Keywords: 
• industrial photogrammetry /
• target center offset /
• determination

Abstract: The image of the circular target will appear as an ellipse if the target plane is not parallel to the image plane. There is an offset between the image of circular target center and the center of the ellipse in the image. The offset can be larger than the measurement accuracy of industrial photogrammetric measuring systems if the target is oversized. In general, the influence of the offset can be reduced to a negligible quantity by choosing suitable target size. Nevertheless, the natural target, can not be changed with regard to its size. Therefore, other ways should be found. One of the methods is the rotational transformations of image, i.e. transformations of tilted image to horizontal image. The method is suitable to the case in which all the targets lay in only one plane or several parallel planes. Other one is offset correction method. In order to find out the practical method of the offset correction, the authors researched the case of the offsets distribution. If the offsets of points on the principal line can be determined, the offsets of the other points in the image can be determined. The quadratic equation for computing offset of point on the principal line was derived through the collinearity equations, and the steps of computing offset were given.