From i° to the 3 + 1 description of spatial infinity

By carrying out a 3 + 1 decomposition of the spi framework, the expressions of the conserved quantities, defined at / °in terms of the Weyl curvature, are recast in terms of the initial data of the physical space-time. In particular, the analysis brings out the supertranslation ambiguities in the usual 3 + 1 definitions of angular momentum and clarifies, within the 3 + 1 framework, the meaning of the stronger boundary condition needed to remove these ambiguities. The discussion is so arranged that only a minimal acquaintance with the spi framework is necessary to appreciate these issues.