Mistakes in Computations
An involved computation, such as determining an
area by DMDs, involves a large number of calculations
that present the possibility of a large number of errors.
Some of the most common types of mistakes are
discussed below in the hope that, if you know what they
are, you may be able to avoid them.
MISTAKES WITH SIGNS.— You must be
extremely careful to give a value (such as a latitude or
departure) its correct sign in the first place and to apply
the sign correctly in addition, subtraction, multi-
plication, and division. The matter of signs is such a
fertile field for mistakes that a good idea is never to write
a value without including the sign. The practice of
omitting plus signs is a correct procedure, but it is safer
to write in the plus signs. Then, if you find a value
without a sign, you know that you forgot to put the sign
in and that it might just as possibly be a minus as a plus.
WRONG COLUMN.— A WRONG COLUMN
mistake may be an entry made in a wrong column or a
reading taken from a wrong column. To avoid such
mistakes, make both entries and readings with
deliberation; that is, without undue haste and always
with close attention to the column in which it should be
entered or read.
WRONG QUADRANT.— When you mistake the
quadrant in which a line lies, you get a bearing that may
have the correct angular value but that has the wrong
compass direction. The usual mistake of this kind is to
set down the compass direction of the back bearing
rather than of the front bearing.
A common cause of this mistake is viewing the
direction of a line from the wrong station. In figure 740,
the direction of AB is northeast but the direction of BA
is southwest. AB and BA are, however, the same traverse
line. But if you are determining the direction of AB, that
direction is northeast. But if you are determining the
direction of BA, that direction is precisely the opposite,
or southwest. To minimize direction error, you may
place arrows on the diagram showing the direction of
the line.
WRONG AZIMUTH.— The same consideration
applies to azimuths. Suppose that the bearing of AB in
figure 7-40 is N46°E. Then the azimuth of AB is
(measured from north) 46°. BA is the same traverse line;
but the azimuth of BA is definitely not 46°, but 226°.
LEAVING OUT A TRAVERSE LINE.— A
common source of mistakes is leaving out (commonly
called dropping) a traverse line, either in the field notes
Figure 7-40.—Proper compass direction of a closed traverse.
or in computations. If you get an outsized angular and
linear error of closure, you check first to make sure that
you have not dropped one of the traverse lines.
WRONG DECIMAL PLACE.— The incorrect
placement of a decimal point is a common mistake.
Suppose, for example, you are determining an
approximate double area by multiplying a DMD of
+841.97 feet by latitude of –153.53 feet If you were to
mistakenly use a value of –1535.3 instead of the correct
–153.53, you obviously will not arrive at the correct
result.
Locating Mistakes
If you cannot locate and correct a particular mistake,
you must rerun the whole traverse to find it. However,
this can often be avoided if you know a few tricks for
locating mistakes.
OUTSIZED ANGULAR ERROR OF CLO-
SURE.— The size of an outsized angular error of
closure may be a clue to the location of the particular
mistake. Suppose, for example, that for a six-sided
closed traverse, you measure interior angles as follows:
90°18'
118°48'
154°42'
147°18'
101°12'
612°18'
The interior angles in a six-sided closed traverse
should add up to 720°00'. The difference between
720°00' and 612°18' is 107°42'. This large difference
suggests that you dropped an angle measuring about
7-27
