Overlay the radar images on the electronic navigation chart can enhance the safety of maritime navigation. The coordinate transformation is necessary, ellipsoidal model is High precision, but low real time. Planar model can simplify the calculation, but it only can use in some condition. In order to solve this problem, compare the ellipsoidal model and planar model, and obtain the limit scale can use planar model, it is useful in overlay radar images on the electronic navigation chart.