The Pierre Auger Observatory (PAO) [1] calls for two sites with multiple semi-autonomous detection systems. For example, the site currently under construction comprises 24 fluorescence telescopes pointing in different directions and 1600 surface particle detectors spaces 1.5 km apart. Each component, and in some cases each event, provides a preferred coordinate system for simulation and analysis. To avoid a proliferation of coordinate systems in the offline software [2] of the PAO, we have

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... oped a geometry package, implemented in C++, that allows the treatment of fundamental geometrical objects in a coordinateindependent way. This package makes transformations between coordinate systems transparent to the user. The geometry package allows easy combination of the results from different sub-detectors, at the same time as ensuring that effects like the Earth's curvature, which is nonnegligible on the scale of a single Auger site, are dealt with properly. The internal representations used are Cartesian. For interfacing, including I/O, the package includes support for Cartesian coordinates, geodetic (latitude/longitude and UTM), and astrophysical coordinate systems.