VL_mid is an analysis export variable.
It reports the distance (measured by vessel log) from the first ping in the analyzed variable to the middle ping in the domain which was analyzed.
This variable is exported only if it is selected on the Export page of the EV File Properties dialog box.
About analysis
About analysis variables
About ping position
About VL_End
About VL_Start
About Dist_E
About Dist_M
About Dist_S