Volume sizes, positions and orientations are stored in the GEANT ZEBRA data structure JVOLUM (see the GEANT [2] manual for a description of JVOLUM). In EOSGX the position and orientation of the major detectors may be loaded into JVOLUM from two possible sources: the FFREAD file, or the TAS ROTM table.
The GEOMETRY command determines which source of positioning information is to be used. The command
GEOMETRY FFspecifies the FFREAD file, while GEOMETRY TT (the default) selects a special TAS table, the ROTM table.
The two choices have a different functionality. The FFREAD file is read by the program during initialization, so that the position data cannot be modified afterwards without re-running the program. This is not a problem if the detector positions are well-known, and are not under study. However, this method of positioning the detectors has generally been superseded by the use of the ROTM table.
The handling of the ROTM table information is dynamic. If the positioning source is set to TT, the GEANT geometry reflects the contents of the ROTM table at all times. If the ROTM table is modified, the changes are automatically loaded into the JVOLUM data structure. If the data structure is modified by means of the MOVE command, the ROTM table is automatically updated to correspond with the new positions.