A method in a map server for providing map data to a client device via a communication network, the method comprising: receiving, from the client device, a request for map data corresponding to a specified geographic region; generating a first plurality of vector descriptors for rendering and displaying, at the client device, a first map image for the specified geographic region, wherein each vector descriptor indicates a geometry of a respective map element in accordance with a vector graphics format, and wherein the map data includes a depth indication that specifies an initial order in which a first of the map elements is rendered relative to a second of the map elements; causing the first plurality of vector descriptors to be transmitted to the client device via the communication network; receiving an indication that a second map image for the same specified geographic region is to be rendered at the client device at a same zoom level, wherein the second map image is different from the first map image; generating a modification indication (122, 152, 202, 252, 302, 304, 306) that indicates a set of one or more vector descriptors; causing the modification indication to be transmitted to the client device via the communication network for rendering and displaying the second map image using at least some of the first plurality of vector descriptors and the modification indication; characterised in that generating a modification indication (122, 152, 202, 252, 302, 304, 306) includes: identifying a first map element that is rendered as a part of the first map image at a first depth relative to one or more overlapping map elements and is rendered as a part of the second map image at a second depth relative to one or more overlapping map elements; providing, in the modification indication, an indication of the second depth that corresponds to the identified first map element; and providing, in the modification indication, a change depth indication to indicate that the identified first map element is to be rendered according to the second depth. receiving, from the client device, a request for map data corresponding to a specified geographic region; generating a first plurality of vector descriptors for rendering and displaying, at the client device, a first map image for the specified geographic region, wherein each vector descriptor indicates a geometry of a respective map element in accordance with a vector graphics format, and wherein the map data includes a depth indication that specifies an initial order in which a first of the map elements is rendered relative to a second of the map elements; causing the first plurality of vector descriptors to be transmitted to the client device via the communication network; receiving an indication that a second map image for the same specified geographic region is to be rendered at the client device at a same zoom level, wherein the second map image is different from the first map image; generating a modification indication (122, 152, 202, 252, 302, 304, 306) that indicates a set of one or more vector descriptors; causing the modification indication to be transmitted to the client device via the communication network for rendering and displaying the second map image using at least some of the first plurality of vector descriptors and the modification indication; characterised in that generating a modification indication (122, 152, 202, 252, 302, 304, 306) includes: identifying a first map element that is rendered as a part of the first map image at a first depth relative to one or more overlapping map elements and is rendered as a part of the second map image at a second depth relative to one or more overlapping map elements; providing, in the modification indication, an indication of the second depth that corresponds to the identified first map element; and providing, in the modification indication, a change depth indication to indicate that the identified first map element is to be rendered according to the second depth. identifying a first map element that is rendered as a part of the first map image at a first depth relative to one or more overlapping map elements and is rendered as a part of the second map image at a second depth relative to one or more overlapping map elements; providing, in the modification indication, an indication of the second depth that corresponds to the identified first map element; and providing, in the modification indication, a change depth indication to indicate that the identified first map element is to be rendered according to the second depth.