横断 and 縦断: crossing a target and crossing lengthwise
Compare crossing a road, continent, or island with a line running lengthwise through a building in a plan, using each target’s axes and stated direction.
Identify the target crossed in 横断
横断 (おうだん) refers to crossing a target. “大通りの横断には歩道橋を使いました。” describes using a pedestrian bridge to cross the main road.
“探検隊は大陸を東から西へ横断しました。” says that the expedition crossed the continent from east to west. The continent is the target, and 東から西へ gives the direction stated for this crossing.
Read the lengthwise axis in 縦断
縦断 (じゅうだん) refers to crossing a target lengthwise. “探検隊は島を北から南へ縦断しました。” says that the expedition traversed the island from north to south.
“図面の中央に建物を縦断する線を引きました。” describes drawing a line in the middle of the plan that runs lengthwise through the building. This use concerns the direction of the line, rather than a person traveling.
Combine the stated compass direction with the target’s axis
The continent example states east to west, and the island example states north to south. These directions do not make east–west a universal condition of 横断 or north–south a universal condition of 縦断.
Having a building as the target does not turn 縦断 into climbing upward through it. The action stated in the plan sentence is drawing a line, and 縦断する describes that line’s direction.
Are the directions in the two expedition examples fixed conditions for every use of 横断 and 縦断?
Show the answer and why
No. The sentences explicitly give east to west for the continent and north to south for the island. Read the direction with each target’s axes, and do not turn the line in the plan into upward travel.