A method of correcting accelerometer and magnetometer measurements made in a well, comprising: making a series of triaxial measurements with magnetometers and accelerometers in an interval of the well to derive measured values of gravitational acceleration g, magnetic filed intensity B and the sine of the magnetic inclination sinl; obtaining known values of g, B and sinl for the interval; determining values of a correction to be applied to the measured values by simultaneously minimising the difference between the measured values and the known values of g, B and sinl for the interval; and applying the correction to the measured values to obtain in situ corrected values for g, B and sinl.