We are all familiar with the "Then & Now" approach to comparing contemporary and historic landscapes and urban settings. First we see a photograph of a building (or street or whatever) as it looks today. Then we compare it to another photo of the same site 50, 75, 100 years ago. It is a compelling way to emphasize change (or permanence, I suppose, though it is seldom used that way).
Two examples are the phone apps for Canadian cities developed by Jim Marsh and his team at Historica Canada. Check out Toronto in Time and Vancouver in Time.
But I had never seen a "Then & Now" like the Museum of London has developed. Called Streetmuseum, it is an app that blends the then and the now in a single, hybrid image. You can see how it works at the BBC site. Very cool.