In geography, a plane is a flat, two-dimensional surface used to represent or measure features on the curved Earth. You’ll encounter the term most often in three contexts: map projections that flatten the globe onto a sheet, coordinate systems that assign grid locations on a flat surface, and vertical datums that establish a baseline plane for measuring elevation. Each use takes the same core idea, a perfectly flat reference surface, and applies it to a different geographic problem.
Planes in Map Projections
The most common use of “plane” in geography involves turning the three-dimensional Earth into a flat map. A map projection is a mathematical formula that converts latitude and longitude on a sphere into x and y coordinates on a plane. Every flat map you’ve ever seen, whether on paper or a screen, is the result of projecting curved geography onto a plane.
One major family of projections, called azimuthal (or planar) projections, does this in the most literal way possible. Imagine holding a flat sheet of paper against a globe and shining a light through the globe onto that paper. The shadows traced onto the sheet are the projected features. The flat sheet is the plane. Where the plane touches the globe is called the point of tangency, and distortion is lowest at that point. The farther you move from it, the more shapes, distances, and areas stretch.
Three well-known azimuthal projections differ based on where the imaginary light source sits. In a gnomonic projection, the light is at the center of the sphere. In a stereographic projection, it’s on the far side of the globe from the plane. In an orthographic projection, the light source is treated as infinitely far away, producing the effect of looking at Earth from deep space. Each arrangement bends the geography differently onto the plane, making each projection better suited to specific tasks. Gnomonic projections, for example, show the shortest flight path between two points as a straight line.
The plane can also be secant rather than tangent. A tangent plane barely touches the globe at a single point. A secant plane slices into the globe, creating an entire circle of contact rather than just one point. This spreads the area of lowest distortion over a wider region, which is useful when mapping a large zone.
Planes in Coordinate Systems
Geographic coordinates (latitude and longitude) describe locations using angles on a curved surface. That works well for global positioning, but it’s awkward for everyday tasks like surveying a property line or measuring the distance between two buildings. For those jobs, geographers and engineers project coordinates onto a flat plane and switch to simple linear measurements in meters or feet.
A projected coordinate system pairs a geographic coordinate system with a map projection to create a flat grid. Once the math converts angular degrees into positions on a plane, you can measure distances with a ruler instead of trigonometry. This is what GIS software does every time it displays data on your screen: it takes curved-Earth coordinates and mathematically flattens them onto the plane of your monitor.
The State Plane Coordinate System (SPCS) is a practical example. Created in the 1930s for U.S. surveying and engineering, it divides each state into one or more zones, each with its own projection and flat reference plane. By keeping each zone relatively small, the distortion introduced by flattening stays negligible, often less than one part in 10,000. Surveyors can then work with straightforward x and y grid coordinates instead of wrestling with the Earth’s curvature on every measurement.
The Vertical Reference Plane
When you see a sign that says a mountain peak is 4,392 meters above sea level, “sea level” is acting as a reference plane. A vertical datum defines the flat surface from which all elevations are measured, essentially setting the zero point for height.
In North America, the standard is the North American Vertical Datum of 1988 (NAVD 88). It was established by tying together leveling measurements across the United States, Canada, and Mexico, all anchored to a single tidal benchmark at Rimouski, Quebec, where local mean sea level was recorded at a specific height. Only one tidal station was used as the anchor because mean sea level is not actually the same height everywhere. Ocean currents, temperature, salinity, and atmospheric pressure cause the sea surface to vary from place to place, so using multiple tide gauges would introduce inconsistencies. By fixing one point and building outward, the datum creates a single, consistent plane of reference across the continent.
The Reference Ellipsoid as a Plane
At the largest scale, geographers need a mathematical model of Earth’s overall shape to serve as the foundation for everything else. The Earth isn’t a perfect sphere; it bulges slightly at the equator and is flattened at the poles. The model that captures this shape is called a reference ellipsoid, and it functions as the fundamental surface (or generalized plane) from which all geographic measurements begin.
The most widely used model today is WGS 84, the World Geodetic System established in 1984 and refined since. It defines the Earth’s shape with a semi-major axis (the equatorial radius) of 6,378,137 meters and a flattening factor of about 1/298.26, meaning the poles are roughly 21 kilometers closer to Earth’s center than the equator. Every GPS receiver in the world uses WGS 84 as its reference surface. When your phone reports your location, it’s calculating your position relative to this mathematically defined ellipsoid.
Why the Concept Matters
The idea of a plane in geography is fundamentally about simplification. The Earth is irregular, curved, and constantly shifting. By defining flat reference surfaces at different scales, geographers make it possible to draw accurate maps, assign precise coordinates, and measure elevation consistently. Without agreed-upon planes, two surveyors measuring the same hilltop could report different heights, and two cartographers mapping the same coastline could produce maps that don’t align.
Every time you open a navigation app, check a topographic map, or look up the elevation of a city, you’re relying on planes: the projection plane that flattened the globe for your screen, the coordinate plane that placed your blue dot on the right street, and the vertical datum plane that tells you how far above sea level you are.