A multitemporal TM data set (scene 191/31/2, Rome) has been analyzed in order to determine the best image chip characteristics for use in a ground control point (GCP) library. We have attempted to identify the optimum selection of image features, bands, chip size, and season of the year for the reference image. The value of correlation coefficient between chips (small subimages centred at the GCP) of the same feature, but acquired at different times, is taken as a component to measure chip quality. Also, the high contrast in a chip is taken to indicate better quality of the GCP. The "Optimum Chip Factor" (OCF) is a measure of these two aspects of GCP quality. ro determine the optimum bands, chip size and features, the OCF was computed in different spectral bands, over different window sizes and for different features. The best season was found by taking each acquisition date as a reference image and comparing the number of successfully relocated GCPs in all other images. The results can be taken as a guideline for the use of GCPs in a GCP library. We found that the best band is band 5, the best features are features with land/water boundaries, and the best chip size is 15 pixels square. To determine the best season of the year, some further investigations will be necessary.