Celestial Observations for Land Surveyors

Solar observation (sun shots) or Polaris (North Star) can be used to determine azimuths. However, with the advent of GPS they are much less frequent now than in the past. It is beyond the scope of this lesson to teach you how to do a celestial observation but a very basic understanding can be imparted.

Requirement for these processes are a good theodolite or transit, the latitude of your area (that can be scaled from a 71/2′ quad map), the sun’s declination from an ephemeris, and a calculator.

Astronomical computations make use of spherical Trigonometry’s Law of Sines and Law of Cosines. The sides of a spherical triangle are arcs of great circles. In the determination of an azimuth by solar observation, the general scheme for finding the azimuth from point A to point B (any backsight of your choosing- say between two monuments on your survey), in all cases, the bearing of the sun from north is determined. Then the azimuth of the backsight line is determined by use of the angle turned from the backsight to the sun. The use of a solar prism is recommended so that you can view the sun directly and center it in the optics.

As with the solar method, the bearing of the star from north will be determined. This bearing is used with the measured horizontal angle to the star to determine the direction of the line from the observation point to the backsight mark.

This is a very simplified overview of the process. It is intended to give you a very basic idea of the process. Only working with an experienced party chief and with a lot of practice will equip you to accurately perform these observations.


For additional information, check out Celestial Observations for Land Surveyors (in our portion of the northern hemisphere)