A chart is a miniature graphic representation of a part of the earth's curved surface transferred to a flat surface. A nautical chart shows the natural and man-made features of a specific boating area, including safe channels as well as hazards to navigation.
Grid System: The uniform grid system makes it possible to locate the position of any boat on earth. This system consists of two sets of imaginary lines on the earth's surface; the meridians of longitude, which run north-south through the poles, and the parallels of latitude, which run east-west parallel to the equator.
Geographic coordinates are measured in terms of degrees. By international agreement, meridians of longitude are labeled east or west from the prime meridian, which passes through Greenwich, England at zero degrees longitude, about 5 miles east of the center of London. The longitude of any position on earth is described as x degrees East or West of Greenwich, to a maximum in either direction of 180 degrees. The designation of being "E" or "W" of Greenwich is an essential part of any statement of longitude, commonly abbreviated "Lo."
Parallels of latitude are measured in degrees North or South of the equator for zero degrees at the equator to a maximum of 90 degrees at the poles. The designation of latitude, commonly abbreviated "L," as "N" or "S" is necessary for a complete position.
Chart Scales: Because a chart is a miniature representation of navigable water area, actual distances must be scaled down to much shorter dimensions on paper. This reduction is the "scale" of the chart and it may be expressed as a ratio: 1:80,000 means that 1 unit on the chart represents 80,000 units on the actual land or water surface. This is the "nautical scale" of the chart.