This routine finds the point where the track stored in /ZTSTA/ and /ZTAUX/ intersects the reference surface stored in /ZTSEL/. The intersection point is stored in /ZTTRJ/, Uses fast, approximative tracking.
DSTR : track length from vertex to intersection with reference surface. IERR : error flag : 0 - no error 2 - track turns back before reference surface
from /ZTAUX/ : TANTH, RTRK, RINV : tan(theta), radius and 1/radius of track (calculated elsewhere from values in /ZTSTA/) from /ZTSEL/ : RREF and ZREF : radius of reference surface (for cylinder) or Z-coordinate ( for plane). from /ZTSTA/ : ZVRTX : z-position of vertex.
to /ZTTRJ/ : R(0),Z(0),PHI(0),BETA(0) : track parameters at intersection point; IREF : type of intersection (1 = cylinder, 2=plane)