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
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
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
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
. 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.