Overlay functions is one of the most frequently used functions in a GIS application. They combine two (or more) spatial data layers, comparing them position by position and treating areas of overlap - and of non-overlap - in distinct ways.
Standard overlay operators take two input data layers and assume that they are georeferenced in the same system and that they overlap in the study area. If either of these requirements is not met, the use of an overlay operator is pointless. The principle of spatial overlay is to compare the characteristics of the same location in both data layers and to produce a result for each location in the output data layer. The specific result to produce is determined by the user. It might involve a calculation or some other logical function to be applied to every area or location. With raster data, as we shall see, these comparisons are carried out between pairs of cells, one from each input raster. With vector data, the same principle of comparing locations applies but the underlying computations rely on determining the spatial intersections of features from each input layer.
Using overlay analysis, we can find:
those potato fields on clay soils (select the “potato” cover in the crop-data layer and the “clay” cover in the soil-data layer and perform an intersection of the two areas found);
those fields in which potato or maize is the crop (select both areas of “potato” and “maize” cover in the crop-data layer and determine their union);
those potato fields not on clay soils (use a difference operator of areas with “potato” cover with the areas having clay soil);
those fields that do not have potato as a crop (determine the complement of the potato areas).
Classify and explain spatial analysis functions (measurements, classification, overlay, neighbourhood and connectivity) in a raster and vector environment (level 1 and 2).