## DESCRIPTION

Aerial photographs may be either vertical or oblique. Vertical photographs can be truly vertical (nadir), or slightly tilted (less than 3 degree from the vertical). Usually aerial photos are tilted to some degree. We refer to the term vertical photograph up to a tilt of 3 degree.
Oblique aerial photographs are purposely taken with an angle between 3 and 90 degree from the nadir direction.

The use of i.ortho.init (menu 6) is only required when rectifying a tilted or oblique aerial photo.

i.ortho.init creates or modifies entries in a camera initial exposure station file for imagery group referenced by a sub-block. These entries include: the (XC,YC,ZC) standard (e.g. UTM) approximate coordinates of the camera exposure station; initial roll, pitch, and yaw angles (in degrees) of the cameras attitude; and the a priori standard deviations for these parameters. During the imagery program, i.photo.rectify, the initial camera exposure station file is used for computation of the ortho-rectification parameters. If no initial camera exposure station file exist, the default values are computed from the control points file created in i.photo.2target.

The following menu is displayed:

            Please provide the following information

    	INITIAL XC: Meters                __________
    	INITIAL YC: Meters                __________
    	INITIAL ZC: Meters                __________
    	INITIAL omega (pitch) degrees:    __________
    	INITIAL phi  (roll) degrees:      __________
    	INITIAL kappa  (yaw) degrees:     __________

    	Standard Deviation XC: Meters     __________
    	Standard Deviation YC: Meters     __________
    	Standard Deviation ZC: Meters     __________
    	Std. Dev. omega (pitch) degrees:  __________
    	Std. Dev. phi  (roll) degrees:    __________
    	Std. Dev. kappa  (yaw) degrees:   __________

            Use these values at run time? (1=yes, 0=no)

         AFTER COMPLETING ALL ANSWERS, HIT <ESC> TO CONTINUE
                      (OR <Ctrl-C> TO CANCEL)

The INITIAL values for (XC,YC,ZC) are expressed in standard (e.g. UTM) coordinates, and represent an approximation for the location of the camera at the time of exposure.

- X: East aircraft position;
- Y: North aircraft position;
- Z: Flight altitude above sea level

The INITIAL values for (omega,phi,kappa) are expressed in degrees, and represent an approximation for the cameras attitude at the time of exposure.

- Omega (pitch): Raising or lowering of the aircraft's front (turning around the wings' axis);
- Phi (roll): Raising or lowering of the wings (turning around the aircraft's axis);
- Kappa (yaw): Rotation needed to align the aerial photo to true north: needs to be denoted as +90 degree for clockwise turn and -90 degree for a counterclockwise turn.

If ground control points are available, the INITIAL values are iteratively corrected. This is particularl useful when the INITIAL values are rather rough estimates.

The standard deviations for (XC,YC,ZC) are expressed in meters, and are used as a priori values for the standard deviations used in computation of the ortho rectification parameters. Higher values improve the refinement of the initial camera exposure. As a rule of thumb, 5% of the estimated target extents should be used.

The standard deviations for (omega,phi,kappa) are expressed in degrees, and are used as a priori values for the standard deviations used in computation of the ortho rectification parameters. As a rule of thumb, 2 degrees should be used.

If Use these values at run time? (1=yes, 0=no) is set to 0, the values in this menu are not used.

## SEE ALSO

i.ortho.photo, i.photo.2image, i.photo.2target, i.ortho.elev, i.ortho.camera, i.ortho.transform, i.photo.rectify

## AUTHOR

Mike Baba, DBA Systems, Inc.