Midwest Aerial Devises Custom Collection Strategy to Acquire True Orthoimagery of Canadian Capital
Concerned with the potential for building lean in aerial imagery over downtown Ottawa, Ontario, Canada’s National Capital Commission released an RFP calling for acquisition and delivery of true orthoimagery in which every pavement edge and sidewalk would be clearly visible between tall buildings. TDB Consultants Inc., a Canadian resource management firm, teamed with Midwest Aerial to provide distortion-free orthomosaics that met the Commission’s stringent specifications.
“Building lean is a distortion in which tall structures appear to lean outward from the center of aerial photographs,” said Midwest Aerial President Ken Scruggs. “As a result, the ground surface immediately below the building is obscured from view.”
This creates problems for organizations that need to map infrastructure assets from air photos in downtown areas. Sidewalks, curbs and any features on them, such as manhole covers, utilities, handicap access and fire hydrants, are hidden from view. Such was the frustration experienced by the National Capital Commission, a Crown corporation tasked with developing, maintaining, and improving Ottawa’s Capital Region in and around its famous Parliament Hill.
For the 2011 project, the Commission sought orthoimagery covering 1,380-square-kilometers of the Ottawa metropolitan area at 20-centimeter pixel resolution. Deliverables were to include natural-color orthoimages, each with accompanying digital elevation models, and a seamless mosaic. Horizontal accuracy had to meet Canada’s 1:2000 scale digital topographic mapping database with 90 percent of features located within 0.50 millimeters (at map scale) of their true positions. And the imagery had to be acquired in a tight window before leaf-out.
TDB has a 25-year history specializing in land management, geomatics, and engineering, won the contract to deliver the true orthoimagery. The Canadian company had worked with Midwest Aerial on two earlier projects and sub-contracted the aerial portion of the Ottawa assignment to the Columbus, Ohio, firm specifying use of one of Midwest Aerial’s two Z/I Imaging DMC II-140 digital mapping cameras.
“Based on their prior experiences working with us, TDB knew the DMC II could do the job,” said Scruggs. “The DMC II provided the accuracy and geometry required for this project, and TDB appreciated how easily the digital data integrated into their image processing workflow.”
Having built its reputation in the geospatial industry as a firm that specializes in reliable aerial image and photo acquisitions regardless of tight schedules and complex airspace, Midwest Aerial left nothing to chance for the Ottawa collect. To ensure acquisition of high-quality imagery without building lean, Midwest developed a customized flight plan for the project.
The Midwest team used Google Earth to pinpoint coordinate points for every street intersection in Ottawa’s densely developed city center. These points were entered into the DMC II Z/I Mission Flight Management Software as the precise locations where each image frame would be captured. Taking into account the camera recycle rate of 2.2 seconds, Midwest calculated the frames could be successfully acquired at a ground speed of 125 knots or less from the 8,400-foot altitude above ground required for 20-cm pixel resolution.
“We created two flight plans – one for the central business district with the tall buildings, and another for the outlying Ottawa metropolitan area,” said Scruggs. “For the central business district, we flew down every major street from the southwest to the northeast, capturing images at every intersection which resulted in an as much as a 96 percent side lap and 93 percent forward overlap.”
By collecting images directly over every street intersection with more overlap than typically needed, Midwest Aerial ensured that every roadway, sidewalk, curb and surface feature between the numerous tall office buildings and condominiums would be captured in multiple frames from one or more angles. The goal was to give TDB plenty of true vertical imagery to choose from when generating the orthorectified products.
The acquisition strategy worked as planned. From the 3,300 image frames collected by Midwest Aerial over Ottawa, TDB generated 345 orthorectified image scenes which were also mosaicked into a seamless map sheet, virtually eliminating building lean, ensuring all street surfaces, pavement edges and sidewalks were fully visible even among the tallest buildings in Ottawa.
“Our client was extremely satisfied with the product and informed us these were among the best orthoimages they had ever seen,” said Rob Kragt, TDB Director of Geomatics.
Midwest Aerial now uses the tightly coupled GPS/IMU on all of acquisition flights and finds these data sets are vital deliverables to partners and clients who are performing the image processing portion of a project.
“Attention to seemingly small details like the selection of a GPS/IMU and the creation of a custom collection strategy have a cumulative impact on the overall quality of an aerial imaging project,” said Midwest’s Scruggs.