intermediate TPs. In each case, a “higher” TP
(as TP1) and a “lower” TP (as TP~~) was used,
resulting in two different HIs for each. Computed
by way of the higher HIs, the elevation of BM2
came to 851.98 ft. Computed by way of the lower
HIs, it came to 852.00 ft. The mean (average) of
851.99 ft was taken as the correct elevation.
INDIRECT LEVELING
Indirect methods of leveling encompass
both trigonometric and barometric leveling.
TRIGONOMETRIC LEVELING uses vertical
angles and a horizontal distance to compute
the difference in elevation, BAROMETRIC
LEVELING uses the difference in atmospheric
pressures that are observed by a barometer or an
altimeter to determine the elevation differences.
Indirect methods of leveling will be discussed at
the EA2 level.
PRECISION IN LEVELING;
MISTAKES AND ERRORS
IN LEVELING
Leveling, like any other surveying operation,
is carried out by following a prescribed ORDER
OF PRECISION—meaning that the instruments
you use and the methods you follow have to be
those that can give you the specified standard of
accuracy.
PRECISION IN LEVELING
FIRST-ORDER leveling is used to establish
the main level network for an area and to provide
basic vertical control for the extension of level
networks of the same, or lower, accuracy in
support of mapping projects, cadastral (recording
property boundaries, subdivision lines, buildings,
etc.), and local surveys. Level lines must start and
end on proven, existing BMs of the same order.
New levels must be run between the starting BM
being used and at least one other existing BM and
must show there is no change in their relative
elevations.
SECOND-ORDER leveling is used to sub-
divide nets of first-order leveling and to provide
basic control for the extension of levels of the
same, or lower, accuracy in support of mapping
projects and local surveys. Second-order levels are
divided into two classes: Class I and Class II.
CLASS I is used in remote areas where the line
must be longer than 25 mi because routes are
unavailable for the development of additional or
higher order networks and for spur lines. CLASS
II levels are used for the development of nets in
the more accessible areas. In Class I leveling, it
is required that all lines start and close on
previously established BMs of first or second
order. New levels have to be run between the
existing BM being used and at least one other
existing BM to prove that they have not changed
their relative elevations. The criteria for Class II
are the same as for Class 1, except that Class II
lines are run in one direction only.
THIRD-ORDER leveling is used to subdivide
an area surrounded by first- and second-order
leveling and to provide elevations for the
immediate control of cadastral, topographic, and
construction surveys for permanent structures.
The following criteria should be observed in third-
order leveling:
1. All lines have to start from, and close on,
two previously established BMs of third, or
higher, order of accuracy if the new leveling
indicates they have not changed in their relative
elevations.
2. In the United States, third-order lines
should not be extended more than 30 mi from
BMs of first or second order. In foreign or remote
areas, the distance may be extended according to
the evaluation of the existing control and the
situation. They may be single-run (one direction)
lines but should always be loops or circuits that
close upon BMs of an equal or a higher order.
3. When a line from previously established
third-order marks is extended, the maximum
length of the new line is greatly reduced. The
distance and allowable error are to be carried back
through the existing line to the nearest tie BM of
the second or higher order.
4. Balanced sights should not be greater than
300 ft. BS and FS distances maybe measured by
pacing and approximately balanced between BMs,
Rod readings are read to thousandths and the rod
waved for extended rod readings. The bubble is
checked to make sure it is exactly centered before
each sighting and reading. Turning point pins or
plates or well-defined points on solid objects are
used for TPs.
FOURTH-ORDER leveling is used to sub-
divide an area within a third-order network. This
is the method of leveling used in connection “with
the location and construction of highways,
railroads, and most other engineering works that
concern the SEABEEs in advanced base projects.
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