In theory of errors we details discuss theory of errors, occurrence of error, type of error, gross errors or mistakes, systematic or cumulative errors, accidental or random errors, precision, accuracy, true value, true error, most probable value (M.P.V), principle of least square, residual error, law’s of weights, probable error, error in computed result.

In levelling, we are details discuss about elevation & altitude, reduced level, mean sea level, bench marks(B.M), horizontal line, vertical line, level line, back sight reading, fore sight reading, intermediate sight reading, method to find reduced level, height of instrument, rise and fall method, inverted staff, correction required in levelling work, distance of visible horizon, reciprocal leveling, collimation error, angular / inclination error, sensitivity of bubble tube, the sensitivity of level tube increases with?.