The Cartographer is one of many identically named map facilities on the Ark, updated in real time with the latest dynamic cartographic and diagnostic information. This includes security measures activated on the Ark, any required maintenance of the various Refugias, the structural integrity of the Ark's superstructure and its artificial sun, and the status of the Halo array and any new installations being forged in the Foundry.
In the wrong hands the Cartographer is an incredibly dangerous source of information, so it is hidden deep in a remote wasteland, heavily guarded by Sentinels and protected by huge defensive barriers. The Sentinels guarding the Cartographer are much more likely to attack interlopers on sight rather than wait for provocation and only someone with the appropriate command I clearance can access the Cartographer without a fight.