Wow, this is nice. The Yahoo blog has announced MapMixer, a way to overlay your own maps on to Yahoo Maps, and give your overlays Yahoo-Map-Like functionality. You can try it at http://maps.yahoo.com/mapmixer (you’ll need a Yahoo account.)
You can jump right in by uploading a map but I recommend browsing the existing maps to get an idea of how people are using the feature. Take a look at the Cerritos College map for an example of an overlay that crams a huge amount of information into what was a couple of blank blocks.
Note there’s a “layer opacity” slide bar in the upper right corner of the map that allows you to choose which layer of data is more prevalent. Sometimes the opacity tool is itself hard to find depending on how complicate the overlay is.
There are some limits to what you can do with the overlay. You can switch to satellite zoom without a problem, but I found that for some of the locations, switching to satellite was meaningless as pictures were not available for as close a zoom as the overlay. When the pictures WERE available, the overlay over a satellite image became very interesting (see this Los Angeles Convention Center map to do some zoom experimenting) Of course if you zoomed out too far you’d lose the overlay.
Uploading a map requires logging in to your Yahoo account, specifying an address, and then uploading an image. You can “point match” — specify two matching points on your and Yahoo’s map to allow Yahoo to align them. You can adjust the alignment as well if Yahoo doesn’t get it quite the way you want it. Note that all maps are searchable and public.
This is going to be a great tool for sites that have large areas that aren’t detailed in a mapping program, or businesses who want to create direction/map services with lots of landmarks. From here I’d love to see customized inlay maps — create an overlay, then an persistent inlay (maybe just an uploaded image?) That would provide a building layout, specific parking map, etc.