Hi Al
Wayne implemented circle-fitting in ImageJ:
"1.44m, 1 January 2011
Thanks to Ved Sharma and Michael Doube, added the Edit>Selection>Fit Circle command, which fits a circle to line and multi-point selections. With area selections, it creates a circle with the same area and centroid."
http://rsbweb.nih.gov/ij/notes.htmlSo you have to draw a roi on your partial circle, then run this command.
Michael
>Is there a plugin to find the center from a partial circle?