3.4.5.6.Measurement of anglesDetermination of direction (or azimuth)Base line measurementComputationsReconnaissanceThe first consideration with regard to the selectionof stations is, of cause, intervisibility. An observationbetween two stations that are not intervisible isimpossible. Next comes accessibility. Obviously again,a station that is inaccessible cannot be occupied andbetween two stations otherwise equally feasible, the onethat provides the easier access is preferable.The next consideration involves strength of figure.In triangulation, the distances computed (that is, thelengths of triangle sides) are computed by way of thelaw of sines. Ths more nearly equal the angles of atriangle are, the less will be the ratio of error in the sinecomputations. The ideal triangle, then would be one inwhich each of the three angles measured 60°; thistriangle would, of course, be both equiangular andequilateral.Values computed from the sines of angles near 0°or 180° are subject to large ratios of error. As a generalrule, you should select stations that will providetriangles in which no angle is smaller than 30° or largerthan 150°.Signal ErectionAfter the stations have been selected, thetriangulation signals or triangulation towers should beerected When you erect triangulation towers or signals,remember that it is imperative for these stations to beintervisible. It is also important that the target be largeenough to be seen at a distance; that is, the color of thetarget must be selected for good visibility against thebackground where it will be viewed. When observationsare made during daylight hours with the sun shining, aheliotrope is a very effective target. When triangulationsurveys are made at night, lights must be used fortargets. Therefore, target sets with built-in illuminationsare very effective.Measurement of AnglesThe precision with which angles in the system aremeasured will depend on the order of precisionprescribed for the survey. The precision of atriangulation system may be classified according to (1)the average error of closure of the triangles in the systemand (2) the ratio of error between the measured lengthof a base line and its length as computed through thesystem from an adjacent base line. Large governmenttriangulation surveys are classified in precisioncategories as follows:15-32For third-order precision, angles measured with a1-minute transit will be measured with sufficientprecision if they are repeated six times. As explained inchapter 13 of the EA3 TRAMAN, six repetitions with a1-minute transit measures angles to the nearest 5seconds. To ensure elimination of certain possibleinstrumental errors, you should make half of therepetitions with the telescope erect and half with thetelescope reversed. In each case, the horizon should beclosed around the station.Determination of DirectionAs you learned earlier in this chapter, mostastronomical observations are made to determine thetrue meridian from which all azimuths are referred Infirst-order triangulation systems, these observations areused to determine latitude and longitude. Once the truemeridian is established, the azimuths of all other sidesare computed from the true meridian.To compute the coordinates of triangulationstations, you must determine the latitudes anddepartures of the lines between stations; to do this, youmust determine the directions of these lines. Thelatitudeof a traverse line means the length of the lineas projected on the north-south meridian runningthrough the point of origin. The departure of a traverseline means the length of the line as projected on theeast-west parallel running through the point of originLatitudes and departures are discussed in detail inchapter 7 of this TRAMAN.