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113 CHAPTER 7 DEAD RECKONING DEFINITION AND PURPOSE 700. The Importance Of Dead Reckoning Dead reckoning allows a navigator to determine his present position by projecting his past courses steered and speeds over ground from a known past position. He can also determine his future position by projecting an ordered course and speed of advance from a known present posi- tion. The DR position is only an approximate position because it does not allow for the effect of leeway, current, helmsman error, or gyro error. Dead reckoning helps in determining sunrise and sunset; in predicting landfall, sighting lights and predicting arrival times; and in evaluating the accuracy of electronic positioning information. It also helps in predicting which celestial bodies will be available for future observation. The navigator should carefully tend his DR plot, up- date it when required, use it to evaluate external forces acting on his ship, and consult it to avoid potential naviga- tion hazards. CONSTRUCTING THE DEAD RECKONING PLOT Maintain the DR plot directly on the chart in use. DR at least two fix intervals ahead while piloting. If transiting in the open ocean, maintain the DR at least four hours ahead of the last fix position. If operating in a defined, small operating area, there is no need to extend the DR out of the operating area; extend it only to the operating area boundary. Maintaining the DR plot di- rectly on the chart allows the navigator to evaluate a vessel’s future position in relation to charted navigation hazards. It also allows the conning officer and captain to plan course and speed changes required to meet any operational commitments. This section will discuss how to construct the DR plot. 701. Measuring Courses And Distances To measure courses, use the chart’s compass rose near- est to the chart section currently in use. Transfer course lines to and from the compass rose using parallel rulers, rolling rulers, or triangles. If using a parallel motion plotter (PMP), simply set the plotter at the desired course and plot that course directly on the chart. The navigator can measure direction at any convenient place on a Mercator chart because the meridians are parallel to each other and a line making an angle with any one makes the same angle with all others. Measure direction on a con- formal chart having nonparallel meridians at the meridian closest to the area of the chart in use. The only common non- conformal projection used is the gnomonic; a gnomonic chart usually contains instructions for measuring direction. Compass roses give both true and magnetic directions. For most purposes, use true directions. Measure distances using the chart’s latitude scale. As- suming that one minute of latitude equals one nautical mile introduces no significant error. Since the Mercator’s latitude scale expands as latitude increases, measure distances on the latitude scale closest to the area of interest. On large scale charts, such as harbor charts, use the distance scale provided. To measure long distances on small-scale charts, break the
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