is top down estimation possible even after actulizing activity ?

http://www.brighthub.com/office/project-management/articles/8542.aspx

Avoid Top Down Planning only, it is wrong. It is advocated by a few that cannot build team work or who believe themselves the owners of all truth, the "wannabe Guru". Even if you are to plan it yourself it is wrong, a common misconception when from the very beginning fixing your WBS and not allowing for changes as you get more and more into the details.

If top down planning is wrong and a combination is best this is not necessarily true for estimation, for estimation top down is simply wrong. What, are you to fix your estimate at the top and then by brute force get to the individual parts?