# Busy Road within a City

The aerial photos for the computation of this point cloud could be done without any road blocks and traffic disturbances. Due to the effect that nearly every part of the road is images on a sufficient number of images it does not matter that on nearly each single image vehicels are images, too. The vehicles do not appear in the point cloud, because they are not constant marks on different aerial photos. Only near the left margin of the view holes seem to be in the road. Due to traffic jams in front of traffic lights permanently different cars have been at the places of these holes. So the street surface was on only few or may be on no photos imaged and could not be computed. This traffic jam explains the darker colours near the left margin of the view, because in conjunction with the low sun these cars casted a permanent shadow on the street.

At this place an accident with a car turning into a private drive happened. The final positions of the cars had not been measured, but photos of their final positions exist. With the point cloud of the area the perspective of the pictures could be reconstructed. It is uniquely specified by the place of the camera, its height above street level, the direction of the camera, the tilt of the axis versus the street, the rotation of the image with respect to its axis and the mathematical focus length. Therefore a sufficient number of geometric marks of the point cloud has to be matched with the coresponding marks in the photo. Especially in urban situation the number of geometric marks on the street surface often is unsuffixient. But marks on buildings like corners of windows and roofs are applicable as well.

In the next step 3-D-models of the cars are placed in the point cloud in that way, that they match with their positions in the photo at their best. So the final positions of the cars are reliable reconstructed even if they have not been measured durig acquisition of the accident data. Tests resulted in probable inaccuracies of the cars position in the order of magnitude of 3 cm and concerning the vehicles direction in the order of magnitude of 1 °.

Based on the reconstructed final positions all possible trajectories for the car turning into the private drive could be computed. All these trajectories leaded to a clear answer on the question of evidence. Without this technology this would not have been possible.

A 3-D presentation of this point cloud is available. On most up-to-date hardware – desktops and laptos as well – the presentation runs in your web-browser. Older hardware and most of the smartphones and tablet will not be able to display the point cloud, because the point cloud is to detailed for that hardware. Examples with reduced point density, which should run on weaker hardware, too, will be prepared in future.