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.
Checking Accuracy of Observed Bearings
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.
Recognizing, Reducing, and
Correcting Compass Errors
If a magnetic compass has a bent needle, there
will be a constant instrumental error in all
observed bearings and azimuths. To check for this
condition, set up and level the compass, wait for
the needle to cease oscillating, and read the
graduation indicated at each end of the needle.
If the compass is graduated for bearings, the
numerical value at each end of the needle should
be the same. If the compass is graduated for
azimuths, the readings should be 180° apart.
Similarly, if the pivot supporting the needle
on a magnetic compass is bent, there will be an
instrumental error in the compass. However, this
error, instead of being the same for all readings,
will be variable.
You, can eliminate either of these instrumental
errors by reading both ends of the needle and
using the average between them. Suppose, for
example, that with a compass graduated for
bearings you read a bearing of N45°E and a back
bearing of S44°W. You would use the average, or
The error in the compass should, of course,
be corrected as soon as possible. Normally, this
is a job for an expert. Remember the cause of a
discrepancy in the reading at both ends when there
is one. It is more probable that the needle, rather
than the pivot, is bent. After a bent needle has
been straightened, if a discrepancy still exists, then
probably the pivot is bent too.
If a compass needle is sluggish—that is, if it
moves unusually slowly in seeking magnetic
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