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The heading line is a key concept in two areas of electronic navigation, radar displays and electronic chart displays.
A line on the radar screen from the center to the outer edge that marks the dead ahead direction of your vessel at the moment. For head-up radar this is always a vertical line to the top of the screen, but in north-up and course-up radar this line will point in the direction of your heading and rotate on the screen as your vessel turns to a new course. On most radars, this line can be temporarily hidden to look for weak targets dead ahead. Also called "heading flash" or "ship's heading line." For purposes of radar target classification, it is useful to think of this line as also extending backward to the opposite edge of the radar screen. Thus you can think of targets aft of you as also moving diagonal or parallel to the heading line.
On electronic charting systems
On an electronic chart display, we have the option (highly recommended) to add a line to our vessel icon that points in the direction the vessel is headed, which is often abbreviated CTW (course through the water). Most programs let us set the length of this line in miles, and we change this depending on the type of navigation at hand.
A key setup for safe navigation is to have this heading line showing on the vessel icon as well as the COG predictor line. The latter shows the direction we are actually moving, as opposed to the heading line, which is just the direction we are headed. Controlling the relative lengths of these two lines and zooming in to adequately monitor them is a way to be sure we know if we are not moving in the direction we are headed.
To show your heading in a radar display or on an electronic chart display you must have a dedicated heading sensor, which is a relatively inexpensive and valuable instrument. Some GPS receivers come with a heading sensor installed in the same housing. Unless you have a gyro compass (ships only) or "GPS compass" (basically two GPSs in a line) then the raw heading measurement will be magnetic, so the sensor has to be calibrated for deviation, which can be done electronically in most cases.
See course predictor.