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Why must Earth’s curvature be considered when mapping a large country or continent?

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Diagram showing the deflection of the vertical from the geoid to the ellipsoid or spheroid, with labeled lines for geoid, spheroid, rock surface, ocean, and geoid height in a geology context. — geoid Deflection of the vertical from the geoid to the spheroid.

When mapping a large country or continent, Earth’s curvature must be considered because the planet is not a flat plane but an ellipsoid of revolution. Attempting to represent features of this curved surface on a flat map necessitates the use of cartographic projections, which inevitably introduce distortions in size, shape, or distance.123

In contrast to plane surveying, which assumes a flat surface, geodetic surveying is the discipline that accounts for and applies corrections for the Earth's curvature. Because the geoid—the model of Earth’s figure that coincides with mean sea level—is an irregular equipotential surface influenced by gravity, it cannot be represented as a simple flat plane. Consequently, for large-scale mapping, geodesy provides the necessary mathematical framework to manage the geometry of the Earth's curved surface, ensuring that measurements remain accurate across vast distances.415