An ellipsoid is usually chosen for data processing since observed space distance could not be used directly.
通常以椭球面作为计算基准面，而测量得到的直线距离不能直接用于椭球面计算。
Earth is an ellipsoid and its surface is curved, but the maps are usually drawn to the floor plan on paper. So before drawing you must spread the curved surface surface to the plane first.
地球椭球体表面是曲面，而地图通常要绘制在平面图纸上，因此制图时首先要把曲面展为平面。
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