雷达与电子海图叠加显示可提高船舶航行的安全性，叠加过程需将雷达的极坐标转化为平面坐标.椭球面模型计算精度高，但计算繁琐，实时性不强，平面模型计算简单，但只有满足一定条件才可使用.为解决叠加效率问题，对椭球模型与平面模型计算做了比较，得出在不同纬度，不同长度下适合使用平面模型替代椭球模型的极限比例尺，对雷达与电子海图叠加中坐标变换有一定应用价值.

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.