1. A computer-implemented method comprising: gridding a digital image by dividing the digital image into a plurality of regions; calculating a plurality of image metrics for each region of the gridded digital image; generating an image index file for each region, each image index file comprising the plurality of image metrics for the region of the gridded digital image that the image index file is generated for; storing the image index files in a database; determining similarity between a first gridded digital image and a second gridded digital image by comparing image index files associated with corresponding regions of the first gridded digital and second gridded digital images; assigning the image index files to a cluster; and comparing the image index files to a cluster identifier, wherein the comparing the image index files to a cluster identifier comprises: calculating similarity values by calculating a difference between each image metric of the image index and the cluster identifier; multiplying the respective similarity values by a weighting factor for each image metric of the image index; and summing the weighted similarity values.