这个椭球面的中心和方位就构成了我们所说的大地基准面。
The center and orientation of the ellipsoid is what we call the datum.
海图的基准比例尺是图上一个单位长度与其对应的球面上长度的之比，用分数表示。
The natural scale of a chart is the ratio of a length measured on the chart to a corresponding length measured on the surface of the Earth and is expressed as a fraction.
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。
An ellipsoid is usually chosen for data processing since observed space distance could not be used directly.
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