# Dictionary:Map projection

A scheme for displaying the Earth’s curved surface on a plane surface. Some of the more common projections are shown in Figure M-4. Distortions of one sort or another are inevitable. **Equivalence** is the projection property wherein the product of orthogonal scale factors is maintained constant so that areas are preserved. **Conformality** is the property wherein angles are preserved. **Standard lines** are great or small circles along which the scale is uniform and constant. (**a**) **Tangent projections** are projections onto a surface (plane, cylinder, or cone) that is tangent to the Earth; (**b**) **secant projections** are those made onto a surface that intersects the Earth. (**c**) A **transverse projection** has its axis perpendicular to the Earth’s axis (sometimes merely at any angle to the Earth’s axis). (**d**) An **azimuthal projection** is onto a tangent plane; distortions increase as the distance from the point of tangency. (**e**) A **stereographic projection** is both azimuthal and conformal. (**f**) A **polyconic** has a straight central meridian and each parallel is the arc of a circle and is standard; the scale along meridians is therefore variable and the map is neither equivalent nor conformal. **(g) Transverse Mercator projections** (onto a cylinder at right angles to the Earth’s axis) and *Lambert conic projections* (q.v.) form many official coordinate systems; the *Universal Transverse Mercator* (q.v.) is one standard system in widespread use. **(h)** A **Robinson projection** permits some distortion in size at high latitudes to maintain shapes at middle and low latitudes.