  Ground surface detection in point cloud is widely used as a key module in
autonomous driving systems. Different from previous approaches which are mostly
developed for lidars with high beam resolution, e.g. Velodyne HDL-64, this
paper proposes ground detection techniques applicable to much sparser point
cloud captured by lidars with low beam resolution, e.g. Velodyne VLP-16. The
approach is based on the RANSAC scheme of plane fitting. Inlier verification
for plane hypotheses is enhanced by exploiting the point-wise tangent, which is
a local feature available to compute regardless of the density of lidar beams.
Ground surface which is not perfectly planar is fitted by multiple
(specifically 4 in our implementation) disjoint plane regions. By assuming
these plane regions to be rectanglar and exploiting the integral image
technique, our approach approximately finds the optimal region partition and
plane hypotheses under the RANSAC scheme with real-time computational
complexity.
