Simple Curve Layout To lay out the simple curve (arc definition) just computed
above, you should usually use the procedure
that follows.
1. With the instrument placed at the PI,
the instrumentman
sights on the preceding PI
or at a distant station
and keeps the chainman on the line while the tangent distance is measured to
locate the PC. After
the PC has
been staked out, the instrumentman then trains the instrument on the forward PI
to locate the PT.
2. The instrumentman then sets up at the PC
and measures the angle
from the PI to
the PT. This
angle should be equal to one half of the I
angle; if it is not,
either the PC or
the PT has
been located in the wrong position.
3. With the first deflection angle (3°10’) set on the plates,
the instrumentman keeps the chainman on line as
the first subchord distance (42.18 feet) is measured from
the PC.
4. Without touching the lower motion screw, the instrumentman sets the second
deflection angle (6°55’) on the plates. The chainman measures the chord from
the previous station while the instrumentman keeps the head chainman on line.
5. The crew stakes out the succeeding stations in the same manner. If the
work is done correctly, the last deflection angle will point on the PT.
That distance will be
the subchord length (7.79 feet) from the last station before the PT.
When it is impossible to stake out the entire curve from the PC,
a modified method of the
procedure described above is used. Stake out the curve as far as possible from
the PC. If
a station cannot be seen from the PC
for some reason, move
the transit forward and set up over a station along the curve. Pick a station
for a backsight and set the deflection angle for that station on the plates.
Sight on this station with the telescope in the reverse position. Plunge the
telescope and set
Figure 1111.—Inaccessible PI.
the remainder of the stations in the same way as you would if the transit was
set over the PC. If
the setup in the curve has been made but the next stake cannot be set because of
obstructions, the curve can be backed in. To back in a curve, occupy the PT.
Sight on the PI
and set one half of the I
angle of the plates. The
transit is now oriented so that, if the PC
is observed, the plates
will read zero, which is the deflection angle shown in the notes for that
station. The curve stakes can then be set in the same order shown in the notes
or in the reverse order. Remember to use the deflection angles and chords from
the top of the column or from the bottom of the column. Although the backin
method has been set up as a way to avoid obstructions, it is also very widely
used as a method for laying out curves. The method is to proceed to the
approximate midpoint of the curve by laying out the deflection angles and chords
from the PC and
then laying out the remainder of the curve from the PT.
If this method is used,
any error in the curve is in the center where it is less noticeable.
So far in our discussions, we have begun staking out curves by setting up the
transit at the PI. But
whatPI
is inaccessible? This
condition is illustrated in figure 1111. In this situation, you locate the
curve elements using the following steps:
1. As shown in figure 1111, mark two intervisible points
A and
B on
the tangents so that line AB
clears the obstacle.
2. Measure angles a and
b by
setting up at both A
and B.
3. Measure the distance AB.
4. Compute inaccessible distance AV
and BV
using the
formulas given in figure 1111.
5. Determine the tangent distance from the PI
to the
PC on
the basis of the degree of curve or other given limiting
factor.
6. Locate the PC at
a distance T minus
AV from
the point A
and the PT
at a distance T
minus BV
from point B.
Field Notes
Figure 1112 shows field notes for the curve we solved
and staked out above. By now you should be
Figure 1112.—Field notes
for laying out a simple curve.
familiar enough with field notes to preclude a complete discussion
of everything shown in these notes. You
should notice, however, that the stations are entered
in reverse order (bottom to top). In this manner the data is presented as it
appears in the field when you are sighting ahead on the line. This same practice
applies to the sketch shown on the righthand page of the field notes.
For information about other situations involving inaccessible points or the
uses of external and middle ordinate distance, spiral transitions, and other
types of horizontal curves, study books such as those mentioned at the beginning
of this chapter.
