level, wide
坦 centers on flatness and openness: a surface that is level, smooth, and wide, without obstacles or unevenness.
坦 combines 土 (earth, ground) with 旦, which likely contributes the sound and the idea of something spread out or level. The character suggests flat, open ground.
The left side is earth 土, and the right side 旦 looks like the sun rising over a flat horizon. Picture a wide, level plain of earth stretching out under the morning sun.
For タン, imagine a tan, flat desert plain: tan -> タン, and the earth is perfectly level and wide.