Starting values from scratch

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Starting values from scratch

Starting values from scratch

In most cases, you start with an existing camera calibration file that was used for a previous project. This reduces the effort of the calibration significantly because most of the camera parameters should be stable or very close to previous values.

If the camera is used for the first time with TerraPhoto, you need to create a new camera calibration file. Some system providers deliver calibration values from a system calibration that can be converted into TerraPhoto calibration values. See Tools / Convert from command from the Camera dialog for more information.

If system calibration values can not be used, you need to start the calibration in TerraPhoto from scratch. There are a few calibration parameters for which you have to define initial values before you start with the actual calibration work:

Image width and Image height (image size in pixels)

Orientation

Timing offset and Exposure (usually 0.0)

Lever arm XYZ fields

Principal point z

Zero radius

You should set all other fields to zero.

Lever arm

The lever arm vector is one of the parameters which correlates highly with other parameters. It is best to measure the lever arm vector separately and not try to solve it from an airborne calibration data set.

The lever arm vector defines the offset from the input trajectory information to the focal point of the camera. Depending on the exact position for which the trajectory is computed, the lever arm vector describes the offset between the camera and the IMU, the GPS receiver, or the laser scanner. If the trajectory is computed for the camera, all lever arm components are zero.

An accuracy level of about 1 centimeter is sufficient for the lever arm information. If you fine tune the misalignment angles for each flight session, you can compensate a small inaccuracy of the lever arm vector.

Principal point z

You can compute a reasonable initial value for the Principal point z if you known the approximate focal length of the camera in millimeters and the CCD pixel size. The equation is:

Principle point z = -FocalLength / PixelSize

For example, if focal length is 50 mm and pixel size is 0.0068 mm, the principle point z value is:

-50 / 0.0068 = -7353

Zero radius

The Zero radius function is the preferable model for radial lens distortion. The initial value for the Zero radius should fit inside the image and can be calculated by taking slightly less than half of the image width or height, whatever is smaller.

Examples:

Image size is 7228 * 5428 pixels, use 2500.

Image size is 3056 * 2032 pixels, use 1000.