The Plant Atlas grid square system is the same one used by the San Diego County Bird Atlas and is based upon the historic Public Land Survey that divided much of the United States into townships, ranges, and sections. A section is one mile square. Each township is divided into quadrants (four Atlas grid squares). Each Atlas grid square equals nine sections (three on a side) and is nine square miles in area.

Grid square ID is assigned by a simple index system (numbers 1 through 29 going from left to right across the top of the map, and letters A through W from top to bottom along the side of the map e.g., B5, L7, M28, etc.).