (behavior).

Table D-3
Source Error Type
Personal   Set up Random
  Parallax  Random
  Sight distance  Random
  Rod leveling Random
  Reading or recording  Random/Mistake
 Instrumental Compensator  Random
  Horizontal crosshair Systematic
  Collimation Systematic
  Rod bubble Systematic
Natural Climate Random
  Curvature Systematic
  Refraction Systematic 


Some systematic errors are compensated by balancing BS and FS distances. The distances need only be within several paces of each other in order to remove the error effects. While point elevations are correct when BS and FS distances are balanced, the EI (elevation of the instrument) is still subject to error since the BS error is present without the compensating FS error.

To minimize most systematic errors when differential leveling:

• Always balance BS and FS distances within a few paces, and,

• Each instrument set up should have a single BS and a single FS - do not take multiple FS readings off a single BS reading.

What's left after systematic errors (and mistakes) have been eliminated? Random errors. Recall that random errors tend to be small and compensate. These are the only things which prevent perfect closure on a closed level network.