The common base maps such as World_Topo_Map (MapServer) define zoom levels down to 23. In reality the tiles only go to level 19, but because they are defined to 23, some clients such as ArcGIS.com map are able to scale the lower level tiles so that users see something better than "No Data Available". I want to provide this functionality in my applications, but haven't been able to figure out how to make use of it.
One part that is especially interesting is that zooming past the "real" tile level, the clients know to make a different request that returns a description of the tile rather than an image. A normal image tile request URL is /tile/level/index/index but an extended level tile request URL is /tilemap/level/index/index . I don't see the /tilemap functionality described in any server API documentation so I'm not sure how to interpret what it returns.
Are there any examples or documentation that would help me use this in custom applications?