In field astronomy, a celestial body provides the reference direction. So, from the geographic location (latitude and longitude) of the station, ephemeris data of celestial body and either time or altitude of the same celestial body, the azimuth of the celestial body is computed by solving astronomical triangle. If time is used, the procedure is known as the hour-angle method. Likewise, if altitude is measured, the procedure is termed as the altitude method. The basic difference between these two methods is that the altitude method requires observation of approximate time and an accurate vertical angle of the celestial body, whereas the hour angle method requires observation of accurate time.

Recent developments of time receivers and accurate timepieces, particularly digital watches with split-time features, and time modules for calculators, the hour-angle method is more accurate, faster. It requires shorter training for proficiency. It has fewer restrictions on time of day and geographic location and thus is more versatile. The method is applicable to the sun, Polaris, and other stars. Consequently, the hour-angle method is emphasized, and its use by surveyors is encouraged.

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