1. A method in a map server for efficiently transmitting image map data to client devices, the method comprising: receiving, from a client device, a request for map data for rendering a digital map of a certain geographic area; selecting map data for rendering the digital map; generating multiple map image layers using the selected map data, each covering the same geographic area with no overlap, including generating (i) a first map image layer including a first type of geographic data and having a first resolution, and (ii) a second map image layer including a second type of geographic data and having a second resolution different from the first resolution; and transmitting the multiple map image layers to the client device via a communication network for generating a single digital map of the geographic area.