What do you mean by 'not regular'? Are the cells not all perfect squares, is the distribution of the cells not rectangular, or are there cases where the 3 x 3 groupings will leave left-overs? If the cells are squares and the distribution of squares lends itself to 3 x 3 groupings across the surface then... Use the create fishnet tool to generate a feature class of cells that are 1500m x 1500m. Then use a spatial join to populate the original cells with the OID of the 1500 x 1500 cells. Exactly, the cells are not perfect squares and their distribution is not rectangular, it is the Italy shape, so left-overs are likely to be created. Do you have any idea in this case?
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