A method of determining the characteristics of a scene around a vehicle comprises capturing a stereo pair of images of the scene and processing the images to produce a depth map of the scene. Each pixel in the depth map is assigned a value that corresponds to the range of a corresponding region in the scene, the pixels being arranged in a grid of rows and columns with each column of pixels in the grid corresponding to a vertically oriented set of regions in the scene and each row a horizontally oriented set of regions in the scene. The values for one or more columns of pixels in the depth map are binned to form a corresponding histogram, each bin in the histogram having a count value that corresponds to the number of pixels in the column that have a depth within the range assigned to the bin. The or each of the range bin histograms are processed to determine for the or each histogram the lowest range bin that is indicative that an object that represents a non-drivable region is present at a depth that lies in the range of depths assigned to the bin, thereby identifying the location of one or more boundary points that lie on a boundary of a drivable space in the scene.