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TRIANGULATION PROCEDURES A triangulation survey usually involves the following steps: 1. Reconnaissancemeaning the selection of the most feasible points for stations 2. Signal erection on these points 3. Measurement of angles 4. Determination of direction (or azimuth) 5. Base line measurement 6. Computations The first consideration with regard to the selection of stations is, of cause, intervisibility. An observation between two stations that are not intervisible is impossible. Next comes accessibility. Obviously again, a station that is inaccessible cannot be occupied and between two stations otherwise equally feasible, the one that provides the easier access is preferable. The next consideration involves strength of figure. In triangulation, the distances computed (that is, the lengths of triangle sides) are computed by way of the law of sines. Ths more nearly equal the angles of a triangle are, the less will be the ratio of error in the sine computations. The ideal triangle, then would be one in which each of the three angles measured 60; this triangle would, of course, be both equiangular and equilateral.Values computed from the sines of angles near 0 or 180 are subject to large ratios of error. As a general rule, you should select stations that will provide triangles in which no angle is smaller than 30 or larger than 150. Signal Erection After the stations have been selected, the triangulation signals or triangulation towers should be erected When you erect triangulation towers or signals, remember that it is imperative for these stations to be intervisible. It is also important that the target be large enough to be seen at a distance; that is, the color of the target must be selected for good visibility against the background where it will be viewed. When observations are made during daylight hours with the sun shining, a heliotrope is a very effective target. When triangulation surveys are made at night, lights must be used for targets. Therefore, target sets with built-in illuminations are very effective. Measurement of Angles The precision with which angles in the system are measured will depend on the order of precision prescribed for the survey. The precision of a triangulation system may be classified according to (1) the average error of closure of the triangles in the system and (2) the ratio of error between the measured length of a base line and its length as computed through the system from an adjacent base line. Large government triangulation surveys are classified in precision categories as follows:
For third-order precision, angles measured with a 1-minute transit will be measured with sufficient precision if they are repeated six times. As explained in chapter 13 of the EA3 TRAMAN, six repetitions with a 1-minute transit measures angles to the nearest 5 seconds. To ensure elimination of certain possible instrumental errors, you should make half of the repetitions with the telescope erect and half with the telescope reversed. In each case, the horizon should be closed around the station. |
 
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