# Glossary¶

Ballpark transformation

For a transformation between two geographic CRS, a ballpark transformation is a coordinate operation that only takes into account potential difference of axis orders (long-lat vs lat-long), units (degree vs grads) and prime meridian (Greenwich vs Paris/Rome/other historic prime meridians). It does not attempt any datum shift, hence the “ballpark” qualifier in its name. Its accuracy is unknown, and could lead in some cases to errors of a few hundreds of metres.

For a transformation between two vertical CRS or a vertical CRS and a geographic CRS, a ballpark transformation only takes into account potential different in units (e.g. metres vs feet). Its accuracy is unknown, and could lead in some cases to errors of a few tens of metres.

Note

The term “Ballpark transformation” is specific to PROJ.

Pseudocylindrical Projection

Pseudocylindrical projections have the mathematical characteristics of

\begin{align}\begin{aligned}x &= f(\lambda,\phi)\\y &= g(\phi)\end{aligned}\end{align}

where the parallels of latitude are straight lines, like cylindrical projections, but the meridians are curved toward the center as they depart from the equator. This is an effort to minimize the distortion of the polar regions inherent in the cylindrical projections.

Pseudocylindrical projections are almost exclusively used for small scale global displays and, except for the Sinusoidal projection, only derived for a spherical Earth. Because of the basic definition none of the pseudocylindrical projections are conformal but many are equal area.

To further reduce distortion, pseudocylindrical are often presented in interrupted form that are made by joining several regions with appropriate central meridians and false easting and clipping boundaries. Interrupted Homolosine constructions are suited for showing respective global land and oceanic regions, for example. To reduce the lateral size of the map, some uses remove an irregular, North-South strip of the mid-Atlantic region so that the western tip of Africa is plotted north of the eastern tip of South America.