I wanted to overlay an old map of DC onto a current map, Leaflet makes it easy to do so but you need to know the geographic coordinates of two corners of the map image to place it correctly. This app allows you to find those coordinates by manually adjusting the scale and position of the map image or automatically fitting the image by choosing two landmarks.
"L'Enfant plan" by Andrew Ellicott, revised from Pierre (Peter) Charles L'Enfant; Thackara & Vallance sc., Philadelphia 1792 - Library of Congress. Licensed under Public domain via Wikimedia Commons.