|1.2 GIANT Support for the Triangulation Process|
|The years of research and development behind GIANT make it
a tool that integrates seamlessly into, and effectively expedites, most of the standard
triangulation process steps enumerated above in Section 1.1.
GIANT does not include a module to control the collection of triangulation data from any specific piece of equipment. Instead, GIANT implements a flexible, non-proprietary, ASCII interface that accepts data from any source. The user may gather data from any instrument that is judged to render sufficiently accurate photo coordinates for the project at hand. This includes the entire range of classical photogrammetric measurement devices such as analog stereo restitution instruments equipped with coordinate encoders, stereo comparators, analytical stereoviewers and modern softcopy digital photogrammetric workstations.
GIANT assumes that most systematic errors, with two qualifications, have been removed from the image coordinates that are input to the program. Models are included for optionally correcting for atmospheric refraction, aircraft cabin pressure refraction and, for underwater photogrammetry, the refraction effects due to the water. GIANT also includes a self-calibration mode wherein refinements to the elements of interior orientation (principal distance and principal point coordinates) can be determined. The self-calibration can also optionally determine refinements to the parameters of radial and asymmetric lens distortion models. The user is urged to exercise caution when these optional self-calibration modes are used, and to carefully study the discussion in Section 3.3.
In support of Step 1, data capture, it was mentioned that GIANT could receive data from any--in fact, any combination--of photo measurement devices. The user has two options to prepare the input data stream from the source data. The arguably preferable option would require the data collection module to form the input ASCII file directly. The second, more error-prone, option would involve the manual creation of the file using some standard text editor. Of course, a hybrid approach, where the collection instrument would generate the file contents and a text editor used to check and ensure format compliance, is a third possibility. Regardless of the method used, it is important to know that GIANT contains several facilities for checking the integrity of the input data stream, and errors can often be found without needlessly wasting time waiting for a large triangulation to run which terminates abnormally because of a data format problem. Refer to Chapters 2 and 4 for further discussion of the input data stream integrity checking features.
Configuration of the triangulation run is simple and intuitive with GIANT's Windows-compliant Graphical User Interface. Options are grouped according to function and are easily accessed and modified with simple point-and-click mouse operations. Intelligence built directly into the program prevents incompatible options from being selected or for measurement errors to cause a non-convergent triangulation to continue to execute needlessly. A simple mouse button press also triggers execution of the triangulation run.
When the triangulation adjustment completes the user can inspect the ASCII output file for the quality of the adjustment. A large degree of flexibility permits the inclusion of any or all of the original input source data, as well as several statistical measures in the output file to simplify the location of errors in the data stream. After errors are detected and removed, the triangulation can be executed any subsequent number of times until the adjustment is judged to be satisfactory. Once a satisfactory convergence is reached, all data echoes and statistical information could be deactivated and only, say, adjusted station point coordinates and frame stations are printed.
GIANT offers several graphics functions to assist and enhance the analysis of the triangulation process. In a broad sense, the graphics functions might be classified as providing either geometric information or statistical information. The geometric class of functions permits a user to inspect useful geometric properties of the block such as the distribution of control points or pass points, the locations of different frame groups or frames within a group, and the positions of exposure stations and approximate frame footprints. Certain types of adjustment weaknesses are readily identified when viewing these geometric features. A more in-depth analysis can be gained through the examination of the statistical graphics functions. The standard deviations of pass points or camera station positions, for instance, can indicate subtle control or geometry deficiencies. The graphics functions also support querying capabilities, so points or frames that are graphically seen to be suspect, can be identified and dealt with appropriately