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Geodetic Astronomy Solutions

# Geodetic Astronomy Solutions - angle Polaris at Elongation...

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Lecture: Geodetic Astronomy Solutions Time Systems: Watch time Watch Corrections WWV and www.time.gov 24 hour time hours of correction from Greenwich Greenwich Mean Time, Universal Time Star tables for GHA and declination of sun and polaris: www.cadastral.com 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

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Unformatted text preview: 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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