Measuring and drawing existing structures for preservation work can be accomplished with the aid of a variety of technologies – laser scanning and photogrammetry chief among them – that increase both project accuracy and the speed of data collection in the field. Increasingly, both of these leading technologies are sought after at the same time in order to capitalize on each technology’s individual strengths. Rich laser-scan-data in dense clouds of 3D points provides a great point of departure for the creation of a “BIM” model. Meanwhile, mosaics of scale-able, rectified photographic images – created through photogrammetry – offer the users of the project documentation direct information about the physical conditions of the structure. “Hybrid drawings” consisting of building-information-modeling (BIM), rectified photography and record measured drawings provide an efficient and definitive means to quantitatively describe the structure.
So in cases when a project requires a combination of lidar sourced point cloud data and photogrammetry, how do the two systems talk to each other? The current answer is through the identification of registration points/conjunction points. These are “smart points” that correspond to a specific location on the structure being measured and drawn. Sometimes these locations or points are somewhat “theoretical”, existing as an idea as much as a point in space – for example “the northwest corner of the building”.
There are some important distinctions between point clouds created automatically by machine and those created through a process of selection based on the experienced judgement of a human.