바탕으로 and 토대로 can be used interchangeably sometimes, but there is a difference too. 바탕 has a fairely broad meaning, including "foundation", "basis", "background", or anything something is based on or added to. 토대 means a foundational structure or groundwork. It is the supporting base something builds on, so 토대로 works better with more substantial things, like constructing something big or complex. For example, they are not interchangeable in these examples.. - 하늘색 바탕의 보라색 무늬 = A purple pattern on the azure background. - 그의 업적은 회사의 경이로운 발전에 토대가 되었다 = His accomplishments served as the foundation for the company's remarkable growth.