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This is a line we can set on our boat icon in an electronic charting system (ECS) that shows which way the boat is actually moving, as opposed to which way it is headed. Namely, it points in the direction of the course made good (COG), which is a derived GPS parameter.
Another key aspect of the COG predictor, common to all ECS, is the user can define the length of this line to be equal to a predicted position some time in the future, such as 6 min from now or 1 hr from now. The length of the COG predictor is then figured as the speed made good (SOG) times this time interval. If the time is set to 6 min, then the length of the line will be one tenth of the boat speed. qtVlm refers to the time interval as the reckoning time.
Usually there is a dot or other marker at the end of the line as a reminder that this is the position the boat will be at the designated time interval.
On type approved ECDIS systems and some few ECS, such as qtVlm and TimeZero, the user can change the line into a cone so it can then be used to trigger alarms at hazards that lie ahead. See danger cone.