The combined error due to curvature and refraction (ecomb ) is thus given by
ecomb = 0.0675 D2 m where D is the distance in km
It is finally subtractive in nature as the combined effect provides increase in staff reading. In Figure 16.1, x l x a represents the combined error due to curvature and refraction and in Figure 16.2, it is AL .
In most ordinary leveling operation, the line of sight is rarely more than 2 meter above the ground (where the variation in temperature causes substantial uncertainties in the refraction index of air). Fortunately, most lines of sights in leveling are relatively short (< 30 m) and B.S. & F.S. are balanced. Consequently, curvature and refraction corrections are relatively small thus insignificant except for precise leveling.
