Thematic data layers can be overlaid to create a composite layer. The output layer contains either the minimum or the maximum class values of the input layers. For example, if an area was in class 5 in one layer, and in class 3 in another, and the maximum class value dominated, then the same area would be coded to class 5 in the output layer, as shown in the figure below.
The application example in in the figure above shows the result of combining two layers—slope and land use. The slope layer is first recoded to combine all steep slopes into one value. When overlaid with the land use layer, the highest data file values (the steep slopes) dominate in the output layer.
Use the Overlay (GIS Analysis) function in ERDAS IMAGINE Workspace or Spatial Modeler to overlay layers.