1. The optical aberrations of the lens. The aberrations are errors of the simple lenses used with standard equipment. The most interesting kind of aberration for this case is optical distortion.
The distortion of the image is resulting from variation of magnification over the field of the lens. This effect doesn't appear in the center of the frame but only towards the edges, doesn't affect the sharpness of the image but only the shape. The parts of the image near to the edge of the frame are in different scale.
To avoid big distortion errors it is recommended to use lens from normal up to midrange wide-angle (35 mm for 135-size film) and to overlap the frames approximately 20 %.

2. Perspective distortions. This error generally comes from displacement of the camera from its right position, on axis with the center of the object quadrant. If the displacement is constant the error is as bigger as the distance to the object and focal length of the lens are smaller. The perspective distortion could be corrected by digital rectification.

The errors of optical and perspective distortion could be avoided or corrected in some ratio in every single image before or during the process of assembling the complete image. But in the final image persist, some degree of error resulting like difference between the scale factors of the two dimensions - X and Y. One more time this error could be reduced using software like AutoCAD Overlay, Hitachi Image edit, etc., applying rubber-sheeting.

We measure the error in few cases after applying 4 points rubber-sheeting. The error is approximately 1%.