Transform coordinates from a specified source to a specified target map projection. This uses the PROJ library directly, via the PROJ package. The reproj() function is generic, methods may be added to remove the need for an explicit source definition. This is for use primarily to transform generic data formats and direct leverage of the underlying PROJ library. (There are transformations that aren't possible with PROJ and that are provided by the GDAL library, a limitation which users of this package should be aware of.) The PROJ library is available at <https://proj.org/>.