Why map projections exist
The Earth is roughly spherical. Maps, screens, and printed plans are flat. There is no way to flatten a curved surface onto a plane without introducing some form of distortion. A map projection is the controlled mathematical process that manages that distortion.
What projections distort
Because a sphere cannot be perfectly flattened, every projection makes trade-offs. The four main properties that can be affected are shape, area, distance, and direction.
| Property | What it means | Preserved by |
|---|---|---|
| Shape | Local angles and shapes stay accurate | Conformal projections (e.g. Mercator, Transverse Mercator) |
| Area | Relative sizes of regions stay correct | Equal-area projections (e.g. Albers, Mollweide) |
| Distance | Distances from one or two points are accurate | Equidistant projections |
| Direction | Bearings from a central point are correct | Azimuthal projections |
How projections affect the coordinates you see
Projections do not just change how a map looks. They change how positions are described numerically. A geographic coordinate system uses latitude and longitude in degrees. A projected coordinate system uses Easting and Northing in metres.
Common map projections you will encounter
| Projection | Description |
|---|---|
| Transverse Mercator | The projection used as the basis for systems like UTM and British National Grid. |
| Web Mercator | The projection used by Google Maps, OpenStreetMap, and most web mapping platforms. |
| UTM (Universal Transverse Mercator) | A global system that divides the world into 60 zones, each using a Transverse Mercator projection. |
| British National Grid | A projected coordinate system based on a Transverse Mercator projection, used across Great Britain. |
| Lambert Conformal Conic | A projection commonly used for regional mapping in North America and parts of Europe. |
Projection vs coordinate system
A map projection and a coordinate system are closely related, but they are not the same thing. The projection is the mathematical method that flattens the Earth. The coordinate system is the broader framework.
Projection vs datum
Projection and datum are two of the most frequently confused concepts in mapping. A projection controls how the Earth is flattened onto a surface. A datum controls which model of the Earth is used as the starting point.
Practical guidance
- If your destination is a web map, phone, or GPS app, convert to latitude/longitude (WGS84).
- If your destination is a GIS project, CAD drawing, or survey report, check which projected coordinate system it expects.
- If projected values look nothing like latitude/longitude, that is normal.
- If a converted coordinate is close but slightly offset, the issue may be the datum rather than the projection.
Takeaway
A map projection is the mathematical method that turns the curved Earth into a flat, usable map. Understanding which one your data uses is essential for accurate conversion, measurement, and mapping.