surveying iii (topographic and geodetic surveys) - Modern Prepper

second-order, Class I net. The length closures, as compared to these lines or measuredbaselines, must not exceed 1 part in 20,000 after all side and angle conditions have beensatisfied.(1) Second-order, Class II triangulation is used to subdivide areas of higher control and toprovide networks for mapping, cadastral, and local surveys. Second-order, Class II triangulation muststart from and tie to lines of a first-order triangulation net or to lines of an adjusted net of second-order,Class I or Class II. Length closures, as compared to these lines or measured baselines, must not exceed 1part in 10,000 after all side and angle conditions have been satisfied.(2) The 0.2-second theodolite is also used for second-order, Class I and Class II triangulation.Observations are made at night using a 1.0-theodolite and signal lamps. Daylight observations usingheliotropes are also acceptable.(3) The methods used in the observation of second-order, Class I and Class II, are designed togive a maximum triangle closure of 5 seconds, with an average triangle closure for the net that doesnot exceed the required average of 1.5 seconds for Class I or 3 seconds for Class II.2-35. A Typical Triangulation Net. A typical triangulation net is shown in Figure 2-2. In first-ordertriangulation, the known lines AB and CD are always baselines measured to such a precision as to ensurethat errors do not exceed 1 part in 300,000. For second-order triangulation, the known lines such as ABand CD are either second-order baselines or adjusted lines of first-order triangulation. The measuredbaseline AB is the known length. Once all the angles are observed, use the law of sines to compute thelengths of the other lines in the figure. There are various ways of computing line EF, such as through thetriangle ABE, as shown in Figure 2-3. After computing the distance BE, which is then used as theknown line of the triangle BEF, line EF is computed. Another route to compute the length of EF isthrough the triangles AFB and AFE. One of the routes is geometrically superior to the other, dependingon the angles used. Carry this system through from one figure to another until the next base (CD) isreached. Obtain the check on the discrepancy between the measured length of that line and its length ascomputed through the triangulation net from the preceding base (AB).Figure 2-2. A Typical Triangulation NetEN0593 2-20