benchmark and at least one other existing benchmark and must show there is no change in their relative
(2) All first-order lines are divided into 1- to 2-kilometer sections. Each section must be run
forward and backward. The two runnings of a section must not differ by more than ...3 millimeters √K,
where K is the length of the section in kilometers. On all sections that are 0.25 kilometers or less in
length and which require two or more setups in each direction, a discrepancy of not more than 2.0
millimeters between the backward and forward measurement is considered a satisfactory check. In the
case of a single-setup section, the checks between forward and backward runnings seldom exceed 1.0
millimeters, averaging 0.6 millimeters or less.
(3) When additional runs are made due to excessive divergence, the indiscriminate mean of all
measured differences in elevation for that section is computed, excluding obvious blunders. If any
measurement of the difference in elevation for that section is more than ...6.0 millimeters √K from the
indiscriminate mean, that observation is rejected. No rejection should be made on because of a residual
smaller than ...6.0 millimeters √K, unless there is some good reason for suspecting an error in that
particular measurement. In such cases, the reason for the rejection must be stated in the record. After
the rejection has been completed, the mean of all remaining forward measurements is computed and
compared with the mean of all remaining backward measurements to determine if the required accuracy
has been obtained. The mean of the means is then the final value.
(4) The method and equipment used in first-order levels are designed to yield a maximum of ...1
millimeter for the probable accidental error and ...0.2 millimeter for the probable systematic error in a
distance of 1 kilometer.
b. Second-Order Leveling. The uses of second-order leveling are also quite important, so the
criteria are only slightly less strict than those for first-order leveling. Second-order leveling is used to
subdivide nets of first-order leveling and to provide basic control for the extension of levels of the same
or lower accuracy. This order leveling is used to provide data for mapping projects, local surveys, and
special projects, which include the positioning of radar equipment and stellar camera pads. Second-
order leveling is also used for initial missile site surveys. Second-order levels are divided into two
classes, Class I and Class II.
(1) Class I is used in remote areas where the line must be longer than 40 kilometers due to the
unavailability of routes, for the development of additional or higher order networks, and for spur lines.
All lines must start on previously established benchmarks of first or second order. New levels must be
run between the starting benchmark and at least one other existing benchmark to prove that they have
not changed their relative elevations. Failure to check within the limit of ...8.4 millimeters √K (where K
is the distance between benchmarks in kilometers) may indicate that at least one or both of the
benchmarks must be tied in to prove the starting elevation. All Class I lines are divided into 1- to 2-
kilometer sections which are run both forward and backward. The discrepancy between these runs must
not exceed ...8.4 millimeters √K, where K is the length of the section in kilometers. When