difference between an asset revaluation decrease and impairment

I would suggest that one way in which it is different is impairment exists if is that the recoverable amount is determined as being the higher of value in use or fair value less costs to sell. If this recoverable amount is less than the carrying amount it is impaired. In other ways the process is indeed very similar in terms of accounting treatment. With revaluation the whole class of similar assets would also have to be re-valued which would not necessarily be the case with impairment.