# American Practical Navigator by N. Bowditch

By N. Bowditch

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One may want to identify a location or area simply by alpha-numeric rectangular coordinates. This is accomplished with a grid. In its usual form this consists of two series of lines drawn perpendicularly on the chart, marked by suitable alpha-numeric designations. A grid may use the rectangular graticule of the Mercator projection or a set of arbitrary lines on a particular projection. The World Geodetic Reference System (GEOREF) is a method of designating latitude and longitude by a system of letters and numbers instead of by angular measure.

On such a chart the border scale near the latitude in question should be used for measuring distances. Of the various methods of indicating scale, the graphical method is normally available in some form on the chart. In addition, the scale is customarily stated on charts on which the scale does not change appreciably over the chart. The ways of expressing the scale of a chart are readily interchangeable. 39 inches. If the natural scale of a chart is 1:80,000, one inch of the chart represents 80,000 inches of the earth, or a little more than a mile.

Soundings Charts show soundings in several ways. Numbers denote individual soundings. S. and foreign surveys, different datums, or smaller scale charts. Large block letters at the top and bottom of the chart indicate the unit of measurement used for soundings. SOUNDINGS IN FATHOMS indicates soundings are in fathoms or fathoms and fractions. SOUNDINGS IN FATHOMS AND FEET indicates the soundings are in fathoms and feet. A similar convention is followed when the soundings are in meters or meters and tenths.