We went up to the roof, from where we had a good view of the procession.

这个句子中，from where引导非限制性定语从句。where为关系副词作介词from的宾语。

from不可以移到定语从句末尾。

Across the street from where we were standing was a little park.

这个句子中，from后接名词性关系分句作介词宾语，from where we were standing整体是一个介词短语，修饰前面的介词短语across the street, 表示马路对面以哪里为基准。注意from where we were standing并不是定语从句，也不是修饰street这个名词的。

from where we were standing=from the place wherer we were standing.

这个句子中from也不可以放在从句末尾。