The illustrations in Figure 2-3 are quadrilaterals. This is the simplest and most economical figure used

to give a double determination or check on the length of the line, as computed through the quadrilateral.

This figure will be used as the *go forward *line or *base *for the succeeding figure. In first-order

triangulation, the figures must be such that they afford a double check and all stations are occupied.

Single triangles should not be used in first-order triangulation and only rarely in second-order

triangulation.

between bases, and almost equal in importance, are the requirements limiting the acceptable error of

angle measurements. The limits imposed on angular errors serve to maintain a uniform accuracy along

the chain of triangles. The specifications for procuring the required accuracy make use of other criteria,

such as the number and *strength *of the geometrical figures between adjacent bases, the observation of

astronomical azimuths at specified intervals, and the accuracy of baseline measurements. All of these

requirements are subsidiary to the controlling tests of the agreement between the measured and the

computed length of a base and the limits specified for angle errors, and they are essential in securing

sustained accuracy and control.

field observations in order to obtain consistent results in procuring the required precision. The

requirements will vary, depending on the classification of the triangulation, be it first order or second

order.

Baseline-measurement accuracy.

The strength of a single figure in a network.

The sum of the strengths of figures (R1) between baselines.

The maximum single-triangle closure.

The average triangle closure between bases.