1. A method for automatically generating large-scale tonally-balanced orthomosaic images, the method comprising the steps of: receiving, at a tone-balancing server computer comprising program code stored in a memory, a plurality of orthorectified input images associated with a geographic region; obtaining a reference image for the geographic region; for each orthorectified input image, performing the steps of: subdividing the orthorectified input image into a plurality of patches; computing a pixel-level tonal mapping function for each patch to match a corresponding set of pixels of the reference image; computing a first global tonal mapping function for each orthorectified input image by, for each pair of patches of the respective orthorectified input image, computing a function that smoothly connects the respective pixel-level tonal mapping function for the pair of patches; and applying the first global tonal mapping function to each pixel of the each orthorectified input image to create a plurality of tonally-balanced orthorectified output image; computing a second global tonal mapping reference by smoothly matching each first global tonal matching function for neighboring pairs of orthorectified output images; and applying the second global tonal matching function to each pixel of each tonally-balanced orthorectified output image to create a single new, global orthorectified tonally-balanced output image of the geographic region encompassed by the plurality of orthorectified input images.