Photogrammetry is the practice of extrapolating spatial relationships from photographic imagery. While the concept of photogrammetry is centuries old, today’s powerful software quickly generates a 3-dimensional point cloud of any scene or object captured with a half dozen or up to several hundred overlapping digital photographs. Two recent projects illustrate very different applications for this technology in expanding what Vertical Access can do for clients in hard to reach areas.
Developing Background Drawings from 3D Imagery
Work on existing structures begins with accurate as-built documentation. For most conditions survey projects in difficult-to-reach areas, we bring our industrial rope access teams armed with the Tablet PC Annotation System (TPAS®) to digitally capture data and photographs of representative and notable conditions. TPAS® links each condition and photograph to a location on a pre-loaded AutoCAD drawing.
But what if there is no background drawing on file for use in the field? In the past our project teams have created background drawings from photographs taken from a boom or aerial lift, from neighboring buildings, or with other means of measurement (such as deploying VA technicians on rope with measuring tape).
Enter photogrammetry. On a recent project, the main façade of a building could not be surveyed with an aerial lift due to site features, leading the project team to investigate the use of an unmanned aerial vehicle (drone) to capture close-range imagery of the façade conditions as well as photographs for use in the creation of scaled, detailed background drawings.
Vertical Access’ licensed drone pilots operated a DJI Phantom 4 Pro drone to obtain medium-range imagery for the creation of a photogrammetric 3D point cloud, as well as close-range 4K video for the conditions documentation of the structure. The field work was accomplished by a team of two pilots in less than two days and required only partial closure of pedestrian walkways to safely conduct the drone flights.
Back in the office, VA used Pix4D Mapper Pro photogrammetry software to generate the point cloud, textured mesh, and 2D orthoimages (elevations) of the areas to be surveyed.
The orthoimages were scaled and placed as background images in an AutoCAD drawing, and fault conditions such as cracks, spalls, and displaced masonry units were annotated digitally using TPAS®. Condition quantities were extracted from the AutoCAD drawing into an Excel spreadsheet to facilitate repair cost estimates.
A close-range visual inspection as well as image capture for photogrammetry was completed in much less time than it would have taken to inspect the façades and obtain hand measurements using an aerial lift. The field work was accomplished with minimal disruption to pedestrians.
Modeling Decorative Features of Historic Structures With Photogrammetry Restoration of historic decorative terra cotta, cast stone, or natural stone often requires extensive replacement of individual units. Erecting scaffolding for hands-on access to measure and remove existing units adds significant cost to project budgets. Vertical Access recently used photogrammetry to aid in a pilot project as an alternative option for acquiring dimensions of existing units.
VA technicians photographed a terra cotta spandrel panel and parapet, using existing scaffolding to gain hands-on and close-range access. The photographs were used to create 3D photogrammetric models of individual units, achieving a level of detail sufficient to be transformed into specifications for replacement units.
With VA’s demonstrated rope access capability, individual units or entire assemblies can be photographed and modeled without the need for scaffolding, yielding a cost savings for the building owner.