The particle picking centering method The so-called center of gravity method where the image is scanned and the intensity peaks calculated to find the center of the particle 'mass'. The image is then shifted by the correct amounts to maximize the number of peaks near the center. Another centering method averages all the picked particles together, and then cross correlates each individual particle to the average. The cross correlation function is used to determine by how much to shift each particle when trying to center it. When all the particles have been cross correlated to the average and shifted, a new average is generated. Once again, all the particles are compared to the new average, and shifted as necessary to center them as best as possible. This iterative process is repeated until no significant shift is necessary for all the particles Crosscorrelating to a global average is but one variation on this theme. Similar methods also use an external model or a rotational average of the particle itself as the centering reference. Unfortunately, it can be difficult to obtain a reasonable external refernce, so a global average or a rotational average are most often used.