Close Range Aerial Photogrammetry utilizing Small Unmanned Aircraft Systems (sUAS):
Acklam’s Aerial Survey Team recently flew a 9.5-mile corridor for a project requiring up-to-date close range orthophoto’s for a defined width along the corridor. In order to maintain accuracy, +/-50 ground control points (GCP’s) were set in various locations evenly throughout the project area. Over the 88.4 miles of flight lines and 2,400 photos were taken throughout the course of the project corridor flight. This flight was performed using a SenseFly eBee Plus fixed-wing Unmanned Aircraft System (UAS). This UAS allows the Aerial Survey Team to fly large areas in short amounts of time. The final product deliverables included a surface put together from a point cloud obtained from the imagery along with ortho-rectified photography that the client utilized for multiple purposes.
Ortho-rectified photos help with planning/design and when used with time lapse can document multiple phases of construction. Photographs and models of large sites can be shared with anyone via secure cloud networks. Surfaces can be used to accurately determine cut/fill volumes and show existing topography conditions of a site at a specific time.
The Aerial Survey Team started planning the flight using aerial photos readily available to the public. Optimally spaced GCP’s were marked then adjusted and measured on site. The American Society for Photogrammetry and Remote Sensing recommends that 20% of GCP’s be used for check points. Check points can validate accuracies during a project and in this case, the Root Mean Squared (RMS) Error was under 0.06’ horizontally and 0.11’ vertically. There were several other aspects of the planning process that had to be considered including, but not limited to, battery usage, tall structures, weather, and lighting. Fluctuations in the wind and light can distort the imagery and cause confusion for the software while processing. Complex math formulas to account for collinearity, strip, and bundle adjustments can now be ran in the background with software programs like Pix4D which significantly cuts down on error and increases efficiency.