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Latitude and Longitude: The Grid of the Earth

Latitude measures how far north or south a location is from the equator, which sits at 0°. The North Pole is 90° N and the South Pole is 90° S. Longitude measures how far east or west a location is from the Prime Meridian — an imaginary line running through Greenwich, England — with values ranging from 0° to 180° in each direction.

Together, any location on Earth can be described as a coordinate pair, such as 48.8566° N, 2.3522° E for Paris, France. This decimal format is what GPS devices and mapping apps use internally. But for centuries, navigators and surveyors used a different notation.

Degrees, Minutes, Seconds (DMS)

The traditional system divides each degree of arc into 60 minutes (written with the symbol ′) and each minute into 60 seconds (written ″). This mirrors the same 60-based system used in time — not a coincidence, since early astronomers measured both angles and hours using the same tools.

You'll encounter DMS notation on printed maps, in aviation charts, and in surveying documents. A typical DMS coordinate looks like: 48° 51′ 29″ N, 2° 17′ 40″ E (the Eiffel Tower).

Converting DMS to Decimal Degrees

To work with modern software or compare coordinates numerically, you need to convert DMS to Decimal Degrees (DD). The formula is straightforward:

Worked example: Convert 48° 51′ 29″ N to decimal degrees.

For southern latitudes or western longitudes, the result is negative: 33° 52′ S becomes −33.8667°.

How Big Is a Degree? Distance on Earth's Surface

The Earth's equatorial circumference is approximately 40,075 km (24,901 miles). Since a full circle is 360°, one degree of latitude equals roughly:

This means one minute of latitude ≈ 1.855 km, and one second of latitude ≈ 30.9 meters. Longitude degrees are the same length only at the equator — they shrink as you approach the poles, reaching zero at 90° N or S.

Nautical Miles and Knots

The nautical mile is the standard unit of distance in aviation and maritime navigation. It is defined as exactly 1,852 meters (approximately 1.151 statute miles). Its definition is rooted in geography: 1 nautical mile equals 1 arc-minute of latitude along any meridian.

Knots are the unit of speed used at sea and in the air. One knot equals one nautical mile per hour. A ship cruising at 20 knots travels 20 nautical miles per hour, or 37.04 km/h. When a pilot says the aircraft is flying at "450 knots," they mean 833 km/h.

Altitude: Measuring Height Above the Earth

Altitude — the vertical distance above a reference point — is measured differently depending on who's doing the measuring and why.

Meters vs. Feet

This is one of the most persistent measurement divides in the world. Pilots use feet for altitude: aircraft altimeters in most countries are calibrated in feet, and the standard altitude transition levels (like FL350, meaning 35,000 feet) are universally understood in aviation. Climbers and scientists use meters: Mount Everest's official height is 8,848.86 m, not 29,032 feet, in scientific literature.

In practice, a cruising altitude of 35,000 feet is approximately 10,668 meters — or roughly 10.7 km above sea level.

Barometric Altitude

Aircraft don't measure altitude by bouncing signals off the ground — they measure atmospheric pressure. As you climb, the air thins and pressure drops. The standard rate of pressure drop is approximately 12 pascals (Pa) per meter at sea level, though this varies with temperature and weather.

The standard atmosphere defines sea level pressure as 101,325 Pa (or 1013.25 hPa / millibars). Pilots set their altimeters to the local pressure setting (QNH) to get an accurate altitude reading. This is why you hear pilots request "QNH 1013" from air traffic control — they're asking for the local barometric pressure.

Map Scales and Cartographic Measurements

A map scale tells you the ratio between a distance on the map and the corresponding distance in the real world. A scale of 1:25,000 means 1 unit on the map equals 25,000 of the same units in reality. So 1 centimeter on the map = 250 meters on the ground (or 1 inch = ~0.395 miles).

Hikers using 1:25,000 maps know that 4 cm of trail on the map represents a 1 km walk on the ground — useful for estimating walking time.

The UTM Coordinate System

While latitude/longitude uses angular measurements, the Universal Transverse Mercator (UTM) system uses flat grid coordinates measured in meters. It divides the Earth into 60 vertical zones, each 6° of longitude wide, and assigns each location a northing (distance from the equator) and easting (distance from the zone's central meridian) in meters.

UTM is widely used in military mapping, GIS software, and scientific fieldwork because it makes distance calculations trivial: the difference between two UTM northings in meters is the actual distance between the points (within the same zone). The Eiffel Tower in UTM is approximately: Zone 31N, Easting 448,296 m, Northing 5,411,803 m.

GPS Accuracy and Practical Precision

Modern GPS (Global Positioning System) consumer devices achieve accuracy of 3–5 meters in ideal conditions (open sky). Differential GPS and WAAS (Wide Area Augmentation System) can improve this to under 1 meter. Survey-grade GPS receivers using real-time kinematic (RTK) correction can achieve centimeter-level accuracy.

For context: 1 second of latitude ≈ 30 meters. A GPS accurate to 5 meters is resolving a position to about 1/6 of an arc-second — remarkable precision from satellites orbiting 20,200 km above Earth.

Geo-Measurements Across Industries

Aviation

Pilots work in feet for altitude, knots for speed, and nautical miles for distance. Aircraft cruise at altitudes like FL350 (35,000 ft / 10,668 m), navigate using waypoints defined in decimal degrees, and measure visibility in statute miles in the US but meters or kilometers elsewhere.

Maritime Shipping

Ships use nautical miles, knots, and fathoms (for depth — 1 fathom = 6 feet = 1.829 m). Electronic chart displays (ECDIS) show positions in latitude/longitude. Deep ocean trenches like the Mariana Trench are measured in kilometers (10.994 km deep).

Hiking and Mountaineering

Trekkers use 1:25,000 or 1:50,000 maps, contour lines measured in meters, and altimeters. The summit heights of famous peaks — Kilimanjaro at 5,895 m, Mont Blanc at 4,808 m, Aconcagua at 6,961 m — are given in meters universally, even in countries that use feet for other purposes.

Cartography and GIS

Digital cartographers work in decimal degrees for coordinate storage, meters for distance calculations, and map projections that translate the curved Earth surface onto flat screens. Web maps like Google Maps use the Web Mercator projection (EPSG:3857), measuring coordinates internally in meters from a reference point.