Bottom-up architecture of Warehouse development

What is Bottom-up architecture of Warehouse development?

The second data warehousing systems architecture, the "Bottom-up" architecture became popular because the Top-down architecture took too long to implement, was often politically unacceptable, and was too expensive.

The central idea in Bottom-up architecture is to construct the data warehouse incrementally over time from independently developed data marts. The process begins with ETML for one or more data marts. No common data staging area is required. There is generally a separate area for each data mart. There may not even be standardization on the ETML tool.