Table 13-1.-Control Traverse Order of Precision
FOURTH ORDER control surveys. The FIRST
is 90°, or 5,400´. The precision of a 1-min transit.
ORDER is the highest and the FOURTH
ORDER, the lowest standard of accuracy.
Because of the type of instruments available
in the SEABEEs, most of your surveys may not
require a precision higher than a third order
survey. When the order of precision is not
specified, you may use table 13-1 in this training
manual (TM) as a standard for a horizontal
control survey when using the traverse control
method. For surveys that call for a higher order
of precision, you will have to use theodolites to
obtain the required precision.
The triangulation control method is discussed
fully in Engineering Aid 1 & C, NAVEDTRA
10635-C. At present, however, you may have
survey problems that require the use of the
triangulation method. In such a case, you may
use table 13-2 in this TM as a guide for the order
of precision if it is not specified in the survey.
The practical significance of a prescribed or
implied order of precision lies in the fact that the
instruments and methods used must be capable
of attaining the required precision. The precision
of an instrument is indicated by a fraction in
which the numerator is the inherent error. (In a
1-min transit, the inherent error is 1 min.)
The denominator is the total number of units
in which the error occurs. For a transit, this last
then, is 1/5,400, adequate for a third order survey.
Precision of a tape is given in terms of the
inherent error per 100 ft. A tape that can be read
to the nearest 0.01 ft has a precision of 0.01/100,
or 1/10,000—adequate for second order work.
Attaining Precision with a
Linear Error of Closure
For a closed traverse, you should attain a
RATIO OF LINEAR ERROR OF CLOSURE
that corresponds to the order of precision
prescribed or implied for the traverse. The ratio
of linear error of closure is a fraction in which
the numerator is the linear error of closure and
the denominator is the total length of the traverse.
To understand the concept of linear error of
closure, you should study the closed traverse
shown in figure 13-27. Beginning at station C, this
traverse runs N30°E300 ft, thence S30°E300 ft;
thence S90°W 300 ft. The end of the closing
traverse,
BC, lies exactly on the point of
beginning, C. This indicates that all angles were
turned and all distances chained with perfect
accuracy, resulting in perfect closure, or an
error of closure of zero feet.
However, in reality, perfect accuracy in
measurement seldom occurs. In actual practice,
13-22
