In the LAMOST project, the unit-holes on the Focal Plane Plate are the final installation location of the optical fiber
positioning system. Theirs precision will influence the observation efficiency of the LAMOST. For the unique
requirements, the unit-holes on the Focal Plane Plate are composed by a series of tiny angle dimensional holes which
dimensional angle are between 16' to 2.5°. According to these requirements, the measurement of the tiny angle
dimensional holes for the unit-holes needs to less than 3'. And all the unit-holes point to the virtual sphere center of the
Focal Plane Plate. To that end, the angle departure of the unit-holes axis is changed to the distance from the virtual
sphere center of Focal Plane Plate to the unit-holes axis. That is the better way to evaluate the technical requirements of
the dimensional angle errors. In the measuring process, common measuring methods do not fit for the tiny angle
dimensional hole by CMM(coordinate measurement machine). An extraordinary way to solve this problem is to insert a
measuring stick into a unit-hole, with a target ball on the stick. Then measure the low point of the ball center and pull out
the stick for the high station of center. Finally, calculate the two points for the unit-hole axis to get the angle departure.
But on the other hand, use this methods will bring extra errors for the measuring stick and the target ball. For better
analysis this question, a series experiments are mentioned in this paper, which testify that the influence of the measure
implement is little. With increasing the distance between the low point and the high point position in the measuring
process should enhance the accuracy of dimensional angle measurement.