Sanborn keymaps: location priors, if you can read them

Over the past few months I’ve been working on mapsnap, a program to automatically georeference old insurance maps. My previous post introduced the project and explained why it’s so interesting and worth doing. This post gets into some more of the technical details. You can browse all the georeferenced maps at mapsnap.org. Most volumes of the Sanborn Fire Insurance Maps come with some special sheets that we can use to guide georeferencing. The most important one is the keymap. This is a graphic depiction of which blocks are covered by which pages. Here’s part of one in Brooklyn: The large black numbers here refer to pages: page 30 covers the Manhattan Bridge approach, page 34 is just to its east, etc. When you manually georeference a Sanborn map on OldInsuranceMaps.net, placing the keymap is typically one of the first steps. With the keymap in place, you can quickly get a sense for where each of the other pages is, how it’s oriented, and what its scale is. At first glance, the keymap…

Commenting on Bubbles has been removed. Why? If you want to talk about this post, write to the author, use the comments on their blog if it has them, or reply with a "RE:" post on your own blog. Suggestions? feedback@bubbles.town