The Digital Orthophoto Index layer is a 4,000 by 4,000 meter grid in MA State Plane coordinates which serves as an index for the digital orthophotos. The MA State Plane projection splits the State into Mainland and Island Zones, and the orthophoto images as produced are referenced to the correct Zone. Thus, two orthophoto grid templates exist for the Commonwealth. This represents a change from other MassGIS datalayers which are registered to the Mainland Zone only. This layer represents the Massachusetts mainland.The naming convention for each grid cell is based on the NAD83 meters coordinate pair for the lower right corner of the cell. The Grid ID is composed of the first 3 digits of each coordinate in this xy pair. For example, a grid cell with xy coordinates of 253000m 942000m at the lower right corner would have an ID of 253942. All datalayers registered to these index covers will have the grid number ID as part of their name.