等とう偏へん角かく線せん

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Meanings

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isogonic line; isogonal line

Technical term in geomagnetism and navigation: a line on a map connecting points of equal magnetic declination.

等とう偏へん角かく線せんは磁じ気き偏へん角かくが等ひとしい地ち点てんを結むすんだ線せんです。
An isogonic line is a line connecting points of equal magnetic declination.