Aerotriangulation

Acquiring ground control can be a significant expense in any mapping, DTM or orthophotography project. Aerotriangulation is used to reduce the amount of field survey work required by extending control to each stereomodel photogrammetrically. For larger projects, this saving can be considerable. It can also be used to extend ground control into areas inaccessible to the surveyor.

During the aerotriangulation process, tie-points, some times called pass-points, are measured between adjacent images. Image rays projected from two or more overlapping photographs intersect at the common ground points to define the three-dimensional position of each tie point. Once the ground control points have been measured, the entire block of images is adjusted to fit the ground control coordinates in a bundle block adjustment. Thus, the true ground coordinates of the tie-points are determined.

The results of this aerotriangulation are used to control the subsequent photogrammetric mapping. For analytical stereo plotters, each stereomodel is scaled and levelled using the adjusted coordinate values of the pass points; where as for digital stereo plotters, the adjusted photo locations and rotations are utilised to form the stereo models.

BLOM Aerofilms produce both analytical aerotriangulation, using a Wild PUG 4 point transfer device and Leica SD3000 / SD2000 analytical stereo plotters, and digital aerotriangulation, using Z/I Imaging SSK (ISAT), Socet Set (Orima) and KLT Atlas digital stereo plotters.

The final bundle block adjustment calculation is made using Inpho's PATB-NT and inBLOCK. These programs allow for the use of both ABGPS (Airborne GPS) and IMU (Inertial Measurement Unit) data in the adjustment, thus further reducing the amount of ground control required.