Cartographer

The Cartographer, also known as Map Room, is a Forerunner structure on their various megastructures, such as the Halo Array, which features a schematic of the entire installation when activated.

The purpose of the Cartographer structure is to show a holographic map which can be used to navigate the massive expanse of area that a Halo is made of. These were most likely used by the Forerunners themselves when they would visit one of these installations for any such reason. These obviously aren't used by the Monitor of the installations itself as they have built-in mapping programmed. Almost every Forerunner structure encountered so far has one of these centers.

Installation 00
The Cartographer of Installation 00 was located on top of a massive spire structure, the map room itself lying in a wide open area giving a panoramic view over a massive waterfall. The map room is somewhat larger than Alpha Halo's Cartographer structure and also much more open to the outside. The Cartographer is a much bigger version of the one seen in Halo: Combat Evolved level, Silent Cartographer. There are many structural similarities between the two. They also play the same role, although each on a different scale.

The structure is explored in the Halo 3 level The Ark when the player(s) crosses through the Forerunner wall, following 343 Guilty Spark who hacks into the structure to allow him to pass through.

Installation 04
On Installation 04, the Cartographer was located in an immense underground facility, below a large island. It was protected by a security system accessible by another underground facility on the other side of the island. These facilities were a part of the installation's larger tunnel network, with a massive shaft leading to the depths of the megasturcture in between them. During the Battle of Installation 04, John-117 battled his way to the Cartographer in order to locate the installation's Control Room.

Onyx
Onyx also featured a map room, deep below the Forerunner city in what the Office of Naval Intelligence rererred to as Zone 67. Dr. Halsey used this Cartographer structure during the Battle of Onyx, when she was trying to find the way to the portal that led to the Shield World in the core of the artificial planet.

Trivia

 * The Cartographer/Silent Cartographer always seem to appear over a chasm.
 * Monitors only have built in map systems of their own installations, (eg. 343 Guilty Spark only has a map of Installation 04) which is why you have to visit a Cartographer on the Ark in Halo 3.
 * The multiplayer level Construct takes place on the cartographer building of Installation 00.
 * The waterfall in Installation 00's Cartographer could be leading to the cooling systems, or could just be a decoration.