Part 2: Theoretical Core of Image Processing that Interprets Drone Imagery

Exterior Orientation

In the 1st blog of the series, I wrote about Interior Orientation (Check it out, if you haven’t read it) which is the internal geometry of a camera or sensor as it existed at the time of data capture. Now, I will put down my theoretical knowledge on Exterior Orientation which is another part of the image processing.

Exterior Orientation

Exterior Orientation is the position and angular orientation associated with a drone image.

Elements of Exterior Orientation

The variables defining the position and orientation of an image are referred to as the elements of exterior orientation. They define the characteristics associated with an image at the time of exposure or capture.

1. The positional elements of exterior Orientation include Xo, Yo and Zo. The coordinates define the position of the perspective center with respect to the ground space coordinate system X, Y and Z. Zo is commonly referred to as the height of the camera above the sea level which is commonly defined as “datum”.
2. Angular or rotational element of exterior orientation describes the relationship between the ground space coordinate system (X,Y and Z) and the image space coordinate system (x,y and z). Three rotation angle is commonly used to define angular orientation. They are Omega, Phi and Kappa.

Calculating the position of Ground Point.

Omega is a rotation about the photographic x-axis, Phi is a rotation about the photographic y-axis and Kappa is a rotation about the photographic z-axis, they are defined as being positive if they are counterclockwise when viewed from the position end of their respective axis. The photographic z-axis is equivalent to the optical axis (focal length).

The x’, y’ and z’ coordinate are parallel to the ground space coordinate system. Using the three rotation angle, the relationship between the image space coordinate system (x,y and z) and ground space coordinate system (X,Y and Z) can be determined. Relationship between the two systems can be made using a matrix and is referred as orientation or rotation matrix.

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