3.2 Coordinate Systems
The fundamental principle that underlies photogrammetric triangulation, the condition of collinearity, relates two coordinate systems: the photo or image space and the ground or object space. GIANT offers the user considerable flexibility in defining both coordinate systems.
3.2.1 The Image Coordinate System

Flexibility in image space definition is derived from GIANT's Camera Definition feature and from its self calibration option. The Camera Definition function is discussed in Section 3.3, and the user is referred there for details. In GIANT the image coordinate system can be defined through the three parameters of interior orientation: the camera's principal distance (focal length) and the two coordinates of the principal point. The Camera Definition feature permits the modification of these values.

Alternatively, the parameters of interior orientation can be determined through GIANT's self calibration feature. Briefly stated, a self-calibrating bundle adjustment assumes that the parameters of the camera's interior orientation are only approximately known. They are included as additional parameters of the triangulation and solved as part of the least-squares adjustment. In a similar manner, the self calibration concept can be further extended to include the six parameters of radial and asymmetric lens distortion models. Since the self-calibration options are accessed through the Camera Definition screen, the user is referred to Section 3.3 for the detailed discussion of their use.

3.2.2 The Object Space Coordinate System
GIANT offers the user a wide array of object space coordinate systems ranging from conventional rectangular systems to user-definable geodetic systems. All object space coordinate system controls are found on the Process Options screen.


To establish an object space coordinate system, first use the mouse to press the Rectangular or Geographic button of the Object Space field. Second, specify the units by selecting the Feet or Meters button of the Object space units field. If the system selected is geographic, then the appropriate datum parameters must be set. Beneath the Object space units field is found a window with a drop down arrow icon to its right. Pressing the arrow icon will display the two pre-defined datums, the National Horizontal Datum of 1927 and the National Horizontal Datum of 1983, from which to choose. If one of these pre-defined datums is suitable, select it by double clicking on it. When a datum is selected, the Major Axis and Minor Axis windows to the right will automatically be filled with corresponding values for the semi-axes of the datum's reference ellipsoid. The above illustration shows a configuration with a Geographic object space system in units of meters. The National Horizontal Datum of 1983 is selected.

 Alternatively, it is possible to construct a user-defined geographic datum. To do so, select the datum designated as Unknown. The semi-axes will clear. Key in appropriate values for the semi-axes. Datums are often specified by a semi-major axis (a) and a flattening (f) or an eccentricity (e2). GIANT contains no provision for the specification of a datum by other than the two semi-axes. Most standard textbooks in photogrammetry, land surveying, or geodesy contain the simple equations that relate the ellipsoidal flattening and/or eccentricity to the semi-axes. The user is encouraged to consult such a text when required to determine the semi-axes from flattening or eccentricity information.

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