Digital zenith camera has high precision in astrogeodesy. Identifying the star partially and setting the approximate position are needed when using digital zenith camera to orient. The scope is around the approximate position during star identification partially. Generally, the scope is usually replaced by field of view angle of digital zenith camera. Actually, this is not correct. The scope of star identification partially was deduced based on the analysis of the field of view angle of digital zenith camera. Then, the approximate position was analyzed according to the scope. The experimental data demonstrates that the range of longitude and latitude of identified stars should be above the field of view angle of digital zenith camera. Also, the recognized star number will rise when the approximate position is close to the true position.