<P> The gnomonic projection is used for charts intended for plotting of great circle routes . NOAA uses the polyconic projection for some of its charts of the Great Lakes, at both large and small scales . </P> <P> Positions of places shown on the chart can be measured from the longitude and latitude scales on the borders of the chart, relative to a geodetic datum such as WGS 84 . </P> <P> A bearing is the angle between the line joining the two points of interest and the line from one of the points to the north, such as a ship's course or a compass reading to a landmark . On nautical charts, the top of the chart is always true north, rather than magnetic north, towards which a compass points . Most charts include a compass rose depicting the variation between magnetic and true north . </P> <P> However, the use of the Mercator projection is not without its drawbacks . Mercator's technique was to make the lines of longitude parallel . On the real globe, the lines of longitude converge as one goes north or south away from the equator, until they meet at the pole . This distortion means that horizontal distances are exaggerated . Mercator's solution, imperfect as it might be, was to increase the distance between lines of latitude in proportion; in a Mercator's projection map, a square is a square no matter where you are on the chart, but a square on the Arctic Circle is much bigger than a square at the equator, even though both occupy the same number of degrees on the globe . The result of this is that scale in a nautical chart is dependent on latitude . In practical use, this is less of a problem than it sounds . One minute of latitude is, for practical purposes, a nautical mile . Distances in nautical miles can therefore be measured on the latitude gradations printed on the side of the chart . </P>

A harbor chart could have a scale of