Whis Whistle Buoy. Long flashing light, period 10 seconds. For now, just remember that all this information can be found in the nautical chart. Angles are true and distances can be measured using the vertical scale. Like the Mercator projection use the vertical scale to measure distances.
Great circles appear as straight lines on the chart.
A line which makes the same angle with all meridians. Theoretically not the shortest route, but a handy straight line on a Mercator chart. Defines the relationship between the ellipsoid adopted as the model of the Earth's shape, and the Earth itself. Coordinates which refer to, for instance, AIA should be corrected before plotting them in a chart based on another horizontal datum. If your GPS reciever consistently disagrees with known positions by a constant amount and direction, then check you have set it to display the correct horizontal datum.