地图绘制工作者发有了一些网格线，这些网格线纵横交错地分布在整个球体上。
Mapmakers, or cartographers, have devised a grid of lines which criss-cross the globe.
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。
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.
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