Abstract: In this paper, an adjustment method is given by which the optimal probability values of displacement vector and strain parameter are esti-mated together under the condition of V^TPV=min, thus the analysises of vector field and strain field are unified.At the same time, if the point (or neighbourhood) with least or lesser strain is regarded as quasi-stationary point to set up reference system of free network, the problem for selecting onset data which is used to optimally determine a free network can be solved.