The Crown Hill structure (the north bound Highway 400 bridge that passes over Highway 11) has various elements that are not simple, involving arches as the abutments transition into the girders. In addition, the horizontal alignment of the structure is also a curve. To top it all off, lane closures are required to survey this structure as the shoulders underneath the bridge do not allow enough clearance to survey from a safe distance. We decided to increase our chances of success by incorporating an innovative method of data collection, LiDAR scanning. A LiDAR scanner is capable of capturing thousands of points a second. Colour photos are also taken which allows each of the data points to be colourized. We rented a LiDAR scanner for a day and performed 6 scans of the underside of the bridge. These scans were referenced together and produced what looks like a surface made out of points (in fact it looks like a 3D model of the bridge complete with colour). This allows a technician to trace the outlines of the bridge elements. This line work can then be exported and brought into AutoCAD. We performed a direct comparison between the LiDAR line work and the data from our second round of total station surveying. The results were in most cases within 2 cm of each other.
Although the processing time can be a bit higher with LiDAR scanning than with total station, the advantages are abundant. Much more data can be collected in a shorter period of time and the tracing can be done in the office without the pressures associated with traffic control and fieldwork. With total station methods it is difficult for the field crew to know whether the results are good while on site, whereas with LiDAR scanning, a lot of the work is done in the office with only the scans themselves being done in the field. The LiDAR scans provide abundant data for re-sampling if necessary, where with total station methods an additional field visit would be required. In addition there are more types of deliverables that can be produced including line work, point clouds, and mesh surfaces.