The GPROJECT procedure performs three different types of projection: Albers' equal-area projection with two standard parallels (the default method), Lambert's conformal projection with two standard parallels, or the gnomonic projection (an azimuthal equidistant projection).

The Albers' projection is a conic projection from the surface of the sphere to a cone secant to the sphere, cutting it at two standard parallels of latitude. The axis of the cone coincides with an extension of the polar axis of the sphere. Each section of the resulting map bears a constant ratio to the area of the sphere. In general, distortion in shape tends to increase toward the poles in latitudes outside of the two standard parallels.

Albers' Projection illustrates an Albers' equal-area projection of the northern hemisphere.(footnote1)

The Albers' projection is suitable for portraying areas of large and small east-to-west extent and produces satisfactory results in most cases. However, both standard parallels must lie on the same side of the equator, so this method might not be suitable for map data sets of large north-to-south extent that span the equator. For those map data sets, use the gnomonic projection method.

The Lambert's projection is obtained from a secant cone in the same manner as Albers' projection. In the Lambert's projection, meridians of longitude are straight lines that radiate from the apex of the cone, while parallels of latitude are concentric circles. The Lambert's projection is somewhat better than the Albers' projection at representing the original shape of projected unit areas, while the Albers' projection is somewhat better at representing relative sizes of projected unit areas.

Lambert's Projection illustrates a Lambert's conformal projection of Europe.

The Lambert's projection is ideal for navigational charts and maps of relatively small east-to-west extent. However, as in the Albers' projection, both standard parallels must lie on the same side of the equator, so this method might not be suitable for map data sets that span the equator. For those map data sets, use the gnomonic projection method.

The gnomonic projection is a planar projection from the surface of the sphere directly onto an imaginary plane tangent to the sphere at the map projection pole. By default, the projection pole is placed at the center of the map data set that is to be projected, but you can specify the projection pole to be anywhere on the surface of the sphere. (See the POLELAT=latitude POLELONG=longitude.)

Gnomonic Projection illustrates a gnomonic projection of Africa.

In the gnomonic projection, distortion increases as the distance from the map pole increases. Because of this distortion, the PROC GPROJECT procedure deletes all of the observations that lie more than 85 degrees from the map pole. The gnomonic projection is best suited for mapping areas of small east-to-west extent.