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Manual leveling requires an appropriate display of the accelerometer outputs.

If high accuracy is required in preflight leveling, it is usually necessary to integrate or doubly integrate the accelerometer outputs ( this also minimizes the noise problem ).

With integration, the effect of a small acceleration ( or small platform tilt angle ) can be seen after a time.

However, skill is required on the part of an operator to level a platform to any degree of accuracy.

Also, it requires more time as compared to the automatic approach.