Azimuths in Control Surveys

With the advent of GPS for control surveying, a problem has arisen. Many existing control stations established by conventional methods (i.e. first order triangulation stations) are in locations not suitable for direct occupation by GPS equipment. Eccentric observations are often possible, with the eccentric station located up to 100 m away. In order to connect the eccentric station to the main station, an azimuth is necessary. Occasionally, a companion GPS station is set, and the angle is measured. More often, it is necessary to observe an astronomic azimuth. Various methods are explored for determining azimuth, with a discussion of the theory of each method, equipment and procedures, and accuracy attainable. The different methods examined include astronomic, gyrotheodolite, azimuth pairs by GPS, and use of azimuth marks. Existing Standards and Specifications address the issue of azimuths only in the context of controlling traverses and triangulation networks, with accuracies of 0.5” to 2” the goal. This paper addresses the issue of lower accuracy azimuths, in the range of 5” to 20”. Also discussed is the difference between astronomic, geodetic, and grid north. A discussion is made for potential inclusion in the "Input Formats and Specifications of The National Geodetic Survey Data Base" of the different types of azimuths besides the presently accepted polaris azimuth, when the distances are short and the required accuracy is lower. INTRODUCTION Over the years, various methodologies have been used to obtain azimuth control when extending horizontal control whether by traverse or triangulation. In the eastern part of the US, old photographs show that in the early part of this century, much of the area had few trees, having been timbered and cleared for farming. Many of the triangulation stations, located on high hilltops, had good visibility to an adjacent station or at least to an azimuth mark. Over the years, most of the rural areas have reforested, and the urban areas have become built up. It is a rare occurrence now that a recovered 1 order station has any kind of available (i.e. visible from the ground) azimuth reference. This was probably one of the main reasons why local surveyors often did not make ties to this control. Instead, many surveys used assumed datums with assumed azimuths. Some surveyors simply backsighted the nearby reference marks, thereby introducing large errors into their surveys. When GPS arrived in the mid 1980’s, there was precious little satellite coverage in a day. When establishing a survey network to be used by conventional equipment, the clients often specified that an intervisible azimuth mark was to be established. Because of the high cost of GPS equipment and the short GPS window, usually stations were established individually, and astronomic observations were then made at some or all of the stations to establish azimuth marks. Now, with 24 hour coverage and shortened observation times, stations are frequently established in pairs. Still, it is not always possible to set an intervisible station a sufficient distance away in a location that is suitable for GPS.