About the Photogrammetry Unit​

  • ​The Photogrammetry Unit acquires aerial imagery and aerial LiDAR elevation data and generates geospatial information products used for transportation planning, design, and construction. The unit also partners with other government agencies for statewide orthophotography and aerial LiDAR elevation data programs serving as NCDOT's subject matter experts for aerial surveying.

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    Photogrammetry is the art, science, and technology of deriving three-dimensional (3-D) information from photographs. This is done by photographing the same scene from two different points by at least two photographs (stereo pair) thus allowing stereoscopic 3-D vision. These photographs can be obtained two ways: terrestrial or aerial. The NCDOT Photogrammetry Unit acquires aerial photography with a large format metric camera mounted on a maned aircraft or a small format non-metric camera mounted on an unmanned aircraft, commonly called a drone. The 3-D information obtained from these aerial photographs is used to generate products that aid in the completion of transportation facilities from inception through construction. Photogrammetry is an important process in transportation project development and construction as it provides a broad view of the project area in a short time. Benefits of photogrammetry include: mapping and elevation data collection that would take much longer and be more expensive for a ground surveying crew to complete traditionally; an alternate means for ground based surveying in dangerous areas; traffic free disturbance during entire process; and high resolution photos of broad areas which are publicly available due to the Photogrammetry Unit’s record retention policy.

    Photogrammetric products include digital mosaics, orthophotography, mapping horizontal (planimetric) features, elevation data in the form of digital elevation models (DEM) or digital terrain models (DTM), and construction earthwork pay quantities. The process to create these products involves highly skilled engineers, technicians, photographers, pilots, a metric camera on a manned aircraft or a non-metric camera on a drone, powerful computers with state-of-the-art hardware, and specialized software that computes complex mathematical equations. Please note that work on these products should always be done under the responsible charge of an NC Professional Land Surveyor following the laws and rules set forth by the NC General Assembly and the NC Board of Examiners for Engineers and Surveyors. The vast majority of the work to create these products is done after aerial photography acquisition. Each mapping project typically goes through the following workflow from start to finish regardless if the aerial photography is acquired using a manned aircraft or a drone:

    ​Higher level engineers in the Photogrammetry Unit work with NCDOT Project Managers to establish mapping requirements. This is done in early fall as part of the Photogrammetry Unit’s Annual Requesting Mapping & Surveys cycle since it is recommended to acquire all aerial photography between January to mid-April moving east to west across the state. The acquisition during this period is important because this is when the leaves are off the trees in NC. This yields higher ground visibility from all acquired aerial imagery resulting in more accurate mapping data. The most critical piece of information that is needed from the Project Manager is a project boundary because this tells the Photogrammetry Unit where to fly and what to map. Once the project boundaries are decided upon the purpose of the photogrammetric services must be discussed; preliminary design, final design, alternates selection, earthwork quantity determination, imagery for environmental purposes (color infrared), or imagery for planning purposes (Red-Blue-Green). Lastly, the mapping scale and pixel resolution of the photography must be decided on because this dictates the flying height of the airplane.

    During the flight planning and control planning phase the flying height, flight layout, and control layout is established. The flying height is dependent on factors such as required scale (pixel resolution), height of ground above mean sea level, and height of structures within the project boundary. The layout depends on the project limits and maintaining enough image overlap for 3-D stereoscopic viewing. Flight layouts are organized as flight lines (strip of overlapping photographs) in both block (two or more parallel flight lines) or tangent strip configurations depending on the corridor’s shape. Once the flying height and flight layout is created, the control plan can be started. The control plan consists of approximate horizontal locations where the Photogrammetry Unit would like a ground control point (panel). Panels are distinct, photo-identifiable targets placed at highly accurate ground surveyed points which are used to control aerial photography. Panels are laid out in a minimal, well-spaced network around the project limits to contain cost and are in the shape of a white “V” so they can easily be seen in the aerial photography. Later during the aerotriangulation phase these panels are used to extend the project control network such that each aerial photograph covers several control points. Upon review and acceptance of aerial photography by Photogrammetry Unit engineers, the controlled aerial photography is available for product generation.

    The NCDOT Location and Surveys Unit is responsible for setting panels at the project site in accordance with the control plan supplied by the Photogrammetry Unit and providing accurate ground coordinates for the point of the “V” shaped panel. They are responsible for coordinating with property owners, laying the panels (typical with paint or vinyl), and obtaining the ground coordinates of the panels typically using GPS surveying techniques. The Location and Surveys Unit is also responsible for the exact placement of each panel due to safety, access limitations, and making sure each panel has clear visibility from the sky. Once all panels are placed on the ground, the aerial acquisition phase can proceed.

    Each project flown to acquire aerial photography is called a “Mission”. Missions are completed by the Photogrammetry Unit working closely with the NCDOT Aviation Division for manned aircraft acquisition while only Photogrammetry Unit staff acquires non-metric camera aerial photography with the unit’s drone. For manned aircraft missions, Photogrammetry Unit aerial photographers work with Aviation from deciding when to fly to the completion of the mission. Manned aircraft aerial photography is acquired from the most accurate large format metric camera commercially available. This metric camera is mounted on the belly of a specially modified NCDOT Aviation Division aircraft. The metric camera includes a highly accurate GPS position and orientation system that provides a relatively accurate position and orientation direction for each photograph. Missions cannot be flown anytime: there must be enough light to get a proper exposure and it is preferred to fly when all the leaves are off the trees. Missions must also be flown under the right weather conditions: little to no clouds, low wind, no fog, no rain, no snow, no flooding, no smoke, and an optimal sun angle to reduce shadows. Once the manned aircraft aerial acquisition is complete the GPS data from the camera position and orientation system is post processed by Photogrammetry Unit staff. During the post processing phase, raw airborne GPS position and orientation data are adjusted and yield relatively accurate position and orientation data that are used in the Aerotriangulation phase.

    Aerotriangulation is the process where the minimal, well-spaced network of ground control panels and the relatively accurate post processed position and orientation data are used to extend the project control network such that each aerial photograph covers several control points. This process is completed by Photogrammetry Unit engineering and technical staff using sophisticated 3-D hardware for stereoscopic viewing and analytical software that iteratively computes the complex photogrammetric collinearity equations​ to densify the control network. The advantage of aerotriangulation is it reduces the need for field-surveyed ground control by utilizing an office procedure which reduces the cost. It also improves safety conditions for ground surveyors due to selecting panel locations in easily accessible areas, as opposed to dangerous locations in high traffic areas or rugged terrain.

    Stereo compilation is the collection of 3-D data with vectors and points from 2 overlapping aerial photographs, commonly called a stereo pair. Stereo compilation is done behind a computer screen with 3-D monitors and 3-D glasses using a sophisticated 3-D hand held controller along with software that solves a constrained version of the photogrammetric collinearity equations in real time. Horizontal (planimetric) features are stereo compiled manually from the aerial photography. Elevation data in the form of a DEM or DTM is produced from manual collection of break lines (change of slope in terrain, i.e. bottom of a ditch) and points from the stereo pair, complemented with NC’s statewide Quality Level 2 LiDAR (Light Detection And Ranging) elevation data. The statewide Quality Level 2 (QL2) LiDAR elevation data has a non-vegetated vertical accuracy at a 95% confidence level less than 0.6 feet and the accuracy of stereo compiled elevation data in open terrain is similar. Photogrammetry Unit engineers are NCDOT’s subject matter experts on NC’s statewide LiDAR and orthophotography programs. The unit maintains the statewide bare earth QL2 LiDAR data on NCDOT servers, and provides training and tools for NCDOT staff to directly access the data in either DEM or bare earth (decimated)​ formats for project development.

    Orthophotos are an accurate image map with horizontal coordinates produced with mosaicked aerial photographs that have been corrected for tilt and relief displacement. The Photogrammetry Unit produces preliminary orthophotos, called digital mosaics, and final orthophotos, called orthophotos. The difference between the digital mosaic and an orthophoto is the quality of the position and orientation data used, the quality of elevation data used, and the level of image editing completed prior to delivery. Digital mosaics can be produced rapidly because they are generated using relatively accurate post processed GPS position and orientation data from the camera system, best available elevation data, and minimal image editing. Orthophotos take longer to produce because they are generated from aerotriangulation position and orientation data, updated elevation data, and thorough image editing. As a result of these process steps, orthophotos are more accurate than digital mosaics and have significantly less image artifacts. The Photogrammetry Unit typically generates digital mosaics and orthophotos with a 1/8th (0.125) of a foot pixel size.

    The Photogrammetry Unit supports NCDOT construction projects by providing earthwork pay quantities upon request from NCDOT Resident Engineer’s Offices. This type of project includes multiple missions, each flown at a different stage during the construction process, to quantify how much earthwork has been moved. New location projects, borrow pits, stockpiles, or construction projects in low travel regions are good candidates to consider for drone aerial photography missions for earthwork quantity determination due to FAA drone operation restrictions​. Using DTM or DEM elevation data from two different dated aerial photography missions, Photogrammetry Unit staff computes the earthwork volumetric quantity using a surface to surface algorithm. Typically, photogrammetric earthwork quantity determination is used for larger construction projects, or when ground crew safety, environmental conditions, or field staffing is a concern. This service requires close communication between the Photogrammetry Unit and the NCDOT Resident Engineer’s Office administering the construction contract.

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