differential leveling. The theodolite, other measuring equipment, and the trigonometric functions of a
triangle are all used for trigonometric leveling. In point of accuracy leveling, trigonometric leveling is
inferior to precise spirit leveling, particularly in flat areas. In mountainous country, the trigonometric
method is of great value, and the results are comparatively more accurate.
There are many accuracy classifications and requirements for leveling. This lesson begins by discussing
the orders of accuracy and requirements and continues by describing the equipment, methods,
procedures, records, and computations for precise differential leveling. The final portion of this chapter
covers the process of determining elevations by the indirect method of trigonometric leveling.
PART A - ACCURACY AND REQUIREMENTS
4-1. Spirit Level Lines. Spirit level lines usually determine elevations referred to the datum. These
lines are run according with rigid specifications that prescribe the conditions under which the line may
be run and the tolerances that are permissible for the recognized orders of accuracy. When planning
new level lines, the surveyor must take into consideration the ties to all previously determined elevations
of acceptable orders of accuracy, the best distribution of new elevations for the contemplated survey, the
nature of the terrain involved, and the most practical routes to run the new line. All pertinent
information must be assembled for study, and suitable guide maps must be prepared to show the routing
of the proposed lines. Generally, the level man is not responsible for such planning, but he should
understand the established requirements as he will be in a position to apply sound judgment if local
conditions force slight deviations from the initial plan.
4-2. Benchmarks. The value of a line of levels depends upon the accuracy, distribution, and
permanency of established benchmarks or those tied in during leveling. Benchmarks are permanent
objects, natural or artificial, bearing a marked point whose elevation above or below the adopted datum
is known. Temporary benchmarks (TBMs) are intended to serve for short time periods.
4-3. Leveling. Leveling is classified as the degree of accuracy according to the methods and
instruments used and the closure specified. The orders of accuracy are first, second, and third order.
First order is the most precise. This lesson will only address the required accuracies of first- and
a. First-Order Leveling. Since the uses of first-order leveling are so important, the criteria are very
strict. First-order leveling is used to establish an area's main level net and to provide basic vertical
control for the extension of level nets of the same or lower accuracy. This order of leveling provides
data for mapping projects, special site construction projects, cadastral and local surveys, and geodetic
research involving earth sciences.
(1) First-order level lines must start and end on proven, existing benchmarks of the same order.
New levels must be run between the starting