7. MEASURING AND LAYING OUT HORIZONTAL ANGLES
7.1 Setting Up Over a Point With Plumb Bob
To measure or lay out an angle, set the instrument over a point and level it up. Use
the plumb bob with about six feet of string. Attach the plumb bob string to the
hook under the instrument by means of a large loop fastened by a slipknot and
adjust the plumb bob until it is clear of the ground point. By shifting the entire
instrument, set the tripod (keeping tripod head as level as possible by estimation) so
that the plumb bob appears to be over the ground point. Next, press the legs of the
tripod into the ground and lower the plumb bob until its point is about one-quarter
inch above the point on the ground. The final centering of the instrument can be
made by loosening (about one-half turn) any two
adjacent
(not opposite) leveling
screws and slowly shifting the instrument until the plumb bob is directly over the
point on the ground. Then retighten the same two leveling screws you loosened and
re-level the instrument.
7.2 Measuring Horizontal Angles
To measure horizontal angles such as EFG, (
Fig. 8
), center and level your instrument
over point F in accordance with previous instructions.
Loosen horizontal clamp screw (attached to circle plate); rotate the instrument until
point E is nearly in line with the vertical crosshair. Tighten clamp screw. Turn tangent
screw until vertical crosshair is on point E. By hand set the horizontal circle to read
zero (on some instruments, rotate the circle; on other instruments, set to zero with
movable index). Now, loosen horizontal clamp (
do not touch circle or index
) and
swing the telescope toward point G until the vertical crosshair is exactly on point G.
The horizontal index pointer will have rotated about the horizontal circle by an
amount equal to the angle EFG. The horizontal index and horizontal circle are shown
(
Fig. 10
) after measuring a clockwise horizontal angle of 62°. If your instrument is
furnished with a vernier instead of an index pointer, you will be able to read the angle
closer than a single degree. The use of a vernier is explained later in this manual.
In layout work, it is frequently necessary to set off an angle, usually 90°. Assume that
the 90° angle HIJ is to be laid off and points H and I are shown (
Fig. 9
). Therefore, J is
the point you are to set.
Berger Instruments • 9
Suppose one of your points is below the line of sight and the other above (
Fig. 6B
), C
is 4 feet 6-1/2 inches below the line of sight, and point D, the underside of a floor
beam is 7 feet 9-3/8 inches above the line of sight (the latter reading having been
obtained by holding the rod upside down with the foot of the rod against the beam).
D is then higher than C by an amount equal to 4 feet 6-1/2 inches plus 7 feet 9-3/8
inches, or a total of 12 feet 3-7/8 inches.
6.4 To Measure the Difference in Elevation Requiring More Than One Set-up
If two points are either too far apart or at too great a difference of elevation to be
observed from one set-up, the procedure shown below is recommended (
Fig. 7
).
This example assumes that you want to find the difference in elevation between
points A and D. To make the finding of this difference simple, use the convenient
terms
plus (+) sight
and
minus (–) sight
and carry the readings at each set-up as shown.
The difference of elevation between D and A is found by taking the difference
between the sum of the plus sights and the sum of the minus sights. If the sum of
the plus sights is larger, the final point is higher than the starting point. If the sum of
the minus sights is larger, the final point is lower than the starting point.
6.5 Elevations or Grades
Many constructions, such as buildings and roadways, are required to be built at
specified elevations or grades. To establish these grades, a point of elevation is
necessary, often called a benchmark. Your benchmark should be a firm and definite
point such as a bolt on a water hydrant, a spike in the root of a tree, a corner of a
stone monument, or a chisel square on a ledge, and should be located outside the
construction area. For a large job, several benchmarks in convenient locations are
helpful. The grades may then be carried directly to the job by using the "difference in
elevation" method described in Fig. 6 and Fig. 7. Keep a careful record of your
observations in a notebook so that your values may be checked or used in later work.
8 • Berger Instruments
Difference of elevation of D with respect to A = 12'3"1/8
–19'6"5/8 = –7'3"1/2. Minus sign indicates D is below A.
NOTE: + Sight is rod reading from point to line of sight.
– Sight is rod reading from line of sight to point.
SET UP + SIGHT
– SIGHT
1
2'7"1/8
8'4"1/2
2
1'9"3/4
9'11"5/8
3
7'10"1/4
1'2"1/2
SUM
12'3"1/8 19'6"5/8
Fig. 7
Fig. 8
Fig. 9
E
F
G
62
°
H
I
J
90
°