Map overlay is the combination of several spatial datasets to create a new vector output dataset. You can visualize this by stacking several maps of the same area. Another way to visualize this is to think of the Venn diagram overlay used in mathematics. There are three types of overlays – union, symmetric difference, and intersect overlay – that are used for different reasons, which will not discussed here as they fall outside the scope of this book:
- Union overlay is to combine geographic features and attribute tables of both inputs and output it to a single output
- Intersect overlay is to define the area where both inputs overlap and retains a set of attribute fields for each
- Symmetric difference overlay is to define the output area that includes the total area of both inputs EXCEPT for the overlapping area
Data extraction in GIS is very similar to a vector overlay with the ability to be used in either vector or raster data analysis. Instead of combining properties and features of both datasets, extracting the data can involve using a clip or mask to extract certain features of one dataset that are relevant to the other dataset.
Raster data analysis uses local operation on multiple raster datasets or map algebra. Map algebra has a function that combines the values of each raster’s matrix and can be coupled with another function that can weigh some inputs higher than others through an index model that reflects the influence of various factors from within a geographic phenomenon.