average line in a triangulation system. It is more important to be able to extend the baseline by well-
conditioned figures than it is to have a baseline of great length.
a. A baseline, as used in triangulation, is the measured side of one of a series of connected
triangles. Baselines are used as length control for triangulation, both as starting lines of known length
and as check lines to ensure the required degree of accuracy of the triangulation. They are incorporated
in the triangulation scheme to fulfill the requirements of the order and class of triangulation used.
b. Baseline measurements are conducted using methods designed to obtain high-accuracy readings.
These methods require the use of standardized tapes with small coefficients of thermal expansion or
high-precision, electronic-measuring devices; measurements with standard tape supports and tension;
limiting conditions of alignment and grade; corrections for grade and temperature; and corrections for
variations in alignment, tension, and support.
c. All first-order and second-order, Class I baselines must be accurately measured so that the
computed probable error is not greater than 1 part in 1,000,000. Precautions used in the alignment, such
as the lengths of marking tape, determination of grade corrections, tension, and temperature, should
ensure that any errors in the length of the measured baseline does not exceed 1 part in 500,000. The
total actual error must not exceed 1 part in 300,000. The actual error is the amount of deviation of each
measurement from the mean of all measurements.
d. A baseline with the required degree of accuracy may be measured with tapes at any site where
the grade does not exceed 10, and where there are no gullies, ravines, or similar obstacles over 50
meters wide to be crossed. It may be necessary to build stands or towers in rugged terrain to span
ravines or reduce grades.
3-2. Tape Measurements. The principal baseline measuring tapes, called InvarTM tapes, are
standardized 50-meter ribbon tapes made of nickel or steel. These tapes are used in sets of four, one of
which is used as a standard for field comparison and substitute tape.
a. All base tapes should be standardized by the National Bureau of Standards (NBS) before the start
of a project and immediately after the completion of the last baseline of the project. At the NBS, the 50-
meter Invar tape length is compared with the US standard. The tape is placed, in a specified
temperature, under a tension of 15 kilograms and supported at the 0-, 25-, and 50-meter points; the 0-,
12.5-, 37.5-, and 50-meter points; and on a flat, horizontal surface. The 30-meter steel tape, used for
measuring short distances is standardized for each meter of tape on a flat, horizontal surface. The full-
length value of the 50-meter tape for each of the three methods of support and the values of the meter
intervals for the 30-meter tape described above are listed on certificates of standardization and returned
with the tapes to the organization. Even though all baseline tapes are standardized in the field, make
intercomparisons of the four tapes, using a test section at least four tape lengths long, immediately
before and after measuring each baseline. The terminal