土 (earth) supplies the indexing radical and the core image of flat ground. 坦 is what land looks like when it's level - open, smooth and easy to traverse. The radical links it to 坪 (level plot), 场 (open ground) and 地 (land).
旦 (dàn) supplies the sound, drifted to tǎn. 旦 itself pictures the sun rising over the horizon line - a wide-open vista that quietly reinforces the openness sense of 坦. Same phonetic feeds 但 (but), 担 (carry), 胆 (gall).
