We made a trip to the Napa Library to look at Sanborn maps on microfilm. I made copies of my neighborhood and brought them home to look at them more closely. I was looking at the earliest Sanborn map for Napa which was created in 1886 and noticed my house was not shown on Randolph Street. I then looked at the 1891 Sanborn map, which was the next year it was created. On this map I saw my house and the outline of the structures. The curious thing is there are two dates for the building of my house, one is 1870 from the Napa Planning Department and 1889 from the Historical Register. If my house was built on Randolph in 1871 why wasn’t it on the 1886 Sanborn Map? Then I looked again at the two maps and noticed on Division Street, in an adjoining lot, was a house with a very similar footprint on the 1886 map. Then in 1891 the house on Division wasn’t there but there was a house on Randolph with an almost identical footprint! It looked like the Carriage House on Randolph had been moved to the back of the lot too. So now it looks like it all fits! My house was most likely built in 1870 on Division Street and then moved to Randolph Street in 1889. Apparently it was not uncommon to move houses which seems like it must have been amazing feat to watch! The Sanborn maps are filled with information. The maps were originally created solely for insurance assessment purposes. The maps themselves are large-scale lithographed street plans at a scale of 50 feet to one inch on 21 by 25 inches sheets of paper. The maps were utilized by insurance companies to determine the liability of a particular building through all the information included on the map; outlines of each building; street names; property boundaries; building use; building material, fire department locations, natural features such as rivers and canals; location of water and gas mains; names of most public buildings, churches and businesses; and other local information. The very decision as to how much, if any insurance was to be offered to a customer was often determined solely through the use of a Sanborn map. The maps also allowed insurance companies to visualize their entire coverage areas. Daniel Alfred Sanborn, a civil engineer and surveyor, began working on fire insurance maps in 1866. That year, he was contracted by the Aetna Insurance Company to prepare maps of areas in Tennessee. Within several decades, the company became the largest and most successful American map company. Sanborn’s insurance map business began to decline after World War II, as the insurance industry began modernizing methods of assessing and mitigating risk. Sanborn could no longer afford to maintain its army of surveyors, but continued to sell its maps and perform some updates. Over time, the company diversified into other mapping activities, and is today a major geospatial specialist and holder of electronic GIS assets and systems, though the fire insurance business continue as a niche department.