Photogrammetry - The science and technology of obtaining reliable information about physical objects and the environment by recording, interpreting, and measuring aerial photographic imagery.

The NCDOT Photogrammetry Unit produces digital spatial information products georeferenced to the North Carolina State Plane Coordinate System. Digital imagery mosaics and digital orthophotos produced in the Photogrammetry Unit are used for transportation planning and functional design. Topographic, planimetric, with cadastral information, are produced for preliminary design and public hearings. Highly detailed base plan sheets and digital terrain models are produced for final design, right of way acquisition, and construction activities. Earthwork volumes for selected construction projects are also calculated in the Photogrammetry Unit.

The Photogrammetry Unit is organized into several operational squads responsible for aerial photography and digital geospatial data production. These operational squads are the photographic laboratory, engineering squads, and stereo compilation squads. Duties of each squad are highly specialized and integral to the production of services provided by the unit. All staff is based in the NCDOT Century Center complex located in Raleigh.

Requests for photogrammetric mapping services come from many customers within NCDOT. Primary customers include other units in the Highway Design Branch, the Project Development and Environmental Analysis Branch, and Division Construction offices located across the state. Upon receipt of a request for services, engineering staff perform detailed studies to develop specifications. Flight planning, ground control planning, and coordination with field and aircraft operations are necessary for successful aerial photography acquisition.

The Photogrammetry Unit utilizes Cessna 441 aircraft to acquire aerial photography at altitudes ranging from 1,500 feet to 12,000 feet above mean terrain elevation. Aerial photographs are taken with RC-20 and RC-30 aerial cameras manufactured by LH Systems. These aerial cameras are interfaced with GPS (Global Positioning System) receivers for precision aerial photographic operations. The Photogrammetry Unit also collects GPS-IMU (Inertial Measurement Unit) data during most aerial photography missions. The GPS-IMU data is used to recreate the geometry of the aerial photography mission. Photographs can be acquired using black and white, color, or color infrared film. Aircraft operations are based from the NCDOT Aviation Division hanger located at Raleigh Durham Airport.

After completion of an aerial photography mission, the photographic laboratory processes the aerial film. Inspection for quality, overlap, and coverage is performed. The aerial film is numbered, indexed, and made available to the staff for hard copy reproduction and for conversion to a digital (soft copy) format. The photographic laboratory converts the aerial film to soft copy imagery using an Intergraph precision photogrammetric scanner . The soft copy imagery is then stored and accessed through a computer server and network utilized within the Photogrammetry Unit. Reproductions and digital conversion can be accomplished in black and white, color, or color infrared.

Engineering squads assemble and prepare a variety of materials, including field survey data, and prepare it for use by the stereo compilation squad. Stereo compilation squads use Intergraph ImageStation soft copy stereo plotting instruments to perform aerotriangulation, planimetric map compilation, base plan sheet compilation and digital terrain data collection. The stereo plotting instruments recreate the internal geometry of the aerial camera to produce a three-dimensional (3-D) model of the terrain that is viewed on a high-resolution computer monitor. Horizontal and vertical measurements can be made in the stereo model by means of a reference mark in the plotter. The soft copy stereo plotters support real-time graphics allowing terrain features to be viewed graphically in 3-D over the stereo imagery. Planimetric map data, base plan sheet data, and digital terrain data produced in the stereo compilation squad is then transmitted to the unit's engineering squads for further analysis.

The engineering squads merge and analyze the geospatial data provided by the stereo compilation squad with ground survey data received from the field. Engineering squad staff add descriptive feature information text, field surveyed graphic utility, and property data to produce planimetric maps and base plan sheets. Field surveyed terrain elevation data is incorporated with photogrammetrically collected terrain elevation data to produce digital terrain models used for design, and earthwork volume computation. The engineering squads also produce digital aerial mosaics and orthophotography. These georeferenced digital image products are reviewed for quality using available graphic data and then converted to the customer's specified image format. The engineering squads ensure product quality and deliver all georeferenced digital geospatial information products to the Photogrammetry Unit's many customers.