In the late 1930s the Home Owners' Loan Corporation (HOLC), a New Deal agency created to refinance homes and prevent foreclosures, surveyed real estate trends in the nation's largest cities. Working with local lenders and realtors, they assessed neighborhoods using a number of factors ranging from terrain to income levels to the "infiltration of a lower grade population" (by which they meant African Americans, Jews, and immigrants). . . . red "D" areas were "characterized by detrimental influences in a pronounced degree."
Other scholars are also working on GIS-enhanced versions of the HOLC maps. LaDale Winling of Virginia Polytechnic Institute and State University (and blogger at Urban Oasis), has created a website linking to HOLC maps he has worked on digitizing. And Slate recently published a list of on-line sources for redlining maps, as well as a discussion of the role of the HOLC maps.
More commentary and data on redlining can be found around the Web: see, for example, Evan Tachovsky, "The Legacy of Redlining in Rust Belt Cities," and the recent article in Talking Points Memo's "Primary Source" by N.B.D. Connolly on "How did African Americans discover they were being 'redlined'?"