In the Northern Hemisphere the star Polaris (Declination 89 10' N) is close to the North Celestial Pole. If observation of Polaris are taken the altitude (HO) obtained will be in error by a small amount. Hence a small correction has to be applied to the our angle of the Polaris to get altitude of the North Celestial Pole which is same as latitude of the observer.
That applied to the observed altitudes of the Polaris star. The value of this correction is given in the Air Almanac
, and it is applied to the Polaris altitude to obtain the true latitude.