Projected coordinate system

A projected coordinate system – also called a projected coordinate reference system, planar coordinate system, or grid reference system – is a way to show locations on Earth using simple x and y numbers on a flat surface. This flat surface is made using something called a map projection, which takes the round shape of the Earth and turns it into a flat map.

Each projected coordinate system, like "Universal Transverse Mercator WGS 84 Zone 26N," is made by choosing a map projection, a way to tie the system to real places on Earth called a geodetic datum, a starting point, and a unit of measurement. There are hundreds of these systems, made for different uses and places.

When these systems were first made in the 20th century, such as the Universal Transverse Mercator, State Plane Coordinate System, and British National Grid, people often called them grid systems. But today, the term projected coordinate system is used more often to make it clear what it is. This term is used in international rules like the EPSG and ISO 19111, and in most geographic information system software.

History

The ideas behind maps and ways to mark places on Earth go back a very long time, to ancient times and even more so during the 1700s. But using special flat maps to show exact spots only started in the 1900s.

One of the first systems was made in the United States in the 1930s to help with measuring land. In the United Kingdom, a similar system began in 1938.

During World War II, soldiers needed quick and easy ways to tell where they were, so grids were printed on maps. This helped but caused confusion because each area used its own system. To fix this, a new system called the Universal Transverse Mercator coordinate system was created, along with a simple way to write down locations called the Military Grid Reference System.

After the war, more people began using these systems, especially scientists. Later, as computers became common, many places made their own mapping systems. Even today, these flat map systems are still used a lot, especially for local areas like cities and small countries, while other systems are used for bigger areas like the whole world.

System specification

A projected coordinate system is a way to show places on Earth using simple x and y points on a flat surface. This helps us measure and talk about locations clearly. These systems are used in many computer programs that work with maps.

To make sure everyone understands these points the same way, we need clear rules. These rules include:

• A flat surface with a starting point and directions for measuring left/right (x) and up/down (y). Usually, we measure in meters or feet.
• A method to turn Earth’s curved surface into a flat map. This method has special settings like where the center is, which help change real-world points into x and y points.
• A way to connect the flat map to real places on Earth. This makes sure points on the map match real spots on our planet.

Different map methods, like Mercator or Lambert, are used for various maps. Each method has its own rules to turn Earth’s curves into a flat picture. Some maps even use a "false origin" to keep all points positive, making it easier to work with the numbers.

Grid reference encodings

Locations in a projected coordinate system are shown as pairs of numbers, with the first number showing how far east a point is and the second showing how far north. For example, the top of Mount Assiniboine in Canada is shown as (0594934mE, 5636174mN). These numbers are easy for computers to use but hard for people to remember.

To make things easier, people use shorter codes. For example, instead of writing all the digits, they might only use the important ones. In some systems, letters are also used to stand for parts of the numbers, making the codes shorter and easier to share.

The more numbers added to a grid reference, the more exact the location becomes. Adding two more digits to a four-digit reference creates a six-digit reference, which points to a smaller area about 100 metres across. This level of detail is usually enough for finding places when walking or exploring.

Examples of projected CRS

Further information: List of national coordinate reference systems

• Universal Transverse Mercator (UTM): This is not just one system, but a group of 60 areas, each covering a slice of the Earth 6° wide, using its own special mapping method.
• Universal Polar Stereographic (UPS): These are two systems that cover the Arctic and Antarctica using a special mapping way.
• Ordnance Survey National Grid (OSNG): This system uses a mapping method centered on 2°W to cover Great Britain with its own special code.
• State Plane Coordinate System (SPCS): This is a group of more than 120 systems, each covering a state of the United States or a part of it.
• Swiss coordinate system (LV95): This system covers Switzerland, using a mapping method.
• Irish Transverse Mercator (ITM): This system was made together by the Republic of Ireland and United Kingdom to cover the whole island of Ireland.
• Bermuda National Grid
• Hellenic Geodetic Reference System 1987 (Greece)
• Israeli Transverse Mercator (NIG)
• Swedish grid (RT90)