# 13.2: C.2- Units and their background

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- 16476

Hydrographic charts were meant to provide assistance to the navigators on board sailing vessels who had little more in the way of instruments than a clock and a sextant. Positions were determined with respect to the position of the sun and the stars. The grid of the hydrographic chart is therefore the grid of the degrees latitude and longitude as drawn on the globe. Transformation of this spherical grid to a plane map causes distortions, either in the centre or in the corners of the map. This means that the coordinates of the grid as indicated along the borders of the map are not linear.

Since the mutual distance between the longitudinal coordinates (meridians) varies (they are long at the equator and zero at the poles) only the degrees of latitude (parallels) give a proper indication of the scale of the map. The circumference of the earth is \(40000\ km\), which is divided into \(360^{\circ}\) (degrees), each consisting of \(60'\) (minutes). This means that the \(40000\ km\) are equal to \(360 \times 6 = 21600′\). The sailors used the minute as their unit of distance, the nautical mile, which thus equals slightly more than \(1850\ m\). The early hydrographic charts were not based on the metric system. Their scales therefore appear unusual to people who are familiar only with the metric system of measurement.

The speeds of vessels and the velocities of current are often expressed in nautical miles per hour (also called knots), which is slightly more than \(0.5\ m/s\).

Water depths (soundings) are expressed either in the traditional nautical system, or in the metric system. This is always indicated on the map. The nautical system uses feet, fathoms (Dutch: vadem), or fathoms plus feet. A foot is equal to \(0.3048\ m\); a fathom is equal to 6 feet or \(1.83\ m\).