Page 26 - 4798

Fig. 3.1. Illustration of accuracy and precision

Error Of Closure
The difference between a measured quantity and its true value is called error of
closure. In some cases, the closure can be taken simply as the difference between
two independent measurements.
For example, suppose a distance from point A to point B is first determined to
be 123.25 m. The line is measured a second time, perhaps from B to A, using the
same instrument and methods. A distance of 123.19 m is obtained. The error of
closure is simply 123.25 – 123.19 = 0.06m. It is due to accidental errors, as long as
blunders have been eliminated and systematic errors corrected.

Relative Accuracy
For horizontal distances, the ratio of the error of closure to the actual distance
is called the relative accuracy. Relative accuracy is generally expressed as a ratio
with unity as the first number of numerator. For example, if a distance of 500 ft
were measured with a closure of 0.25 ft, we can say that the relative accuracy of
that particular survey is 0.25/500, or 1/2000. This is also written as 1:2000. This
means basically that for every 2000 ft measured, there is an error of 1 ft. The
relative accuracy of a survey can be compared with a specified allowable standard
of accuracy in order to determine whether the results of the survey are acceptable.
Relative accuracy can be computed from the following formula:
Relative accuracy = 1: D/C
Where D = distance measured.
C = error of closure.

24