https://www.merriam-webster.com/dictionary/imagine

imagine

1: to form a mental image of (something not present)

2: SUPPOSE, GUESS

3: to form a notion of without sufficient basis : FANCY

4archaic : PLAN, SCHEME

https://www.merriam-webster.com/dictionary/visualization

visualization

1: formation of mental visual images

2: the act or process of interpreting in visual terms or of putting into visible form

3: the process of making an internal organ or part visible by the introduction (as by swallowing) of a radiopaque substance followed by radiography

"Imagining" a line is just another word for "visualizing". Some people literally cannot imagine, cannot visualize a line.

So if the line is still at the same level, how would that be possible if the earth curved?

Earths Flat, yo.