As traversing progresses, linear measurements are conducted to determine the distance between stations or points. Generally, the required traverse accuracy will determine the type of equipment and the method of measuring the distance. For the lower orders, a single taped distance is sufficient. However, as the order of accuracy gets higher, DOUBLE TAPING (once each way) is required. Ordinary steel tapes must be compared to an Invar or Lovar tape at specified intervals. For the highest accuracy, electronic distance-measuring devices (EDM) are used to measure linear distances. Linear measurements may also be made by indirect methods, using an angle measuring instrument, like the transit or theodolite with stadia. When the distances are determined by stadia readings, the vertical angles are read and used to convert slope distances to horizontal distances.

If double taping or chaining is required, follow these procedures:

1. Follow a direct line between stations, using a guide, such as a transit and a range pole, for alignment. Start measuring from the occupied station, keeping the front end of the tape aligned with the forward station.

3. Compare the two distances, and if within accuracy requirements, the distance is accepted. If the two measurements disagree by more than the allowable amount, retape the distance.

4. Proceed to the next line measurement, and continue double taping until the tie-in control point is reached.

Horizontal angles formed by the lines of each traverse station determine the relative directions of the traverse lines. These angles are measured using a transit or a theodolite, or determined graphically with a plane table and alidade. In a traverse, three traverse stations are significant: the REAR STATION, the OCCUPIED STATION, and the FORWARD STATION (fig. 13-30). The rear station is that station from which the crew performing the traverse has just moved, or it is a point, the azimuth to which is known. The occupied station is the station at which the crew is now located and over which the surveying instrument is set. The forward station is the next station in succession and constitutes the immediate destination of the crew. The stations are numbered consecutively starting at Number 1 and continuing throughout the traverse. In addition to the number of station, an abbreviation indicating the type of traverse is oftentimes

Figure 13-30.-Traverse stations and angles.

included; for example, ET for electronic traverse or TT for theodolite- or transit-tape traverse.

Horizontal angles are always measured at the occupied station by pointing the instrument toward the rear station and turning the angle clockwise to the forward station for the direct angle, and clockwise from the forward to the rear station for the explement (fig. 13-31). If a deflection angle is to be used, plunge the instrument telescope, after sighting the rear station, and read the angle left or right of the forward station.

NOTE: Office procedures for traverse computations and adjustments, methods of computing traverse area, and plotting horizontal control are discussed at the EA2 level.

Figure 13-31.-Kinds of angles measured at the occupied station.