Projection Types

Map projections may be classified according to the form of the geometric surface on which the projection is made.

Azimuthal

Azimuthal (also called zenithal) projections have their meridians and parallels projected onto a flat screen tangent to the Earth surface, or onto an enclosing box, with the central point as tangent point.

The radial scale is r'(d) and the transverse scale is
r(d)/(R sin(d/R)) where R is the radius of the Earth.

Cylindrical

Cylindrical projections wrap a cylinder around the globe then unroll the cylinder to make a flat map. They consist of horizontal and vertical lines and, unlike azimuthal and conical projections, the whole world can be shown on a single map

Instead of a single standard parallel, some cylindrical projections cut the globe on two parallels — 45°N and 45°S, with the projection lines emanating from a source on the equator diametrically opposite the projection surface.

Between the two parallels, the scale is slightly reduced. To the north and south, the scale is increased till it reaches excessive values at the poles. The polar regions are also badly distorted in the east-west direction to maintain the parallel relationships of the meridians.

Conic

Conic projections transfer the geographic grid from a globe to a cone resting on the globe, then cut and unroll the cone to create a flat map

When the apex lies directly above a pole and the cone touches the globe along a single parallel, the projection is referred to as the perspective conic projection. The parallel that touches the cone is called the standard parallel. On this parallel the scale is exactly as stated for the map or chart and is the same as on the globe from which the projection was made. Everywhere else the scale will be larger on the map and increasing north and south from the standard parallel.

Some conic projections use two standard parallels representing two lines of intersection where the plane of projection cuts the globe. The resulting map has two parallels along which the scale is exactly the same as on the globe. The cone is called the secant cone. In a secant cone projection, scale increments north and south of the two parallels are reduced in proportion to the scale decreases between the parallels.

The position of the two standard parallels should be selected to minimize scale changes in the mid latitudes of a continent or country. The Lambert Conformal Conic projection, used in the US for aeronautical charts, is a perspective conic projection with two standard parallels and all other parallels adjusted so that the map has true conformal properties but also has the property that any line drawn on it is almost a great circle.

Polyconic projection maps are based on a number of cones, each centred on two standard parallels positioned at progressively higher latitudes

For example, USGS uses the polyconic net as a base for its topographic maps as well as various other maps in the United States.

One disadvantage of polyconic maps is that its meridians are curved inwards towards the top. Because of this curvature, adjoining maps, when trimmed along the bounding meridians, do not have an exact fit.

CARIS projections

Maps using the following projections can be used directly by CARIS programs:

Maps using the following projections must be transformed before they can be used by CARIS programs.

Canadian 1:50 000 NTS maps are usually based on the Transverse Mercator Projection in conjunction with the Universal Transverse Mercator Grid (UTM).

The United States Department of the Interior (Geological Survey) 1:24 000 (Quadrangle) maps are usually Polyconic. United States 1:500 000 Aeronautical charts as well as world 1:1 000 000 aeronautical charts are Lambert Conformal.