Map Projection: A mathematical expression that systematically projects locations from the surface of a sphere to a representative position on a planar surface is called a map projection
Conformal Projection: Preserves local shapes. Graticule lines on the globe are perpendicular, so to preserve individual angles describing spatial relationships, a conformal projection must also present graticule lines intersecting at 90-degree angles on the map. Drawback is that area enclosed by a series of arcs may be greatly distorted. No map projection can preserve shapes of larger regions.
Equal-area Projection: This projection preserves the area of displayed features. To do this, the properties of shape, angle, scale, or any combination of these may be distorted. Thus, in such projections, the meridians and parallels may not intersect at right angles. May be difficult, particularly in maps of smaller regions to see that shape has been distorted, and distinguishing an equal-area projection from a conformal projection may be difficult unless documented or measured.
Equidistant Projections: Preserve the distances between certain points. Scale is not maintained correctly by any projection throughout an entire map; however, in most cases, one or more lines on a map have scale maintained properly. Where distance is preserved, those distances are said to be true. In a SINUSOIDAL projection, the Equator and all parallels are true. In other projections, the Equator and all meridians are true. In still others, (e.g. Two Point Equidistant) scale is true between one or two points and every other point on the map. No projection is equidistant to and from all points on the map.
True-direction (Azimuthal) Projections: Shortest route between two points on a curved surface is the spherical equivalent of a straight line on a flat surface; that is, a great circle. These projections are used to rectify some of the great-circle arcs, giving the directions or azimuths of all points on the map correctly with respect to the center.
Projections may be planar, cylindrical, or conic. First step in projecting from one surface to another is to create one or more points of contact. Each point is called a point of tangency. Cones and cylinders contact the globe along a line while planar projections contact only at one point. If the surface of a projection intersects a globe rather than merely touching the surface, then secant calculations are necessary rather than tangential calculations. It is important to know where the projection is tangent because this defines the point or line of zero distortion. This line of true scale is often referred to as a standard line. In general projection distortion increases with distance from the point of contact.
Rhumb Line: The course of a ship that keeps a constant compass direction, drawn as a line on a map, chart, or globe and cutting across all meridians at the same angle: also called a loxodromic curve.
Great Circle: The intersection of the surface of the earth with any plane that passes through the center of the earth.