This problem was solved by, among others, jhartman@scanner.frw.uva.nl.
Maybe he can tell you how it's done...
----\_\_\_--\_----\_------\_-------------------- P. Martijn van Leusen --------
\_ \_ \_\_\_\_ \_ Dept of Pre- and Protohistoric Archaeology
\_\_\_ \_ \_ \_ \_ University of Amsterdam, The Netherlands
\_ \_ \_ \_\_ \_ martijn@scanner.frw.uva.nl
--------\_------\_----\_--\_--\_\_\_\_-----------------------------------------