We have studied the structural properties of multiple layers of self-assembled Ge dots non-destructively by conventional X-ray diffraction and grazing incidence diffraction using reciprocal space mapping techniques. In particular, the influence of the thickness of the Si spacer between the individual dot layers on the strain relaxation in the dots and the lateral and vertical correlation of the dot positions was investigated. Whereas conventional X-ray diffraction averages over the whole multilayer stack, grazing incidence diffraction allows for a position-resolved determination of the strain relaxation.