一条在地球表面上连接磁偏为零的点的假想线。
On the Earth's surface where the magnetic declination is zero.
如果段中存在间隙，则绘制一条假想线来使它封闭。
If there is a gap in a segment, draw an imaginary line to close it.
一条通过的字形的假想线，表示印刷时字符应依此线排齐。
The imaginary line extending through a font and representing the line on which characters are aligned for printing.
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