The workspace Measuring and Positioning System(wMPS) is an indoor large-scale positioning system based on the intersection of rotating laser planes. It can provide 3D coordinates with metrological accuracy, applicable in the measurement and test tasks of manufacturing and assembly. As a large-scale measurement system, the contradiction between its measurement range and measurement precision is more intense than the small-to-medium-scale measurement system. How to find the optimal measurement point or the optimal measurement area centered with this point is an important and significant problem. Moving away from the measurement principle of the measurement system, compared with the traditional theodolite, an evaluation model was presented, which used the condition number of the coefficient matrix of the measurement equation as the indicator of the quality of intersection, then the Particle Swarm Optimization algorithm was introduced to solve the optimal measurement point. The experimental results show that the proposed evaluation model and the problem-solving method are correct and effective; they produce the minimum uncertainty around the calculated optimal point, and lay the foundation for the future research on the transmitter-layout problem.