1. A system for detecting changes by comparing images which cover the same physical area but are collected at different times, the system comprising: at least one input for inputting at least one reference image and at least one test image of a target area; the reference and test images of the target area having signatures representing outstanding features; at least one processor operating to divide the at least one reference image into a plurality of reference subimages; the at least one processor operating to divide the test image into a plurality of test subimages; at least one memory for storing the at least one reference image and the corresponding reference subimages taken at or near the target area at various times, the at least one processor operating to create a sparse image representation of the target area from one of at least one reference image or reference subimages the at least one processor operating to compare the test image to the sparse image representation by matching signatures from the test image to the sparse image representation in order to align the images and detect changes between the test image and the sparse image representation.