Figure 13-8.-Sample field notes from a compass-tape survey.Proceeding in this fashion, the party tookbearings and back bearings, and chained distancesall the way around to the starting point at stationA. The last forward bearing taken, that of EA,has the same numerical value as the back bearingof EA (bearing of AE) taken at the start.Checking Accuracy of Observed BearingsAs a check on the accuracy of the wholebearing-reading process, the size of the interiorangle at each station was computed from theobserved bearings by the method previouslydescribed for converting bearings to interiorangles. The sizes of these angles were entered inthe column headed “Comp. Int. Angle,” and thesum was entered below.The sum of the interior angles in a closedtraverse should equal the product of 180° (n – 2),n being the number of traverse lines in thetraverse. In this case, the traverse has five lines;therefore, the sum of the interior angles should beThe computed sum is, therefore, the same as theadded sum of the angles converted from observedbearings.Recognizing, Reducing, andCorrecting Compass ErrorsIf a magnetic compass has a bent needle, therewill be a constant instrumental error in allobserved bearings and azimuths. To check for thiscondition, set up and level the compass, wait forthe needle to cease oscillating, and read thegraduation indicated at each end of the needle.If the compass is graduated for bearings, thenumerical value at each end of the needle shouldbe the same. If the compass is graduated forazimuths, the readings should be 180° apart.Similarly, if the pivot supporting the needleon a magnetic compass is bent, there will be aninstrumental error in the compass. However, thiserror, instead of being the same for all readings,will be variable.You, can eliminate either of these instrumentalerrors by reading both ends of the needle andusing the average between them. Suppose, forexample, that with a compass graduated forbearings you read a bearing of N45°E and a backbearing of S44°W. You would use the average, orThe error in the compass should, of course,be corrected as soon as possible. Normally, thisis a job for an expert. Remember the cause of adiscrepancy in the reading at both ends when thereis one. It is more probable that the needle, ratherthan the pivot, is bent. After a bent needle hasbeen straightened, if a discrepancy still exists, thenprobably the pivot is bent too.If a compass needle is sluggish—that is, if itmoves unusually slowly in seeking magnetic13-7
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