Master Chief's map is a map that was created by Master Chief using the Halo 3 map, Avalanche. All of the spawn placements were in one part of the map, the weapons were all placed in an isolated area. The same applied for the vehicles and equipment. The remaining forge budget was spent on other random objects - all piled up into different areas.
After completing the map, Chief sent a message to all of his friends, telling them to have a look. One of them joined to have a look, and was amazed by its "greatness". Both Arbiter and Cortana saw the map for how bad it was.