Figure 13-8 shows field notes from a compass-tape survey of a small field. The instrument used was a surveyors compass. The compass was first set up at station A, shown in the sketch drawn on the remarks page. The first bearing taken was that of the line AE. This was actually the back bearing of EA, taken for the purpose of later checking against the forward bearing of EA.

Next, the bearing of AB was taken, and the distance from A to B was chained. The observed bearing (S6220E) was entered beside B in the column headed "Obs. Bearing." The chained distance was entered beside B in the column headed "Dist."

The compass was shifted to station B, and the back bearing of AB (that is, the bearing of BA) was taken as a check on the previously taken bearing of AB. The back bearing turned out to have, as it should have, the same numerical value (6220) as the forward bearing. A difference in the two would indicate either an inaccuracy in reading one bearing or the other or a difference in the strength of local attraction.

Figure 13-8.-Sample field notes from a compass-tape survey.

Proceeding in this fashion, the party took bearings and back bearings, and chained distances all the way around to the starting point at station A. The last forward bearing taken, that of EA, has the same numerical value as the back bearing of EA (bearing of AE) taken at the start.

As a check on the accuracy of the whole bearing-reading process, the size of the interior angle at each station was computed from the observed bearings by the method previously described for converting bearings to interior angles. The sizes of these angles were entered in the column headed "Comp. Int. Angle," and the sum was entered below.

The sum of the interior angles in a closed traverse should equal the product of 180 (n 2), n being the number of traverse lines in the traverse. In this case, the traverse has five lines; therefore, the sum of the interior angles should be 

The computed sum is, therefore, the same as the added sum of the angles converted from observed bearings.