On the basis of research on map projection, this study selects Gnomonic projection to the electronic map of carrier-based early warning aircraft. By calculating and analyzing length and angle distortion of Gnomonic projection, the maximal range of the map of carrier-based early warning aircraft is established. To resolve problem of incorrect calculating length and angle with map ruler, it provides correct length and angle calculation between two points for map ruler. The automatic change of electronic map is also researched.