Methods of Azimuth Determination at a Magnetic Repeat Station: A Comparison.

The determination of geographic azimuths is an essential step in field magnetic surveys. In spite of the relatively low precision required for declination values, it is strongly recommended that those true azimuths should be measured with an error in the order of 0.1' to avoid contamination of desired magnetic values. The applicability and ease of use, as well the deficiencies of the various methods of azimuth determination must be considered in the light of the different field work conditions encountered during repeat station surveys. In general, four classes of methods could be considered: geodetic, astronomical, use of a gyro-theodolite and use of a GPS system. The strengths and weakness of each of these methods are discussed and a comparison between them is made. Notwithstanding a possible worldwide interest of this subject, a special emphasis is given to magnetic repeat station work in developing countries. As a conclusion, we could say that the most suitable method for azimuth determination at a magnetic repeat station strongly depends on regional conditions. For developing countries (latitude range between 30°N and 30°S), mainly for Latin American countries, the gyro-theodolite method proves to be very effective at the present time.