Natural Causes

These are effects of sun and wind. Considerable difficulty could be experienced although taking the staff reading under glaring sun, or sun shining on the objective glass. Accuracy of observation could also be affected while the velocity of wind is large or when the atmosphere is heated. While the sights are long during precision levelling the errors because of effect of curvature and refraction shall be taken into account. The line of level, described as a line of equal altitude, will not remain horizontal in long sights because of earth's curvature (Figure). Aa′ will be the recorded level at A while the real level should be Aa. Therefore, an error e =  aa′ is introduced due to earth's curvature provided as ec = 0.0785 D², where D is the distance in kilometer (km) from the level to the staff station, and e is in meters. In normal levelling, sight length is less than 300 m, therefore e will always be less than 0.007 m.

                                                         Figure : Error Due to Curvature

Errors because of refractions are introduced due to refraction of light passing through layers of air of various densities. The bent light ray to staff to instrument will not remain horizontal (Figure 13) but will be curved introducing error aa′. The effect of refraction is not constant but varies along with atmospheric conditions. Therefore, on an average under normal

                                                              Figure  : Error Due to Refraction

atmospheric conditions the correction for refraction will be aa′. The error, er (in meters) = 0.0112 D² (i.e. roughly about 1/7 the correction due to curvature and opposite in sign).

The combined correction because of curvature and refraction would be eco = ec - er = (0.0785 - 0.0112) D² = 0.0673 D2

As the effect of curvature is to raise the staff reading so the correction for curvature is subtractive. The correction for refraction is additive to staff reading. Thus, the merged correction is subtractive to staff reading.