1. A method comprising: receiving data collected by a first sensor while a vehicle is driving on a roadway, the first sensor having a first sensor range; using the received data from the first sensor to generate a map of the vehicle's environment within the first sensor range; estimating, by a processor, the vehicle's geographic location and heading by comparing the generated map to a stored map identifying the geographic locations of objects including reference objects having particular retroreflective, brightness, or intensity characteristics; receiving data collected by a second sensor while the vehicle is driving on the roadway, the second sensor having a second sensor range greater than the first sensor range; identifying a reference object having a particular retroreflective, brightness, or intensity characteristic from the data collected by the second sensor outside of the first sensor range; correlating the identified reference object to a reference object of the stored map based on the particular retroreflective, brightness, or intensity characteristic; and using the correlation to refine the estimated heading of the vehicle.