Converting egocentric sensory input into an allocentric map is a foundational operation in spatial computation, and the retrosplenial cortex has long been proposed as the hub where it occurs — yet the regional and projection-specific organization of that function has remained undefined. Using two-photon calcium imaging in freely navigating mice, this study demonstrates that the transformation is distributed along the anteroposterior axis. Anterior RSC was dominated by egocentric boundary vector coding, whereas posterior RSC exhibited enhanced allocentric boundary representation, with posterior boundary-proximal cells encoding the identity of a specific boundary rather than distance alone. Posterior RSC contained proportionally fewer spatially tuned neurons but showed markedly greater conjunctive coding, and generalized linear modelling localized the mechanism: allocentric head-direction information converges onto posterior egocentric boundary vector cells, generating specialized conjunctive neurons with higher mean vector length and tuning stability than either pure population within the same subregion. Recording the RSC→MEC projection population revealed that this pathway is selectively enriched for those conjunctive, globally tuned neurons, supplying the medial entorhinal cortex with an integrated, world-referenced spatial signal rather than an assembly of precise local ones. The work rests on miniaturized two-photon calcium imaging (mTPM V2.0.1 with LF headpiece,  Transcend Vivoscope Biotech Co., Ltd.) at 920 nm and collected ~9 Hz, delivered through an implanted GRIN lens and prism, and recorded alongside hardware-synchronized behavioral video. Following 2,717 neurons across three imaging populations in freely moving animals is what allowed the authors to localize a coordinate transformation to a defined subregion and a defined projection pathway.