Map Projection

Maps allow us to communicate about location using a common framework. Without this common framework, the relative distance would be near impossible to measure and share.

But maps don’t provide just one single system, and there are literally infinite ways to talk about distances and locations on the earth’s surface. Which called coordinate systems, and they determine not only how maps look, but also how the data is stored and how the distance is calculated.

Coordinate systems (projected and geographic) are often identified by an EPSG code. EPSG itself stands for European Petroleum Survey Group, an organization involved in best practices for surveying and applied Geodesy.

WGS84

WGS84 is the latest revision of World Geodetic System (WGS), a standard for use in cartography, geodesy, satellite navigation, including GPS. Its comprises a standard coordinate frame for the Earth, a datum/reference ellipsoid for raw altitude data, and a gravitational equipotential surface (the geoid) that defines the nominal sea level

EPSG:4326

4326 is just the EPSG identifier of WGS84.

Web mapping data

For the most part, web maps rely on data stored with WGS84 coordinates (sometimes called “unprojected” data), and then visualize the data using Pseudo-Mercator.

But sometimes, a mapper will say they want to see their data visualized in the WGS84 projection (or perhaps they refer to it as EPSG:4326). As mentioned above, WGS84 is unprojected — there is no visual representation of that data. So if someone says they want to see their data in WGS84, they really want to see their data in a Plate-Carrée projection, which essentially creates a linear Cartesian graph on the page.

In particular, the plate carrée has become a standard for global raster datasets, such as Celestia and NASA World Wind, because of the particularly simple relationship between the position of an image pixel on the map and its corresponding geographic location on Earth.