You are right - such data should not go into OpenStreetMap. I am rather thinking of a new site that allows historical entries - though I wonder how this would be done.

This is a London 1746 map
http://commons.wikimedia.org/wiki/File:Rocque%27s_Map_of_London_1741-5.jpg

imagine historians could annotate that. I'd love to use a software such as you have it here at a new page I am designing with Wikimedia Germany at http://wiki-grid.org/ to allow histotrians to enter annotations.

...and I have many questions: How would one organise this with such different frames such as London 18th-century, London 1740s, London 1746. Open Street Map is simmpe. It only wants to be up to date. Our world would consist of historical layers. At the moment I am rather puzzeld by options and problems (and looking for people with a bit of more practical expertise to say: "that is what it would look like!"

This is something stupid I created in 2001: a map of London useful for all who visit the place in 1720 with the aim to buy novels of the last ten years.

http://www.pierre-marteau.com/resources/novels/places-london.html

We have travellers giving us information abot the city's infrastructure. I would love to connect their information with a map.