What is the accuracy with which we know the field or field integrals between the Vertex and the FMS region
in STAR?  I have integrated the equations of motion for a charged particle in a magnetic field and compared
the results with measurements by Jim Drachenberg in his thesis, and in this link:

http://www.star.bnl.gov/protected/spin/drach/Run8FMS/AutoVeto/

The basic idea is to extrapolate tracks in the FTPC to the position of the FMS and compare
DeltaEta and DeltaPhi.  Jim Drachenberg (and Akio Ogawa and Chris Perkins) all estimated
the DeltaEta from the field map.  Jim Drachenberg measured the FTPC/FMS difference, and
I determined the difference from integrating the trajectories.  Here is the comparison:

   Estimate                        Measured                 Track Calculation
DeltaEta +/-0.27/p_z         0.25-0.28/p_z            0.270/p_z

These are all reasonably close.  I conclude that we know the field integral to +/-3.5%

Jim Drachenberg also found there was a DeltaEta.  He concluded that Eta_towers-Eta_tracks<0
for both positive and negative charged particles and that DeltaEta was the same order of
magnitude as DeltaPhi.  I confirm these results and find:

DeltaEta = Eta_towers-Eta_tracks

   Estimate                        Measured                 Track Calculation
DeltaEta                                                           -0.134/p_z