An additional neutral and more up-to-date indication for network coverage is
http://en.netmeterproject.com/ - after selecting Germany in the upper left corner you can view coverage maps of all four German networks gathered by end user devices. Hence you can only see trails of where users have moved along but not any covered surface areas. As this data comes from real life phones you can often see new LTE deployments sooner than official coverage maps show but it does not represent coverage areas, rather a collection of spots from which you can estimate network availabiliy.