noun
Technical term used in geometry, surveying, and astronomy to refer to a flat surface perpendicular to the direction of gravity.
測量では、地平面を基準にして高さを測ります。
In surveying, height is measured relative to the horizontal plane.
この図では、地平面が点線で示されている。
In this diagram, the horizontal plane is indicated by a dotted line.
Compound of 地 (chi, 'earth') + 平面 (heimen, 'plane'). The reading is straightforward on'yomi.