A system of control stations, local or universal, must be established to locate the positions of various points, objects, or details on the surface of the earth. The relative positions of detail points can be easily determined if these points are TIED IN to a local control station; or, if the control station is tied in to a geodetic control, the positions of other detail points can also be located relative to a worldwide control system.

The main control system is formed by a triangulation network supplemented by traverse. A traverse that has been established and is used to locate detail points and objects is often spoken of as a CONTROL TRAVERSE. Any line from which points and objects are located is a CONTROL LINE. A survey is controlled horizon-tally by measuring horizontal distances and horizontal angles. This type of survey is often referred to as HORIZONTAL CONTROL.

Horizontal control surveys are also conducted to establish supplementary control stations for use in construction surveys. Supplementary control stations usually consist of one or more short traverses run close to or across a construction area to afford easy tie-ins for various projects. These stations are established to the degree of accuracy needed for the purpose of the survey.

In this chapter, we will identify common procedures used in converting angular measurements taken from a compass or transit survey, recognize the methods used in establishing horizontal control, and identify various field procedures used in running a traverse survey.

There are various ways of describing the horizontal locations of a point, as mentioned in chapter 12. In the final analysis, these ways are all reducible to the basic method of description; that is, by stating the length (distance) and direction of a straight line between the point whose location is being described and a reference point. Direction, like horizontal location itself, is also relative; that is, the direction of a line can only be stated relative to a REFERENCE LINE of known (or sometimes of assumed) direction. In true geographical direction, the reference line is the meridian passing through the point where the observer is located; and the direction of a line passing through that point is described in terms of the horizontal angle between that line and the meridian. In magnetic geographical direction, the reference line is the magnetic meridian instead of the true meridian.

The direction of a traverse line is commonly given by bearing. In field traversing, however, turning deflection angles with a transit is more convenient than orienting each traverse line to a meridian. The method of converting bearings to deflection angles is explained in the following paragraphs.