The process of aerotriangulation can be defined as the densification and extension of field control through the use of aerial photography. The advantages of this process are a marked reduction in the amount of field surveying required and more versatility in locating control points.

The phases of the aerotriangulation process are described below:

Preparation and Coding

Preparation consists of dividing the air photo by line, making a work index, checking the stereoscopic coverage of the region and transferring the ground control required to complete the block.

ASDB Index
Map shows fight lines and photo centres around North Bay, Ontario on NTS map sheet 31L.

The coding process consists of the following tasks:

• assign a unique code to all geodetic control in the block
• select and symbolize the areas, on the aerial photographs, that are suitable for the placement of photogrammetric control and assign the proper code
• select and symbolize lake and river points and assign proper code
• prepare the necessary indices and cross reference list that will be required for the analysis of the block

Point Transfer ("Pugging")

The term "pugging" refers to the action of drilling a small hole in the emulsion of the film positive using a point transfer device equipped with a diamond tip drill with a diameter of 60 microns. Marking of the points in this manner provides an unambiguous and permanent identification for the photogrammetric points that were selected in the coding phase.

In order to complete the pugging phase, the technician transfers all the symbols, which consist of circles, squares and triangles, from the aerial photographs onto the corresponding diapositives. Then, under stereoscopic observation, the operator will select and mark an image point within each symbol, the mark left by the removal of the emulsion will become the permanent location of the photogrammetric control.

Taking Measurements ("Bridging")

During this phase of aerotriangulation, the coordinates of both the ground control points and the photogrammetric points are measured and recorded in the model coordinate system. Click for infomation regarding the datum and projection of ASDB control points.

Numerical Adjustment and Evaluation

During the numerical adjustment phase, machine coordinates (in millimetres) are adjusted and transformed to conform, both horizontally and vertically, to known ground co-ordinates. The adjustment will be rerun until it meets the specifications for aerotriangulation or until it has been determined that further runs will not improve the results. The tasks that must be performed during this phase are mostly of analyzing the data and investigating and correcting errors that originated in the previous phases. Possible sources of errors are:

• wrong code assigned to the ground or photogrammetric control
• wrong code read when measuring the points
• point improperly pugged
• wrong geodetic value given to one or several geodetic control points
• distortion in the photogrammetric model
• failure to eliminate systematic errors such as earth curvature

During the evaluation process, the inspector will ensure adherence to specifications and will check that the following criteria have been met:

• proper density and placement of control points
• no residuals of a systematic nature on the control points or the photogrammetric points

Depending on the type of control used and the quality of the results, the inspector will assign a quality rating based on the NATO specifications (STANAG 2215 Edition 5). When the evaluation has been completed, the results of the adjustment are stored in digital format. Aerial photographs and diapositives are sent to the Centre for Topographic Information Resource Centre for storage until requested.