required number of full tape lengths. The
remaining partial tape length is then laid off by
setting the rear chainmans plumb bob at the
appropriate tape graduation.
Making Tape Corrections
A 100-ft tape should, in theory, indicate
exactly 100.00 ft when it is in fact measuring
100.00 ft. However, a tape supported only at the
ends has a sag in it, so when it indicates 100.00
ft, actually the distance measured is less. Even a
tape supported throughout on a flat surface can
be slightly longer under tension than it is without
tension. Also, a tape will be longer when it is
warm than when it is cold.
CALIBRATING A TAPE. All tapes are
graduated under controlled conditions of tempera-
ture and tension. When they are taken to the field,
these conditions change. The tape, regardless of
the material used to make it, will be either too
short or too long. For low accuracy surveys, the
amount of error is too small to be considered. As
accuracy requirements increase, variations caused
by the temperature and sag must be computed and
used to correct the measured distance. In the
higher orders of accuracy, the original graduation
is checked for accuracy or calibrated at intervals
against a standard distance. This standard is
usually two points, a tape length apart, that have
been set and marked using a more precise tape
or a tape already checked. The standard may be
just the precise or checked tape (known as the king
or master tape). This tape is kept in a safe location
and is not used for making field measurements,
but only to check the accuracy of the field tapes.
For the highest orders of accuracy, the tapes are
sent to the National Bureau of Standards, U.S.
Department of Commerce, Washington, DC,
20234, for standardization under exact conditions
of tension, temperature, and points of support.
A tape standardization certificate is issued for
each tape, showing the amount of error under the
different support conditions and the coefficient
of expansion. The certificate (or a copy) is kept
with each tape. For field operations, the tapes are
combined in sets; one is selected as the king tape,
while the others are used as field tapes.
The standard tension for a tape supported
throughout is 10 lb, and the standard temperature
is 68°F. Standard length is, simply, the nominal
length of the tape. A 100-ft tape, for example,
at a temperature of 68°F, supported throughout,
and subject to a tension of 10 lb, should indicate
100 ft when it is measuring exactly 100 ft.
To CALIBRATE a 100-ft tape means to
determine the exact distance it is actually
measuring when it indicates 100 ft, while being
supported throughout, at a temperature of 68°F
and under a tension of 10 lb.
In addition to the National Bureau of
Standards, many state and municipal authorities
provide standardizing service.
RECOGNIZING TAPE OR STANDARD
ERROR. Suppose now that you send a 100-ft
tape to the Bureau of Standards to be calibrated;
the bureau will return a certificate with the tape.
Assume that the certificate states that when the
tape, supported throughout at a temperature of
68°F, and under a tension of 10 lb, indicates 100
ft, it actually measures 100.003 ft on the standard
tape. The tape, then, has a STANDARD ERROR
(also called TAPE ERROR) of 0.003 ft for every
100 ft it measures. This tape reads short.
Depending on the order of precision of the survey,
you may have to apply this as a correction to
measurements made with this particular tape.
CORRECTING FOR STANDARD ERROR.
Whether you add or subtract the standard error
depends upon the direction of the error. The tape
in the above example indicates a distance that is
shorter than it actually measures; in other words,
when you use this tape to lay off a distance of
100 ft, the line is actually 100.003 ft.
The decision to add or subtract the error
depends upon whether you are measuring to
determine the distance between two points or to
set a point at a given distance from another.
Assume first that youre measuring the
distance between two given points, and the
distance as indicated by the tape is 362.73 ft. First,
what is the total tape error? Obviously, it is 0.003
times the number of tape lengths. In this case, it is
0.003 x 3.6273 = 0.0108819 ft,
which rounds off to 0.01 ft.
The next question is: Do you add this total
correction to, or subtract it from, the recorded
distance of 362.73 ft? Well, if you remember that
the tape reads short, you will realize the
reasonable thing to do is ADD the total standard
error to the recorded distance. The correct
distance between the two points, then, is 362.74 ft.
Suppose now that with the same tape, you are
to set a point 362.73 ft away from another point.