one side of a triangle and all of the angles, you can compute the other two sides (Figure 1-8, page 1-12).
Also, if you know the latitude and longitude of one end of the side plus the length and direction of the
side, you can compute the latitude and the longitude of the other end of the side.
Figure 1-8. Principles of Triangulation
(1) The side of the triangle that is measured is called the baseline. It should be measured very
carefully using a bar, a chain, or tape, all of which should be calibrated and periodically checked by the
Bureau of Standards. It is not easy to reduce the baseline to the ellipsoid because our knowledge of both
the geoid and the ellipsoid is so limited. Until recently, it was impossible because the geoid heights
were not known. A compromise is usually necessary, and the baseline is reduced to MSL. The error in
doing this is small in the baseline itself; however, since the baseline establishes the scale of the control
network, this error may propagate throughout the network. While this source of error may be small,
increased knowledge of the undulations of the geoid permits further improvement in the accuracy of
horizontal positioning. With comparatively new instruments, such as the tellurometer (a microwave
system that employs line-of-sight conditions and is capable of measuring as far as 20 to 25 miles) and
the geodimeter (a system that uses light as a carrier and is capable of measuring as far as 2 to 3 miles by
day and 15 to 20 miles by night), you can measure distances much faster but not necessarily with greater
accuracy than with the conventional methods.