Slope Chaining
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SUPPORTING THE TAPE
Horizontal Chaining
Engineering Aid 3 - Beginning Structural engineering guide book
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pole and shifting his body weight until the correct
tension is read on the scale. This position can be
held steadily and comfortably for a comparatively
long time.
Measuring distances less than a full tape length
requires the use of the clamp handle (or “scissors
clamp”), which is attached to the tape at some
convenient point along its length. The handle
permits a firm hold on the tape and furnishes a
convenient attachment for a spring balance. When
properly used, the handle will prevent kinking of
the tape.
READING THE TAPE.—
A chain tape may
be either a PLUS (or ADD) tape or a MINUS (or
SUBTRACT) tape. On a plus tape, the end foot,
graduated in subdivisions, is an extra foot, lying
outside the 0-ft mark on the tape and graduated
AWAY FROM the 0-ft mark. On a minus tape,
the end foot, graduated in subdivisions, is the foot
lying between the 0-ft mark and 1-ft mark and
graduated AWAY FROM the 0-ft mark and
TOWARD the 1-ft mark. As will be seen, this
difference is significant when a distance of less
than a full tape length is being measured.
Suppose that you are measuring the distance
between point A and point B with a 100-ft tape,
and the distance is less than 100 ft. Suppose that
you are the head chainman. To start off, you and
the rear chainman are both at point A. You walk
away from point A with the zero-foot end of the
tape. Because this is a plus tape, the tape has an
extra foot beyond the zero-foot end, and this foot
is subdivided in hundredths of a foot, reading
from the zero.
You set the zero on point B, or plumb it over
point B; then call out, “Take a foot!” When the
rear chainman hears this, he pulls back the first
even-foot graduation between A and B to point
A, or plumbs it over point A. Let’s say this is the
34-ft graduation, The rear chainman calls out,
“Thirty-four!”
You now read the subdivided end-foot gradua-
tion that is on or over point B. Let’s say it is the
0.82-ft graduation. You call out, “Point eight
two!‘’ The rear chainman rechecks the even-foot
graduation on point A and calls out, “Thirty-four
point eight two!”
As you can see, your
subdivided-foot reading is added to his even-foot
reading; hence, the
expression
“plus” tape.
Suppose now that you are measuring the same
distance between the same points, but using a
“minus” tape; that is, a tape on which the
subdivided end-foot lies between the zero-foot and
1-ft graduations. This time when you walk away
with the zero-foot end, you set the 1-ft graduation
on point B and call out, “Take a foot!” When
he hears this, the rear chainman again hauls back
the first even-foot graduation between A and B
to point A—but this time this will be the 35-ft
graduation. So the rear chainman sings out,
“Thirty-five!” When you hear this, you read the
subdivided-foot graduation on point B. This time
this will be 0.18-ft graduation, so you call out,
“Minus point one eight!” The rear chainman
mentally subtracts 0.18-ft from 35.00 ft and calls
out, “Thirty-four point eight two!” When you
are also acting as the recorder, recheck the
subtraction before you record the distance in the
field notebook.
GIVING A LINE.—
The
range pole
is set on
line slightly behind the point toward which the
taping will proceed. Line may be given (that is,
the person with the
range pole
may be guided or
signaled onto the line) by “eyeball” (that is, by
eye-observation alignment by the rear chainman
or someone else at the point from which chaining
is proceeding) or by instrument.
Slope Chaining
The methods used in slope chaining are
basically the same as in chaining on level ground.
There are some differences, however, as follows:
In slope chaining, the tape is held along the slope
of the ground, the slope distance is measured, and
the slope distance is converted, by computation,
to horizontal distance. The slope angle is
usually measured with an Abney hand level and
clinometer; however, for precise measurement, it
is measured with a transit.
In using the clinometer, you take the slope
angle along a line parallel to the slope of the ground
or along the tape that is held taut and parallel to
the slope of the ground. To use the clinometer,
you sight on an object that is usually a point on
a pole approximately equal to your height of
instrument (HI); that is, the vertical distance from
the ground to the center (horizontal axis) of the
sight tube. While sighting the object, you rotate
the level tube about the axis of vertical arc until
the cross hairs bisect the bubble as you look
through the eyepiece. Then, you read either the
slope angle or percentage on the vertical arc and
record it along with the slope distance measure-
ment. The horizontal distance is computed, or in
other words, the tape correction is applied.
If the station points are being marked, the
corrections to the slope distances are applied as
the chaining progresses. These correct ions are
computed either mentally, by calculator, or by
using a table.
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