|
|
Demystifying Advancements in Digital Orthophotography
by Surdex Corporation, St. Louis, MO
About Surdex Corp.
Airborne GPS Control
Photogrammetric control provides the foundation for spatial orientation or referencing of
aerial imagery to the Earth’s surface, thereby allow the imagery to be used in the
photogrammetric compilation of topographic and planimetric mapping features. The
accuracy of the photo control solution is dependent upon the desired accuracy
specifications of the resultant mapping products. Since the ground control solution
provides the framework and accuracy of the entire process, this initial step is the most
critical.
Traditionally, the photogrammetric control solution was accomplished through the
establishment of a basic network of “photo-identifiable” ground control points with
known horizontal and vertical values within a specific ground coordinate system. These
points have known coordinates (x, y, and z) on the Earth’s surface and can easily be
identified in the aerial imagery. For some large scale mapping, it was often necessary to
have a ground control point in at least every third or fourth stereo model. In those cases,
a countywide project would often require as many as 1000 control points for the
photogrammetric control solution.
Surdex was a pioneer of Airborne GPS collection for photogrammetric control, having
undertaken experimental research with Iowa State University and with our first project in
1991 for the USGS. Since that time, Surdex has routinely used this technology to
support, and often to supplant conventional ground control for the Corps of Engineers,
USGS, USAF, FAA, and County/Municipal photogrammetric survey and mapping
projects.
Although Airborne GPS is not a new technology, it is often misunderstood and its
viability in the photogrammetric process is often questioned by many non-
photogrammetrists. Digital Orthophotography is a perfect application for Airborne GPS.
The airborne collection of photo-center control data allows the user to collect an
extremely dense data set of photogrammetric control in a very short period of time.
Essentially, a photo-center control point is established for each frame of photography
flown during the mission, as opposed to establishing control points spanning three to four
models utilizing conventional ground control.
During the photo mission, a minimum of three GPS receivers are simultaneously
collecting GPS observation data at 1-second intervals. The airborne unit collects both at
one-second and at an event epoch, which is the center-point of the exposure cycle for
each frame. Two receivers, or “base stations,” are generally co-located with previously
established, or to be established, high accuracy ground control points. A third receiver,
the “rover” is located in the aircraft and utilizes an L1/L2 antennae which has been
surveyed into the aircraft’s coordinate system, all referencing the nodal point of the aerial
camera’s lens and known focal length.
Surdex has independently executed specific experiments to determine the accuracy of
Airborne GPS as solitary control for a project as well as evaluating the affects of minimal
perimeter conventional ground control on the vertical accuracy's.
As a result of these tests, Surdex has determined that Airborne GPS is adequate for
horizontal control. Even without additional ground control, Airborne GPS data does not
reflect any serious effects of deformation in the triangulation. The expected problem
would result in tortional deformation longitudinally along the flight line, since airborne
control is collected along a straight line through the air. However, there is enough
excursion variation, such as drift and altitude change, along the flight line to suppress any
adverse effects. Additionally, the geometric strength of normal strip triangulation
minimizes the tortional deformation as well. The combination of triangulation and flight
variation guarantees elimination of the problems expected from linear collection in the
air.
Next
|
|
Sponsored by:
For information
regarding
advertising rates
Click Here!
|
