It's very likely that you have not calibrated your camera.

All camera lenses provoke some deformation of the image. This type of deformation is known as "Distortion". When you calibrate your camera, this lens distortion is measured by Pixdim, stored in a configuration file and then used by Pixdim to automatically correct these errors when calculating 3D points and the positioning of the 3D image.

If you don't calibrate your camera, Pixdim reads data concerning the focal length, the size of the CCD sensor ... stored in the image file (known as EXIF data) and use automatic "Un-distortion" algorithms. These algorithms improve 3D calculations and positioning of the images but are not nearly as precise as the real values measured during the calibration process.

In reality, EXIF data can be quite imprecise. Our own tests have shown that errors of up to 6% exist between EXIF data and data generated by the calibration process ! In this case it is impossible to have precise measurements and 3D image positioning.

When Pixdim runs 3D point calculations, it 'straightens out' the 3D image and positions it correctly (the 3D points are displayed in Google SketchUp without distortion). If the image has been 'straightened out' using data that is not sufficiently reliable, then they cannot correspond exactly with the 3D points.

This phenomenon should not occur with a calibrated camera.