The global coordinate system relies on latitude and longitude to pinpoint any location on the Earth’s surface. A full circle is divided into 360 degrees, but this unit is too broad for practical navigation, surveying, and modern mapping. To find a specific location, such as a street address or a ship’s position, a far greater resolution than a single degree is required. This necessitated a standardized method for subdividing the degree into smaller units of angular measure.
The Standard Subdivision: Degrees, Minutes, and Seconds
The traditional method for achieving precision uses the sexagesimal system, or base 60, inherited from ancient Babylonian mathematics. This framework subdivides one angular degree into 60 smaller units called minutes of arc (‘). Each minute is then further divided into 60 even smaller units called seconds of arc (“).
This Degrees, Minutes, Seconds (DMS) notation allows coordinates to be expressed with significant detail, such as 34° 03’ 16” N. This format is historical and still widely used in nautical and aeronautical charts where traditional methods and tools are often employed. The system treats the degree as the whole number, with minutes and seconds representing fractions of the degree. Because one degree equals 3,600 seconds, the DMS system offers high mathematical precision for mapping and navigation.
The sexagesimal structure mirrors the way time is measured, making the system intuitive for human users who manually read and calculate positions. When greater precision is needed, the seconds unit is expressed with a decimal fraction rather than being further subdivided into sixty parts.
Understanding the Scale of Angular Measurement
Translating the angular units of DMS into real-world distances illustrates the practical need for subdivision. An arc minute of latitude is approximately equal to one nautical mile (about 1.85 kilometers). This relationship is consistent everywhere on Earth because lines of latitude are parallel. Navigators historically relied on this equivalence for calculating distances traveled at sea.
The arc second represents an extremely small distance on the ground. A single second of latitude is roughly equivalent to 30 meters, or just over 100 feet. This resolution enables modern GPS devices to locate an object with high accuracy.
The scale of longitude, however, is not constant because the lines of longitude converge at the poles. While an arc minute of longitude is one nautical mile at the equator, that distance shrinks to zero at the North and South Poles. For example, at a mid-latitude of 45 degrees, the physical distance covered by one degree of longitude is significantly less than it is at the equator.
Decimal Degrees: The Alternative Subdivision Method
The Decimal Degrees (DD) method is a modern alternative to the traditional DMS system and is the standard format for most digital applications. This approach expresses the degree and its fraction as a single number using a decimal point, eliminating minutes and seconds altogether. For example, a coordinate expressed as 40° 30′ 00″ in DMS is written as 40.5° in DD.
This notation is mathematically simpler for computers to process, store, and calculate, making it ideal for technologies like Global Positioning Systems (GPS) and Geographic Information Systems (GIS). Digital systems rely on standard floating-point arithmetic instead of performing sexagesimal conversions. High precision is achieved by extending the number of digits after the decimal point, where each additional digit represents a smaller fraction of the degree.
A coordinate using five or six decimal places can locate a point with sub-meter accuracy, which is more than enough for almost all commercial and personal uses. This shift to a decimal format is not a change in the underlying measurement but rather a change in how that measurement is expressed and recorded. The DD system offers a streamlined and efficient way to handle the necessary precision for today’s location-based technologies.