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course through the water This is in effect an alternative name for heading, but with some nuanced differences that are more convention than critical. Course through the water (CTW) and heading (HDG) are for the most part interchangeable. When referring to DR or grib reckoning (qtVlm) we would use heading (HDG) as we refer to an idealized concept. The actual direction the boat is pointed at any moment is its heading, but when we display this heading we are steering that we read with a heading sensor it would be called and displayed on meters as course through the water (CTW). In actual sailing with live NMEA input, we can average the instantaneous headings to get a more accurate CTW, sometimes called "data smoothing." When you want to know what the compass reads (ie which way the boat it pointed at the moment), you would probably ask the helm for your heading, not what your CTW, but when entering this in the logbook it could be called either one. Likewise a meter label could be HDG or CTW. When writing equations dependent on heading and speed, we would likely call them CTW and STW to be consistent. See also course over ground and course made good. Abbreviation:  CTW

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