We have also developed and evaluated a constrained 3D navigation technique which applies street topology in order to limit the navigation space. Similarly to, we assume that the road network is given as a graph with the set of nodes representing the terminal and intersection points of the streets and the set of edges connecting the nodes that represent the streets. Traveling in this mode consists of moving along streets and selecting from available streets at crossings with user-drawn gestures as commands for moving the camera. Contrary to the path drawing technique [9] where a path is drawn in the 3D space, the stroke-based gestures in our case operate in the screen space (like in [7]) to avoid problems of inaccuracy as mentioned above