Table 15-7.—Figure Adjustment for Chain of Triangles, Figure 15-27measured angles around each station, determine theextent to which it differs from 360°, and distribute thisdifference over the angles around the station accordingto the number of angles.Figure 15-27 shows a chain of triangles. Stationadjustment for this chain of triangles is given in table15-6.At station A, as you can see, the sum of the observedinterior angles 3, 5, and 8 plus the observed exteriorclosing angle 12 comes to 360°00´25´´. This differsfrom 360° by 25 seconds. The number of anglesaround the station is four; therefore, the correction foreach angle is one fourth of 25, or 6 seconds, with 1second left over. The sum of the observed angles is inexcess of 360°; therefore, 6 seconds was subtractedfrom the observed value of each interior angle and 7seconds from the observed value of the exterior angle.The angles around the other stations were similarlyadjusted, as shown.The next step is the figure adjustment for each ofthe triangles in the chain. For a triangle, the sum of theinterior angles is 180°. The figure adjustment for eachof the three triangles illustrated in figure 15-27 is shownin table 15-7.As you can see, the sum of the three adjustedobserved interior angles in triangle ABC (angles 1, 2,and 3) comes to 179°59´40´´. This is 20 seconds less than180°, or 20/3, or 6 seconds for each angle, with2 seconds left over. Therefore, 6 seconds was addedto the station adjusted value of angle 1, and 7 secondseach was added to the measured values of angles 2and 3. The angles in the other two triangles weresimilarly adjusted.ADJUSTING A CHAIN OF QUADRI-LATERALS.— The station adjustment for a chain ofquadrilaterals is the same as that for a chain of triangles.The next step is a figure adjustment like that for a chainof triangles, except that the sum of the interior angles ofa quadrilateral is (4 - 2) 180°, or 360°.Next, for a quadrilateral, comes another figureadjustment, based on the four overlapping triangleswithin the quadrilateral. To understand this figureadjustment, study the quadrilateral shown in figure15-28. The diagonals in this quadrilateral intersect toFigure 15-28.—Quadrilateral.15-35

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