1. A navigational system comprising: means for periodically generating a first pattern of terrain features from a sensed image of ground terrain, said generating means including: means for periodically sensing an image of ground terrain; and means for extracting the first pattern of terrain features contained in the sensed image; means for prestoring a second pattern of terrain features contained in a reference image, said sensed image substantially corresponding to a portion of the reference image; means selectively responsive to the first and second patterns of terrain features for determining the relative coordinate and orientation displacements of the first pattern within the second pattern, said determining means including: means responsive to the features of the first and second patterns for selectively developing a feature neighborhood of nearby features surrounding each feature in the first and second patterns; means selectively responsive to the feature neighborhoods of the first and second patterns for developing a plurality of neighborhood comparison signals indicative of the closeness of match and coordinate and orientation displacements between each feature neighborhood of the first pattern with respect to each feature neighborhood of the second pattern; and means responsive to the neighborhood comparison signals for computing the relative coordinate and orientation displacements of the first pattern within the second pattern; and means responsive to the relative coordinate and orientation displacements for updating position and heading coordinates of the navigational system.