IDENTIFYING PARTY PERSONNEL
Home
Download PDF
Order CD-ROM
Order in Print
TRANSIT-TAPE SURVEY
Attaining Precision with a Linear Error of Closure
Engineering Aid 3 - Beginning Structural engineering guide book
Page Navigation
424
425
426
427
428
429
430
431
432
433
434
disturbed). Last is the STRENGTH OF INTER-
SECTION, which generally means that the angle
between two tie lines should be as close to 90° as
possible. The more acute or obtuse the angle is
between tie lines, the less accurate the location of
the point defined by their intersection.
IDENTIFYING PARTY PERSONNEL
A typical transit-tape survey party contains
two chainmen, a transitman, a recorder
(sometimes the transitman or party chief doubles
as recorder), a party chief (who may serve as either
instrumentman or recorder, or both), and axmen,
if needed. The transitman carries, sets up, and
operates the transit; the chainmen do the same
with the tapes and the marking equipment.
When the transitman turns an angle, he calls
out the identity and size of the angle to the
recorder, as “Deflection angle AB to BC, 75°16´,
right.” The recorder repeats this, then makes the
entry. Similarly, the head chainman calls out the
identity and size of a linear distance, as “B to C,
265.72 ft,” then the recorder repeats this back and
makes the entry at that time. If the transitman
closes the horizon around a point, he calls out,
“Closing angle, such and such.” The recorder
repeats this and then adds the closing angle to the
original angle. If the sum of the angles doesn’t
come close to 360°, the recorder notifies the
party chief.
The party chief is in complete charge of the
party and makes all the significant decisions, such
as the stations to be marked on the traverse.
ATTAINING THE PRESCRIBED
ORDER OF PRECISION
The important distinction between accuracy
and precision in surveying is explained as follows:
l
Accuracy
denotes the degree of conformity
with a standard. It relates to the quality of a result
and is distinguished from precision, which relates
to the quality of the operation by which the result
is obtained.
The accuracy attained by field surveys is the
product of the instructions or specifications to be
followed in doing the work and the precision in
following those instructions.
For example, the “accuracy of a surveyor’s
tape” means the degree to which an interval of
100 ft, as measured on the tape, actually agrees
with the exact interval of a standard 100-ft tape.
If a tape indicates 100 ft when the interval it
measures is only 99.97 ft, the tape contains an
inaccuracy of 0.03 ft for every 100 ft measured.
The accuracy of this particular tape, expressed as
a fraction, is 0.03/100, or approximately 1/3,300.
l Precision denotes degree of refinement in
the performance of an operation or in the state-
ment of a result. It relates to the quality of
execution and is distinguished from accuracy that
relates to the quality of the result. The term
precision
not only applies to the fidelity of
performing the necessary operations but, by
custom, has been applied to methods and instru-
ments used in obtaining results of a high order
of accuracy. Precision is exemplified by the
number of decimal places to which a computation
is carried and a result stated. In a general way,
the accuracy of a result should determine the
precision of its
expression
. Precision will not have
significance unless accuracy is also obtained.
If you measure a linear distance with a tape
graduated in feet that are subdivided into tenths,
you can read (without estimation) only to the
nearest tenth (0.1) of a foot. But with a tape
graduated to hundredths of a foot, you can
directly read distances measured to the nearest
hundredth (0.01) of a foot. The apparent nearness
of the second tape will be greater; that is, the
second tape will have a higher precision.
Completely precise measurement is impossible
in the nature of things. There is always a built-in
or inherent error, amounting to the size of the
smallest graduation. Precision for the first tape
above, expressed as a fraction, is 0.1/100 or
1/1,000 and for the second tape, 1/10,000.
Precision in measurements is usually expressed
in a fractional form with unity as the numerator,
indicating the allowable error within a certain limit
as indicated by the denominator, such as 1/500.
In this case, you are allowed a maximum error
of 1 unit per 500 units measured. If your unit of
measure is in feet, you are allowed 1 ft for every
500 ft.
In general, any survey has to be carried out
accurately, meaning that errors and mistakes have
to be avoided. The precision of a survey, however,
depends upon the
order of precision
that is either
specified or is implied from the nature of the
survey.
The various orders of precision are absolute,
not relative, in meaning. Federal agencies control
surveys. They are generally classified into four
orders of precision; namely, FIRST ORDER,
SECOND ORDER, THIRD ORDER, and
13-21
Integrated Publishing, Inc. - A (SDVOSB) Service Disabled Veteran Owned Small Business