纸上或屏幕上的地图是平面的；它们需要从地球的曲面向平面进行转换：地图投影。

Maps (on paper or on the screen) are flat surfaces; they require a transformation from the curved surface of the earth to the flat plane: a map projection.

youdao

但是事实上它就是，如果你考虑一下的话，就是在给定点上的，最好的近似曲面的平面。

But, it's really, if you think about it, you will see it's really the plane that's approximating the surface in the best way that you can at this given point.

youdao

这就是切平面的，或者说这个曲面的法向量。

OK, and that's going to be the normal vector to the surface or to the tangent plane.

youdao