About VL_mid_from_transect_S
VL_mid_from_transect_S is an analysis export variable.
It is the distance to the middle (VL_M="Middle") ping in the analysis domain (interval or region), from the first ping in transect (Transect_S="Start"), if the grid starts from ping 0. Otherwise, using the platform's vessel log source, it is the distance to the middle ping in the analysis domain from the start of the transect.
This variable is exported only if it is selected on the Export page of the EV File Properties dialog box.