S

HOW
TO
USE
TRANSITS
&
LEVELS
...A concise, easy-to-understand guide to the use of a basic construction tool.
S
urveying, if you check the dictionary, sounds like a discouragingly difficult procedure. In practical
language, however, it simply means
accurate measurement.* To the
contractor, it’s a form of insurance
against a foundation winding up
two inches over somebody else’s
property line.
Surveying instruments, then, are
not the formidable mechanisms
they might seem at first glance.
They are a means of locating a
house properly on a lot, leveling a
foundation, establishing grades,
and all the many other functions involving accurate measurement
which are so important to a contractor.
There are two basic types of
builders’ instruments—the optical
level (sometimes called the dumpy
level) which has its telescope fixed
in a horizontal position, and the
transit-level, which turns not only
sideways, but also up and down.
This enables you to determine
whether a wall is perfectly plumb.
The transit-level also allows you to
run straight lines and to measure
vertical angles.
The optical level consists of a telescope, leveling screws, the vial or
“bubble” and a 360-degree scale.
The telescope lenses magnify so
that a 20-power telescope makes an
object seem 20 times closer than
when viewed with the naked eye.
The greater the magnification, natu-
rally, the greater the distances over
which the instrument can be used.
Cross hairs within the telescope
enable you to center your target.
The clarity of the image within the
telescope will probably surprise you
the first time you examine a precision instrument of this sort. Before
you line up the telescope, though,
you must first level the instrument
by means of the leveling vial and
leveling screws. The vial located on
the telescope works much the same
as the ordinary carpenter’s level, except that it is a great deal more sensitive and accurate.
By turning screws A and B, and
watching the bubble carefully, the
instrument can be leveled on the
A/B axis. The telescope is then given
a quarter-turn to the C/D axis and
leveled again. After rechecking, the
telescope can then be turned
around the complete 360-degree
circle and found to be perfectly level in every direction.
The circle could be considered
like two of the protractors you used
in school, except that it makes a
complete 360-degree circle rather
than the 180-degree half-circle you
had on your protractor. The circle
on a level can be rotated independently so that it can be set at zero no
matter which way the instrument is
pointing. Starting at zero and rotating the telescope on it, we can now
measure any horizontal angle.
The 90-degree angle is the one
most commonly used in construction work, so the circle is divided into four equal parts (or quadrants) to
make it easy to “turn” the telescope
over this angle quickly and accurately. Some instruments are also
equipped with vernier scales, which
break the measurement down into
minutes (with 60 minutes in each
degree). This sort of accuracy is especially important when sighting
over long distances, or engaging in
precision construction.
The tripod, of course, is the allimportant foundation element. On
some instruments, the tripod shoes
also have spurs which allow the tripod to be seated firmly in the
ground.
Now let’s assume that you want to
establish the foundation corners of
a house:
1. You start with an empty lot.
2. You intend to put this rectangular shaped house on it, and you
need to square off the four corners
accurately.
3. Let’s assume we have established the front of the building
line—the line from A to B. We must
now complete the rectangle. We
start by setting up our instrument at
A, leveling it up and pointing toward
B. Then we set the circle or pointer
to 0 degrees.
4. Now, we swing our telescope exactly 90 degrees, which establishes
the direction of point C. We measure
off the correct footage from A to C
* The original material from which this text was adapted is contained in a pocket handbook prepared by C. L. Berger & Sons,
Inc., of Boston, Massachusetts. Much of the text was originally prepared by Herman J. Shea, former Associate Professor of
Surveying, Massachusetts Institute of Technology.
with a tape and drive in a stake at C.
5. Now, we move our instrument
to point C. We sight back on A and
set the horizontal circle at zero.
6. Next, we swing our level 90 degrees and establish point D. Tape
the length and drive in a stake at D.
7. We have now established our
foundation corners. A perfect rectangle has been established in a
matter of minutes.
The line of sight through the telescope is a perfectly straight line for
as far as you can see. You can be assured that it is straight because it is
a line without weight. It cannot sag,
as a length of cord might. Any point
along the line of sight is exactly level with any other point.
To determine the difference in elevation between two points which
can be observed from one position,
set up the instrument about midway between these points. Each
point should not be more than 150
to 200 feet away from the instrument. The height of the line of sight
(horizontal cross hair) above or below each of the points is found by
reading a measuring rod held by a
co-worker. Figure 1-A shows a line
of sight 69 inches above A and 40
inches above B. Simple arithmetic
establishes that B is higher than A
by 29 inches.
To go a bit further, suppose you
are working with one point below
the line of sight, and the other
above. In Figure 1-B, point C is 4feet 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. This reading was determined by holding the
rod upside down with the foot of
the rod against the beam. Again,
arithmetic tells us that point D is
higher than point C by the total of
the two measurements—4 feet 6
1/2 inches plus 7 feet 9 3/8 inches,
or a total of 12 feet 3 7/8 inches.
Differences in elevation requiring more than one set-up may be
determined by the method shown
in Figure 2. To find the difference
in elevation between points A and
D, we simplify it by using the terms
plus (+) sight and minus (—) sight
and carry the readings at each setup 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.
Many projects, especially buildings and roadways, are required to
be built at specified grades or elevations. A point of known elevation
is necessary to establish these
grades, and this known point is
usually called a benchmark. A
benchmark should be a definite
point, and it should be located outside the construction area. It could
be a bolt on a water hydrant, the
corner of a stone monument or
building, or a spike in the root of a
tree. Several benchmarks are helpful for a large job. The grades may
then be computed by the “difference in elevation” method described in Figure 1 and Figure 2.
Figure 3 shows how to measure
horizontal angles such as EFG. The
instrument is centered over point F,
and rotated until point E is in line
with the vertical cross hair and set
the horizontal circle to read zero.
Next, swing the telescope toward
point G until the vertical cross hair
is exactly on point G. The horizontal index pointer will have rotated
about the horizontal circle by an
amount equal to the angle EFG. If
the instrument is equipped with a
vernier scale you will be able to
read the angle more closely than a
single degree.
When setting grades for a driveway, place the grade stakes on both
sides of the driveway, usually at
equal distances from the centerline, as in Figure 4. Assume that the
rod reading for the driveway at this
section is 4 feet 10 inches. Grade
stakes A and B are set at equal
distances from the centerline.
In marking grades on A and B,
mark an even number of feet of
either grade, or a cut or fill. Any
convenient reading, so that the
bottom of the rod is somewhere along the stake, is used
to note the grade. At point A, a
rod reading of 2 feet 10 inches
is obtained, and a line is placed
on the grade stake. This line
represents a cut of 2 feet and is
marked as in Figure 5. In like
manner, a rod reading of 7 feet
10 inches is found at point B.
This is marked as a fill of 3 feet
according to Figure 5. To use
these grade stakes, a line is
stretched from the marking at
A to a point 5 feet above the
marking at B. This line then indicates 2 feet above the driveway grade.
The use of a transit level, of
course, extends the versatility
of the instrument because the
telescope can be tilted up and
down. This facilitates setting
points in line, measuring vertical angles when the instrument is so equipped, plumbing
and defining slopes and
grades. Either the optical or the
transit level may well prove to
be one of the most useful of all
the contractor’s tools, and well
worth the bit of time it takes to
master the techniques.
PUBLICATION #C670337
Copyright © 1967, The Aberdeen Group
All rights reserved