说明：  摄影测量中的后方交会代码 仅在待定点上设站，向三个已知控制点观测两个水平夹角a、b，从而计算待定点的坐标
(Resection in Photogrammetry code point to be determined only station to the three levels are known to control the angle between the two observation points a, b, to calculate the coordinates of points to be determined)