Apparatus and methods according to some embodiments of the present invention use a graphical model, such as a Markov random field model, to represent principal driving directions within an environment. The model has a plurality of nodes representing spatial locations within the environment, and the principal direction for each node is determined probabilistically using linear features detected within an image of the environment. Apparatus and methods according to embodiments of the present invention can be used in improved autonomous navigation systems, such as robotic vehicles.