Currently, the default behavior of the BPF writer is to default to a coord_id of 0 when none is provided. This triggers setting the coord_space to 0 also, which indicates a Cartesian coordinate system. While this is a valid possibility, it is relatively uncommon (at least IMO). This means that a simple pdal translate from LAS to BPF, even with known SRS in the LAS file, will result in a Cartesian output in the BPF. The BPF writer could instead use its existing auto capability to attempt to detect the correct SRS from the source.