Similar set-up to that in Fig. 4. Dotted lines show the SNAPG results without using the centroid error correction and solid lines show the results for covariance matrices measured from the data. The centroid bias algorithm breaks down for very low S/N, so that the solid curves do not reach to S/N< 6. Dashed lines show the SNAPG results when the mean centroid variance in each image is taken to be the covariance matrix for that image. As the dashed and solid lines are almost indistinguishable, the measured covariance matrix is a good estimate of the true centroid covariance. However, it is unable to remove all bias caused by noise, leaving residual biases of percent level for very faint S/N< 10 galaxies.