To measure the difference in elevation between poles, surveyors used an optical level—a telescope on a level base—placed halfway between the poles.
为了测量两极之间的高程差，测量员在两极中间的一个水平基座上使用了光学水准望远镜。
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Why It's Unique: Despite the mountainous topography of the region, trains traveling through the Gotthard Base Tunnel willremain at nearly the same elevation for the entire trip.
独特之处： 尽管该地区属于山地地形，但通过圣哥达基地隧道的火车在整个行程中几乎处于同一海拔高度。
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The Federal Emergency Management Agency (FEMA) publishes maps that designate Special Flood Hazard Areas delineated by a Base Flood Elevation (BFE).