Structural constraints define criteria that describe spatial and semantic properties of the data. Spatial structural constraints
deal mainly with the preservation of typical shapes (on the patch level) or with the preservation of patterns and
alignments if multiple patches are involved. Semantic structural constraints deal with the preservation of the logical
context of patches. For these constraints auxiliary data such as road and river networks or terrain models are necessary
as well as heuristics and domain knowledge about the nature of the data being generalized.