As illustrated in Fig. 1, it is optimal to choose the known points P1 and P3.
The instrument should be set up in such a manner so that the angle between P1 and P3 becomes 90°.
The distances S1 and S2 should be similar.
The accuracy of a calculation result depends on the following:
1) The inner angle between known points is extremely small.
See P1 and P2 on above Fig. 1.
2) The inner angle between known points is extremely large.
See P4 and P6 on above Fig. 1.