1. A method for reducing accumulated systematic displacement errors in an image-correlation-based displacement measuring system, comprising: determining at least one reference-class displacement between the two images of at least one corresponding reference-class image pair, the two images of a reference-class image pair comprising a first image and a second image, each reference class displacement being determined to a sub-pixel resolution; and determining an accumulated displacement based at least partially on the at least one reference-class displacement; wherein: determining the at least one reference-class displacement to a sub-pixel resolution comprises at least one of a) compensating for the apparent displacement between the two images of the at least one corresponding reference-class image pair based on a pre-determined error characteristic of the image-correlation-based displacement measuring system, b) acquiring the second image of the at least one corresponding reference-class image pair based on a prescribed displacement for the second image relative to the first image of the at least one corresponding reference-class image pair and c) selecting an image to be used as the second image of the at least one corresponding reference-class image pair from a set of at least two candidate images based on a prescribed displacement for the second image relative to the first image of the at least one corresponding reference-class image pair.