TOPIC 2: Corrections in Taping Flashcards
Taping operations could either be the following:
taping to determine an unknown length, or taping for the purpose of laying out
a required or specified length
General Rule for applying corrections
“When measuring with tape too long, ADD, tape too short, SUBTRACT. Do the reverse when laying out”
Manufacturers of assorted measuring tapes do not usually guarantee their products to be exactly their correct
length. They do not provide a standardization certificate unless requested, and this usually hast to be paid an extra amount.
These tapes rarely correspond exactly with its specified nominal length since they may actually be slightly shorter or longer.
This is often due to imperfections in their manufacture, stretching, and wear. An incorrect length of tape introduces an
error each time the tape is used and is classified as a systematic error
Correction due to incorrect tape length
The absolute value for the correction per tape length (Corr) is determined
from the difference between the true or
actual length of tape of tape (TL) and the nominal length of tape (NL).
Correspondingly, corrected distances
which are measured or laid out with a tape that is too long or too short can
be determined from the following equations
When distances are measured along the slope, the equivalent horizontal distance may correspondingly
be determined by applying an approximate slope correction
CORRECTION DUE TO SLOPE
For any classification of slopes,
the equivalent horizontal distance (d) is determined by subtracting the slope correction
ch from the measured slope distance (s) or
It is usually difficult to keep the tape in perfect alignment with the end marks when taping through grass or when a
strong wind is blowing. The head tapeman is likely to set the zero end of the tape on one side and sometimes on the other
side of the correct line. The linear error due to inaccuracy in alignment of a tape is similar to the effect of slope and can be
computed in the same manner. It is, however, easier to control and the resulting error is much smaller in magnitude.
CORRECTION DUE TO ALIGNMENT
The tape lengthens as the temperaturerisesmaking the tape too long and shortens as the temperature falls making
the tape too short. Any change in the length of a tape due to variations in temperature is critical when undertaking precise measurements. It can also be significant even for measurements of lower precision as in most engineering-type surveys. In
ordinary taping of short distances it is not usually considered since the amount involved is usually small and negligible. The
correction applied to the length of the tape due to change in temperature Ct is given by the formula
CORRECTION DUE TO TEMPERATURE
During calibration (or standardization) a tape is subjected to a certain amount of standard pull or tension on its ends. When used in the field during taping, it is elongated or shortened accordingly depending on the amount of pull applied on it. If the pull is greater than that for which it was calibrated, the tape elongates and becomes too long. Correspondingly, it will stretch less than its standard length when an insufficient pull is applied thus, making it too short. An error in measurement results whenever the pull applied is different from the standard tension used in calibration. To
account for variations in applied tension, a correction has to be added to or subtracted from the measured length. This correction for pull is expressed as
CORRECTION DUE TO TENSION(PULL)
A tape attains its correct length when it is fully supported and subjected to the pull for which it was standardized.
If the support is only at its ends or at the two points measured, it will sag even if the standard pull is maintained because of
its own weight. The tape takes the form of a catenary when it sags between points of support just as an electric or
telephone wire hangs and swings loosely between two posts. Sag shortens the horizontal distance between end
graduations since the tape length remains the same. Thus, when a stretched tape sags, the actual distance between the
points is something less than the reading on the tape. The correction due to sag is expressed as
CORRECTION DUE TO SAG
A strong wind blowing perpendicular to the direction of taping will move the middle and unsupported portion of
the tape to one side of the line measured. This introduces an error to the measurement which is similar to the effect of sag but usually much less. To avoid this type of error, it is preferable not to undertake any taping work during windy days. If it cannot be avoided and the measurement has to be done on windy days, efforts should be taken to support the tape from being affected by the blowing wind.
CORRECTION DUE TO WIND
By exerting a sufficiently greater amount of pull on the tape when it is suspended and sagged, the tape will be stretched and a considerable decrease in the amount of sag results. The applied pull which will lengthen the tape to equal the shortening caused by sag is referred to as the Normal Tension. The formula for normal tension is expressed as
NORMAL TENSION
The direction of a line is usually defined by the horizontal angle it makes with a fixed reference line or direction. In
surveying, this is done with reference to a meridian which lies in a vertical plane passing through a fixed point of reference and through the observer’s position
MERIDIANS
Types of MERIDIANS:
True meridian, magnetic meridian, grid meridian, assumed meridian
DESIGNATION OF NORTH POINTS
True North, Magnetic North, Grid North, Assumed North
is the north point of the true meridian. In maps and sketches, it is portrayed in the direction of the actual location of the earth’s north geographic pole and is always shown along a vertical line
True north
a north point that is established by means of a magnetized compass needle when there are no local attractions affecting it. At any point on the earth’s surface its direction is indicated by the direction of the magnetic lines of force passing through the point at a particular time. May be located either east or west of true north
Magnetic north
a north point which is established by lines on a map which are parallel to a selected central meridian. It may
coincide with the lines directed toward true north.
Grid north
is used to portray the location of any arbitrarily chosen north point.
Assumed north
The direction of a line is defined as the horizontal angle the line makes with an established line of reference. There are
various kinds of angle which can be used to describe the direction of lines. In surveying practice, directions may be defined by means
of: interior angles, deflection angles, angles to the right/left, bearings, and azimuth.
DIRECTION OF LINES
The angles between adjacent lines in a closed polygon are called interior angles. These angles may be measured clockwise
or counterclockwise. It should be remembered that the sum of the interior angles in a closed polygon is equal to (n-2) 180°.
Interior angles
The angle between a line and the prolongation of the preceding line is called a_______. It may be turned to the
right (clockwise) or to the left (counterclockwise) and it is always necessary to append the letters R or L to the numerical
value to define the direction in which the angle has been turned.
Deflection angles
is the acute horizontal angle between the reference meridian and the line. A quadrant system is used
to specify bearings such that a line may fall under one of the following quadrants: NE, SE, NW, and SW. Each quadrant is
numbered from 0° to 90° from either the North or South end of the reference meridian (N-S Line) to the East or West end of
the reference parallel (E-W Line).
Bearings
of a line is its direction as given by the angle between the meridian and the line measured in a clockwise
direction from either the North or South branch of the meridian. ______ are usually preferred over bearings by most
surveyors because they are more convenient to work with such as in computing traverse date by electronic digital
computers. The azimuth of a line may range from 0° to 360° and letters are not required to identify quadrants.
AZIMUTH
can be expressed in different units, all of which are basically derived from the division of the circumference of a circle. A purely arbitrary unit is used to define the value of an angle. The principal systems of units used are:
Magnitude
The sexagesimal system is used in which the circumference of a circle is divided into 360 parts or degrees. The basic unit is the degree, which is further subdivided into 60 minutes, and the minute is subdivided into 60 seconds. The °, and are used to denote degrees, minutes, and
seconds, respectively.
The Degree
The grad is the unit of measure in the centesimal system. In this system the circumference of a circle is divided into 400 pa rts called grads.
The grad is subdivided into 100 centesimal minutes and a centesimal minute is further subdivided into 100 centesimal seconds. The symbols g, c and
cc are used to denote grads, centesimal minutes, and centesimal seconds, respectively.
The Grad
The circumference is divided into 6,400 parts called mils. The mil will subtend very nearly one linear unit in a distance of 1,000 such units. It
is commonly used in military operations as in fire direction of artillery units.
The MIL
arc length exactly equal to the radius of the circle. One radian is denoted pi by and is equal to 180°. The ____ is sometimes
referred to as the natural unit of angle because there is no arbitrary number in its definition.
The radian
