Geographic coordinate system facts for kids

Lines of latitude run sideways around the globe, while lines of longitude go up and down.

Quick facts for kids Geodesy

Fundamentals
Geodesy · Geodynamics Geomatics · Cartography
Concepts
Datum · Distance · Geoid Figure of the Earth Geodetic system Geog. coord. system Hor. pos. representation Map projection Reference ellipsoid Satellite geodesy Spatial reference system
Technologies
GNSS · GPS · GLONASS
Standards
ED50 · ETRS89 · NAD83 NAVD88 · SAD69 · SRID UTM · WGS84
History
History of geodesy NAVD29
v t e

A geographic coordinate system is an invisible grid used to find positions anywhere on Earth. It uses numbers called latitude and longitude. Instead of using flat, square measurements like a Cartesian coordinate system, it uses angles measured on a round sphere. This tool is the oldest and most popular of all modern spatial reference systems.

Maps use this grid every day for satellite navigation in cars, planes, and mobile phones. Special systems like the EPSG keep these measurements super accurate for scientists.

History of Mapping the Globe
Latitude and Longitude Lines
Understanding the Geodetic Datum
Distances Across the Grid
- How Long Is a Degree?
Fun Codes for Sharing Locations
Cool Facts About Coordinates
See also

History of Mapping the Globe

The angle measurements used for latitude and longitude on a spherical globe.

People have wanted to know where they are for thousands of years. Early explorers needed clever ways to travel across huge oceans without getting lost.

Ancient Greek Mapmakers

The invention of this system started with Eratosthenes of Cyrene in ancient times. He worked at the famous Library of Alexandria during the 3rd century BC and wrote a book called Geography. He realized that drawing grid lines across the world made it easier to pinpoint cities.

Later, an astronomer named Hipparchus of Nicaea improved this map grid. He studied the stars to find locations instead of guessing through dead reckoning. He also used the shadow timing of a lunar eclipse to calculate distances between places.

In the 1st or 2nd century, Marinus of Tyre made an early flat world map. He drew his starting line, or prime meridian, through the Fortunate Isles near the Canary Islands and Cape Verde. He also measured north and south from the sunny island of Rhodes in Asia Minor.

Another famous thinker named Ptolemy wrote his own Geography. He decided to measure north and south starting directly from the Equator.

Medieval Discoveries

During the 9th century, scholars translated these ancient books into Arabic. A brilliant scientist named Al-Khwārizmī wrote the Book of the Description of the Earth. He corrected old errors about the size of the Mediterranean Sea.

Because of his work, medieval Arabic cartography became very advanced. Later, a scholar named Maximus Planudes found Ptolemy's forgotten texts around the year 1300. The book was translated into Latin in the city of Florence by Jacopo d'Angelo in 1407, helping European sailors explore the world.

The Greenwich Agreement

For a long time, different countries used different starting lines on their maps. In 1884, the United States hosted the International Meridian Conference. Twenty-five nations sent leaders to pick one official center line for the whole planet.

Twenty-two nations chose the Royal Observatory in Greenwich, England. The Dominican Republic voted against this, while France and Brazil did not vote. Later in 1911, France officially joined in and adopted Greenwich Mean Time instead of using the Paris Observatory.

Latitude and Longitude Lines

The crisscross grid on a globe has a special name: a graticule. It is made of two different sets of imaginary circles.

Latitude Lines That Wrap Around

Latitude lines run east and west like giant belts stacked up the globe. They are called parallels because they stay the exact same distance apart. The main starting line is the equator, which sits at 0 degrees latitude. It serves as the base circle for the grid. The equator splits the planet into the Northern and Southern Hemispheres.

The very top of the world is the North Pole at 90° N. The very bottom is the South Pole at 90° S. Scientists can check latitude using the stars in an astronomical system with a weighted plumb bob. They can also measure straight from Earth's center in a geocentric model.

Longitude Lines from Pole to Pole

Longitude lines, or meridians, run up and down from the top of the world to the bottom. Each meridian is half of an ellipse shape. They are widest at the equator and squeeze together until they touch at both poles.

The starting line at 0 degrees longitude is the prime meridian in Greenwich, London. Some groups, such as the French Institut national de l'information géographique et forestière, used their own lines in the past.

Directly on the other side of the planet lies the opposite meridian at 180°. Near this line is the International Date Line, where calendar days change. This date line wiggles around places like the Aleutian Islands so that local communities share the same school day.

The Mystery of Null Island

When latitude is zero and longitude is zero, you arrive at coordinates 0°N, 0°E. This imaginary spot is in the ocean in the Gulf of Guinea, south of Tema in Ghana. Geographers jokingly call this empty patch of water Null Island. Computer mapping errors often dump lost data right at this funny spot!

Understanding the Geodetic Datum

Earth is not a perfectly smooth plastic ball. Mountains rise high, valleys sink low, and the planet bulges around the middle. Because of this, scientists use a mathematical tool called a geodetic datum. A datum matches a smooth shape called a reference ellipsoid to the bumpy surface of the planet.

Horizontal and Vertical Datums

To build accurate maps, surveyors use two kinds of datums. A horizontal datum tells you your exact latitude and longitude across the ground. A vertical datum measures height and altitude above a model of the sea called a geoid.

Years ago, surveyors set up metal markers called a control network in the ground. Today, computers use fast measurements from space. These include VLBI radio telescopes, SLR lasers, and DORIS radio satellites.

Why Datums Shift Coordinates

Different datums can give different coordinate numbers for the exact same physical rock. When changing between systems, computers use a datum transformation or a simple shift called a Helmert transformation.

Global systems cover the entire planet. The most common global system is WGS 84, which powers the Global Positioning System (GPS). Another one is the International Terrestrial Reference System and Frame, which tracks tiny shifts across the globe.

Many countries made their own local datums that fit their specific borders best. North America used NAD27 and newer North American Datum models. Europe built ED50, while Great Britain created OSGB36. A British map using OSGB36 can be about 112 meters different from a GPS reading using WGS 84!

The Moving Earth

Earth's outer crust is never sitting completely still. The ground shifts due to continental drift and crustal deformation. Continents slide along like giant bumper cars, moving up to 10 centimeters every year.

Gravity from the Moon and Sun creates an ocean and ground tide that lifts land slightly. High air pressure from a weather system can push the ground down by 5 millimeters. In northern Scandinavia, the ground rises 1 centimeter each year because heavy ice melted after the ice age. Meanwhile, nearby Scotland rises only 0.2 centimeters per year. Modern GPS satellites must track these movements constantly.

Distances Across the Grid

Because Earth is an oblate spheroid that bulges at the center, degrees have different lengths in different places.

How Long Is a Degree?

At sea level along the equator, one degree of latitude equals about 110.6 kilometers. One minute of latitude is 1843 meters, and one second is about 30.7 meters.

Longitude degrees change size dramatically. At the equator, one degree of longitude is 111.3 kilometers wide. As you travel closer to the cold poles, the lines squeeze closer together.

At the equator in Quito, a degree of longitude is 111.3 kilometers wide.
Further north in New Orleans, a degree shrinks to 96.49 kilometers.
In the city of Bordeaux, it drops to 78.85 kilometers.
In Greenwich, it narrows to 69.47 kilometers.
High north in Saint Petersburg, one degree of longitude measures only 55.80 kilometers across!

Fun Codes for Sharing Locations

Long strings of degree numbers can be tricky to say over a walkie-talkie. People created clever shortcuts to share coordinates easily:

The Maidenhead Locator System is used by ham radio hobbyists around the world.
Military planes used the World Geographic Reference System and now use the Global Area Reference System.
Google created Open Location Code, also called Plus Codes, to give simple addresses to remote homes.
Computer experts use Geohash, which turns numbers into a string of letters using a Z-order curve.
Drivers use Mapcode inside navigation units to find streets quickly.
The What3words system names every small square on Earth using three everyday words from a dictionary.

Cool Facts About Coordinates

The International Date Line isn't perfectly straight! It zigzags to avoid splitting countries or islands in half.
The Earth isn't a perfect ball. It's slightly flattened at the poles and bulges a bit at the Equator. This small difference is important for very accurate coordinate measurements.
GPS satellites orbit the Earth and send signals. Your GPS receiver uses these signals to figure out your exact latitude and longitude.
Many online map services, like Google Maps, use geographic coordinate systems to show you locations and give directions.
Being able to tell time accurately was super important for figuring out longitude at sea. Before good clocks, sailors had a very hard time knowing their exact east-west position.