A projection horizon defines the valid area of a particular map projection. Although these are called projection horizons, the valid area of use can change depending on the projection parameters and geographic coordinate system that is used. Because this is the case, a horizon is always created based on a projected coordinate system object. For some projections, the horizon includes the entire world. These include the Robinson, Albers conformal conic, and Azimuthal equidistant projections. Other projection horizons define an area of use that is less than the entire world because of limitations in the projection mathematics. The Transverse Mercator projection, also called the Gauss Kruger projection, is designed for large scale data. Its horizon is around 45 degrees in longitude from the central meridian of the projected coordinate system.

The projection engine can supply two types of projection horizons. The first is a geographic horizon. It defines the valid area of use for a projected coordinate system in latitude and longitude coordinates. The second type is a projected horizon. This is the valid area of use based on the coordinates of the projected coordinate system. The horizon's coordinates will use the same linear unit of measure as the source projected coordinate system.

Horizons can be inclusive or exclusive. Some projection horizons will include all but a small area. In those cases, it's easier to construct an exclusive horizon. Information on whether a horizon is inclusive or exclusive is contained within the horizon structure, PE_HORIZON. Horizons can also be single or multipart and consist of a rectangle (defined as an envelope) or a polygon.