But, in practice, trying to shoot for a higher degree
of accuracy is advantageous if it does not affect
the proper progress of the work. The following
criteria should be observed in fourth-order
1. All lines are to start from, and close on,
previously established BMs of the third or fourth
order of accuracy.
2. Maximum sight distance is about 500 ft.
Rod readings are read to hundredths of a foot.
BS and FS distances are roughly balanced only
when lines of great lengths are run, either uphill
or downhill. TPs are taken on solid or any well-
defined, firm objects.
The instrument commonly used in third- and
fourth-order leveling is the engineers level and
the Philadelphia rod. Always check the proper
adjustments of the instrument before using it.
Order of Precision
The precision of a level run is usually
prescribed in terms of a maximum error of
closure. This is obtained by multiplying a
constant factor by the square root of the length
of the run in miles or in kilometers, depending
upon the system of measurement being used. The
Federal Bureau of Surveying and Mapping
specifies certain requirements and the maximum
closing errors, such as those shown in table 14-1.
You may refer to this standard if the order of
precision is not specified for a particular survey
Calculating Error of Closure
A level run that begins at a particular BM and
is carried back again to the same BM is called a
level loop. A run that does not close on the
initial BM is called a level line. A level line closes
on another BM; but when a level line is carried
back to its origin, it becomes a level loop. Usually,
a level line is carried back to the initial BM to
determine the error of closure.
Error of closure is simply the difference
between the known elevation of the initial BM and
the elevation of the same (BM) as computed in
the level run.
The error of closure that can be allowed
depends on the precision required (first, second,
or third order). The permissible (or allowable)
error of closure in accuracy leveling is expressed
in terms of a coefficient and the square root of
the horizontal length of the actual route over
which the leveling was done.
Most differential leveling (plane surveying) is
third-order work. In third-order leveling, the
closure is usually made on surveys of higher
accuracy without doubling back to the old BM
at the original starting point of the level circuit.
The length of the level circuit, therefore, is the
actual distance leveled. For third-order leveling,
the allowable error is as follows:
By adding the sight distances in the sixth and
seventh columns of the differential level circuit
shown in figure 14-14, you will find that the length
of the level circuit is 2,140 ft. The length in miles is
2140 + 5280 = 0.405.
The allowable error of closure is
Since the actual error is only 0.015 ft, the results
are sufficiently accurate.
First- and second-order levels usually close on
themselves. The leveling party runs a line of levels
from an old BM or station to the new BM or
station, and then doubles back to the old BM for
closure. The actual distance leveled is twice the
length of the level circuit.
For second-order leveling, the allowable error
First-order leveling is still more precise. The
allowable error cannot be greater than
MISTAKES AND ERRORS
As explained in an earlier chapter, the terms
mistakes and errors are NOT synonymous in
Leveling operations, like other survey
measurements, are susceptible to both. Mistakes
can be avoided by a well-arranged system of
operation and by constant alertness by the survey
party members. Checking, as described in some
of the operations, will eliminate many possible