Lecture: Geodetic Coordinate Systems
Lecture: Geodetic Astronomy Solutions
Time Systems:
Watch time
Watch Corrections
WWV and
24 hour time
hours of correction from Greenwich
Greenwich Mean Time, Universal Time
Star tables for GHA and declination of sun and polaris:
Azimuth Measurement by Polaris at any time
In field, measure (1) horizontal angle from survey backsight to star and (2) time of sighting star
From time, interpolate tables for GHA of star and declination of star.
Get latitude and longitude from a quadrangle map.
Calculate angle t from GHA star and West Longitude
Calculate side opposite the t angle – Zenith Distance -- apply cosine law to PZS triangle, knowing t, polar distance, co-latitude.
Calculate the star's azimuth angle Z -- apply sine law, knowing t, ZD, and polar distance.
Calculate the survey mark's true azimuth by combining the measured horizontal angle with the Z angle.
Polaris at Elongation
Polaris makes a counterclockwise circle around the true pole of about one degree radius.
Upper culmination = polaris at its highest point, when it is traveling west (left)
Western elongation = polaris at its western most point, traveling down
Lower culmination = polaris at its lowest point traveling east (right)
Eastern elongation = polalris at its eastern most point, traveling up.
At western or eastern elongation, the star's azimuth is nearly constant for about 10-15 minutes. A large time error will translate into negligible azimuth error at these places, therefore they are preferred for highest accurcy.
At culmination, the star is traveling left or right at is greatest speed, and a time error will translate into the maximum amount of azimuth error. Accurate time is essential here.
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