A description of Sanborn Maps is (from the NEHGS NewEnglandAncestors web site):
Sanborn fire insurance maps are the most frequently consulted maps in both public and academic libraries. Sanborn maps are valuable historical tools for urban specialists, social historians, architects, geographers, genealogists, local historians, planners, environmentalists and anyone who wants to learn about the history, growth, and development of American cities, towns, and neighborhoods.
They are large-scale plans containing data that can be used to estimate the potential risk for urban structures. This includes information such as the outline of each building, the size, shape and construction materials, heights, and function of structures, location of windows and doors. The maps also give street names, street and sidewalk widths, property boundaries, building use, and house and block numbers. Seven or eight different editions represent some areas.
Sanborn Maps exist for many cities and towns for many years, but they can be difficult to find. Only some can be found online. The New England Historic Genealogical Society (NEHGS) has a collection of Sanborn Maps for 8 Eastern States (New England, NJ, NY) for many cities and towns, and many years.
For instance, my Seaver ancestors lived in Leominster MA for many years. The NEHGS site has maps for 1884, 1891, 1896, 1901, 1906, 1912, 1922, and 1931. I was able to find the ancestral homes at 149 Lancaster Street and 290 Central Street, in addition to the locations of the Horn Supply Sompany and Paton Mfg Company where my grandfather and great-grandfather worked. On the NEHGS site, you can select the "Download Map" tab and get a PDF file that you can magnify until you can read it easily, then move around in the map using the browser slide bars. Finally, you can download the map by doing a "File" "Save As" and putting it on your hard drive where you can find it later.
For San Diego, the San Diego Historical Society has the Sanborn Maps in a large format - to get a copy you have to do a portion at a time on the xerox machine. They were quite useful in finding which buildings were extant in a certain year, and by examining maps for different years I could see the community grow up around my ancestral home on 30th Street.
If you have found addresses of your ancestors or relatives in City Directories or census records, then the Sanborn Maps can easily show you the community that they lived in, and how the community changed over time.
Have you looked for Sanborn Maps of your ancestral places in either digital or paper form? Genealogical societies, historical societies, public libraries and academic libraries are probably your best sources.