Aerotriangulation at SBG

The aerotriangulation (AT) process precisely orients aerial imagery to the mapping control. SBG utililizes several methods of AT including analytical and softcopy AT. We use a variety of control methods including conventional ground control and airborne global posistioning system control (AGPS) in conjunction with IMU systems. We use a combination of industry and special custom software to calculate the best solutions for a wide variety of mapping projects.

 

 

 

Airborne Global Positioning System Control

Spencer B. Gross, Inc. (SBG) pioneered kinematic Airborne GPS (AGPS) Control for Photogrammetric Mapping in the Northwest. Using a GPS receiver on board the aircraft, coordinates are recorded for each photo center. This process dramatically reduces the number of ground control points normally required for mapping projects.

This is how it works:

During the normal mapping aerotriangulation procedures, the camera station (exact location of the camera at time of exposure) must be computed as a necessary step in the overall Photogrammetric solution. The coordinates and orientation angles of the camera station are needed before any coordinates can be computed for ground points. AGPS virtually eliminates this computation step as it provides the camera's location as part of the exposure process. For each exposure, the exact coordinates of the camera station are captured and recorded, later to be incorporated in the aerotriangulation process.

The coordinates derived from satellite data during the flight (in reality) apply to the GPS antenna mounted on the exterior fuselage over the camera. After each flight mission, a small survey is performed to determine the position of the camera in relationship to the GPS antenna. Since the camera is "locked-down" during flight, the relationship between the camera and antenna is not changed, and so a consistent computational link between antenna and camera is maintained. This link allows the antenna to be used as an eccentric control point for the camera station. SBG specialized software ( BINGO-F and GEONAP) provides the mathematical computations to determine the camera's location based on the antenna's location.

Each time the camera is triggered for an exposure, it fires a pulse to the on-board GPS receiver. The receiver records the signal and corresponding GPS time in memory. Times are also recorded for the GPS position data derived from satellite readings. Lack of time synchronization between exposure and GPS observation is handled by interpolation methods. Fast data collection rates (half second) and sophisticated 3D curve-fitting routines minimize error.

Since the GPS solution is kinematic, several reference receivers (two minimum) are used to record satellite data over known points on the ground. These receivers are also active during the entire flight mission. The kinematic solution is initialized in mid-air using specialized software. At least 10 minutes of "initialization lock-on" is required. This same software eliminates problems caused by loss of satellite lock during steep banks (turns) between flight lines.

Our AGPS flight missions utilize Trimble 4000SSE receivers: one in the aircraft recording camera exposure timings and satellite data, and one on the ground, over a known point, also recording data.

After the flight mission, we utilize post processing software to resolve the earth coordinates of each camera station. In addition to the AGPS collected coordinate data, a certain number of ground control points are still required as checks to ensure the network is tied into the NAD 83/91 (and local HARN) Datum. The distribution of ground control points is a function of the size and shape of the aerial photography block. For example, a true rectangular block would require four points at the corners. Any deviations in the shape of the rectangle would require an extra point. We encourage our clients to pre-mark "blind points" within the project to quality check our aerotriangulation solution.

SBG uses AGPS as a routine procedure for photogrammetric mapping projects. We easily meet national map accuracy standards, usually exceeding the accuracy required. From 1995 through 1997 we have AGPS controlled over 50,000 square miles of mapping at scales from 1"=100' to 1"=400'.

In 1997, SBG acquired the BINGO-F/GEONAP software solution for AGPS aerotriangulation and bundle adjustment. This software is internationally recognized as the most rigorous and efficient methodology for AGPS control solutions with built-in checks and quality assurance procedures.

SBG utilizes AGPS technology to efficiently acquire control for photogrammetric mapping. This methodology provides the required accuracy for mapping, and results in substantial cost savings to our clients by eliminating costly ground control. We can also provide a comprehensive aerotriangulation report defining the exact parameters of the flight , the aerial photography, the ground control, and the precise solution for aerotriangulation prior to mapping. This process is unique in the Northwest.

 

 

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