Building Footprint Simplification Techniques and Their Effects on Radio Propagation Predictions

Building footprint simplification is of critical importance to radio propagation predictions in wireless communication systems as the prediction time is closely related to the number of both buildings and vertices involved. Intuitively, if the complexity of footprints (i.e., the number of vertices in the footprints) is reduced, predictions can be generated more quickly. However, such reductions often affect the accuracy of results as the simplification error constrains the efficiency that can be achieved. To achieve a good vertex reduction rate for the footprints involved and at the same time preserve the shapes of footprints in terms of their areas, orientations, and centroids, we propose a number of efficient single-pass methods to simplify building footprints. To satisfy constraints on edges, areas, and centroid of simplified footprints, multi-pass methods are suggested. Hybrid methods take advantages of complementary property exhibited by different footprint simplification methods. We assess the baseline effectiveness of our proposed techniques, and carry out an extensive comparative evaluation with real GIS data from different municipalities. Through experimentation, we find that hybrid methods deliver the best performance in both vertex reduction rate and simplification error. We examine the effects that these footprint simplification methods have on the ray-tracing based radio propagation prediction systems in terms of processing time and prediction accuracy. Our experiments show that footprint simplification methods indeed reduce prediction time up to three-folds, and maintain prediction accuracy with high confidence as well. We also investigate the relationship between footprint simplification error and the prediction accuracy. We find that the prediction accuracy is sensitive to the distortion (i.e., change of shape) of building footprints. This helps us to better understand the trade-off between precision of building database and the accuracy of predictions generated by ray-tracing based radio propagation prediction systems. Indexing Terms: Footprint simplification algorithms, simplified maps for radio propagation prediction, experimental evaluation of spatial reduction methods. This work was partially supported by NSF under grant IRI-9733642, the U.S. Department of Commerce under grant 4000186, and The New York Center for Advanced Technology in Telecommunications (CATT).