From Halopedia, the Halo wiki
Grid nineteen by thirty-seven is a Planetary Grid-designated location in the jungle wilderness of the human colony Sigma Octanus IV, just on the outskirts of the city of Côte d'Azur. It is covered by dense foliage but has enough of a clearing to support significant encampment. In 2552, during the Battle of Sigma Octanus IV, Grid 19 x 37 was the site of a firebase, Alpha Headquarters. This base was ambushed and destroyed by the Covenant, as thousands of Unggoy overwhelmed the unit located there.