EM 1110-2-2907
1 October 2003
3-5 Aerial Photography. Aerial photography is a highly useful mapping tool and
maintains the highest spatial resolution of any of the remote sensing systems. Standard 9-
in. (22.9 cm) aerial photos used for mapping and site identification are collected and
made available through commercial companies. USGS generates digital elevation model
(DEM) data and stereo classification of ground cover from aerial photography. These
data are derived from the National Aerial Photography Program (NAPP), formally the
National High Altitude Program (NHAP). The NAPP products are quarter quad-centered
photographs of the entire contiguous US, acquired every 5 years over 2-year intervals
since 1990. NAPP photography is acquired at 20,000 ft (~600 m) above mean terrain
with a 6-in. (~15 cm) focal length lens. The flight lines are quarter quad-centered on the
1:24,000-scale USGS maps. NAPP photographs have an approximate scale of 1:40,000,
and collect black-and-white or color infrared, as specified by state or Federal require-
ments. The St. Louis District of the Corps has several airborne contracts in place as well.
a. Softcopy photogrammetry is the semi-automatic processing of aerial photos after
they have been digitally scanned into files and transferred into a computer. Once in digi-
tal form, the processes of stereo imaging, stereo compilation, aerial triangulation, topog-
raphic mapping, ortho-rectification, generation of DEMs, DTMs, and DSMs and digital
map generation can be carried out.
b. Aerial photos are geometrically corrected using the fiducial marks and a camera
model and projected into the ground coordinates. Images within a stereo overlap are ad-
justed using a triangulation algorithm so that they fit within the constraints of the ground
control point information. At the end of the triangulation, individual stereo models are
mathematically defined between stereo images. Topographic information is extracted
from the images using autocorrelation techniques that match image patterns within a de-
fined radius. By using parallax created by the different angle shots, elevation is measured
from the distance of matching pixels. A terrain model is used to create an ortho-rectified
image from the original photo that is precision geocoded and an ancillary Digital Surface
Model (DSM) is available.
c. Some of the companies that contract with USACE for aerial photography include:
Highland Geographic Inc.
James W. Sewall Company
Alcor Technologies Limited
Aero-Metric Inc.
PhotoScience, Inc.
3-6 Airborne Digital Sensors. The advancement of airborne systems to include high
resolution digital sensors is becoming available through commercial companies. These
systems are established with onboard GPS for geographic coordinates of acquisitions, and
real time image processing. Additionally, by the time the plane lands on the ground, the
data can be copied to CDROM and be available for delivery to the customer with a basic
level of processing. The data at this level would require image calibration and additional
3-8
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