SUPERIMPOSING a historic photo on an up-to-date snap of the same scene is a neat way to bring history to life, is called Computational rephotography. If you want to take a modern photo to effectively contrast with its historical counterpart, you must take it from "same spot, and with same zoom level". If you don't, combined picture ends up looking disjointed, with roofs, walls and roads poorly matched.
"Researchers at MIT in Boston, developed a technique called "visual homing that's used in robotics to send a machine to a precise location, such as a charging station. The team's software runs on a laptop linked to a digital camera. The software compares camera's view to a preloaded historical scene and provides instructions to adjust camera's position and zoom to best match scene. The laptop is a temporary measure, however: "We envision the tool running directly on the camera," the team says."
At Flickr and historypin, you can see the old shots lined up over new, like a window into the past.