intermediate TPs as you need to solve the
problem. A turning point is defined as a point
on which both a minus sight (FS) and a plus sight
(BS) are taken on a line of direct levels.
As shown in figure 14-12, if we assume that
the elevation of the BM is correct, the accuracy
of the elevation you determine for the summit
depends upon how accurately you determine the
elevation of each intermediate TP. This accuracy
depends upon a number of things, the most
important of which are the following:
1. If you are doing leveling of ordinary
precision, FS and BS distances should not exceed
300 ft. Therefore, the first setup point for the
instrument should be not more than 300 ft from
the BM, and the first TP should be not more than
300 ft from the instrument. To balance shots, you
should place the instrument about the same
distance from the BM as the distance to the TP.
2. Obviously, the first setup point must be one
you can observe with a rod held on the BM and
also a rod held on the first TP.
3. Generally, setup points should be used that
make rod readings as small as possible. The reason
small rod readings are desirable is that, for a rod
held out of plumb, each reading on the rod will
be in error. The larger the rod reading, the greater
the error. Suppose, for example, a rod is so far
out of plumb that it indicates 12.01 ft for a
reading that should be 12.00 ft if the rod were
plumb. For a 12.00-ft reading on the rod, the
error is 0.01 ft. For a 2.00-ft reading on the same
rod held in the same manner, however, the error
would be only about 0.002 ft.
4. A TP must have not only visibility and
accessibility, but also stability; that is, it must
furnish a firm, nonsettling support for the base
of the rod. Suppose you select a point in soft,
yielding ground as your first TP. Assume the
elevation of the BM is 312.42 ft. You take a BS
on the BM and read 3.42 ft. Then, HI is
312.42 + 3.42 = 315.84 ft.
The rodman shifts the rod to the TP. You take
an FS and read 5.61 ft. The elevation of the TP
315.84 5.61 = 310.23 ft.
Now, you shift the instrument ahead and take a
BS to carry on the line of levels to a new TP.
But suppose that before you take the BS on
the rod, the TP has settled 0.02 ft in the ground.
Then you take a BS and read 4.74 ft. There is now
an error of 0.02 ft in the new HI, and every
subsequent HI and elevation of TP will be off by
the same amount.
So BE SURE that each TP is stable. When the
use of a point in yielding ground is unavoidable,
you need to base the rod on a turning point pin
or turning point plate. A pin is driven in the
ground; if you dont have a regular pin, a
marlinspike or a railroad spike makes a good
substitute. You should use a plate on soil too soft
to support a driven pin.
METHODS OF LEVELING
Leveling methods are subdivided into two
major categories: DIRECT and INDIRECT.
Direct leveling describes the method of measuring
vertical distance (difference in elevation) directly
with the use of precise or semi-precise leveling
instruments. Indirect leveling methods, on the
other hand, apply to measuring vertical distances
indirectly or by computation. Unlike direct
leveling operations, indirect leveling operations
do not depend on lines of sight or intervisibility
of points or stations. Some of the surveying
instruments commonly used for indirect leveling
methods are the transit and theodolite.
This method of leveling uses the measured
vertical distance to carry elevation from a known
point to an unknown point. Direct leveling is the
most precise method of determining elevation and
yields accuracies of third or higher orders. When
this method is specified for lower accuracy
surveys, direct leveling is sometimes referred to
as spirit or fly levels. Fly levels are leveling
operations used to rerun original levels to make
sure that no mistake has been made. Fly levels
use a shorter route and smaller number of turning
points than the original survey. Lets take a look
at some of the processes involving direct leveling.
Differential leveling (also called direct leveling)
is generally used in determining elevations of
points to establish a chain or network of BMs for
future use. It requires a series of instrument setups
along the survey route; and for setup, a horizontal
line of sight is established, using a sensitive level.
The SEABEEs commonly use this type of leveling
in determining elevation during construction
As shown in figure 14-13, the basic procedure
used to determine elevations in a differential
leveling operation is the same as previously
discussed. First, you take a BS on a rod held on