Data layering is an essential feature of GIS software that allows arranging GIS data into data layers. Data are input themes such as soil, water, and forest based on the analysis requirement of the GIS application.
Different layers of data need to be superimposed to generate the layered data structure. Data layers are user defined and usually based on the requirement of analysis and the availability of data. For a GIS project, data layers or themes such as forest, watershed, and land must be identified in advance. Each layer of a GIS application or software must have both spatial data and attribute data associated with it. Loading a data layer in a GIS application is a complex procedure and involves distinct steps such as graphic conversion, editing, topological building, attribute conversion, linking, and verification. Hence, all data layers
cannot be loaded simultaneously. Layers need to be superimposed with some technique, and overlay is required here. The mathematical and logical approach to superimpose one data layer on another to produce new data layers is called overlay (Figure 3.7).
Topological overlaying is primarily regarded as overlaying polygon data into polygon data. However, combinations of points, lines, or polygons are other options of overlaying, which are also required.
Raster- and vector-based softwares differ in their approach to implement the overlay function.
Overlay for Vector Data
Vector data format represents spatial data using three distinct geometries—point, line, and polygon. To generate a new layer, one can combine all these geometries in different combinations. Overlaying does not involve only areas. Sometimes, areas are overlaid upon lines and points to find out, for instance, how many cities a river flows through or how many wells are situated within a village. Figure 3.8 shows some vector overlay operations involving polygons.
Overlay operations
Superimposing two data layers together to form a merged layer is only the first step in the overlay analysis. After new areas have been created out of the overlapping regions, the next task is to select only those areas that are of interest. For example, one might be interested only in areas where forestland is developed near a particular river watershed. In another analysis, one might be interested only in areas where agricultural land is developed on land not fertile enough for agricultural use.
A map layer involves different kinds of features. When performing an overlay analysis, it is often found convenient to group features into two classes—those satisfying certain conditions and those not satisfying the same conditions.
Figure 3.7 Overlay
Figure 3.8 Vector overlay operations
Union
Union overlay is the procedure to superimpose two polygons’ coverage input and have a union coverage, where the output coverage carries attributional data of both the polygons’ coverage (Figure 3.9). Union is equivalent to the OR operation of Boolean algebra.
Intersection
Intersect overlay is a spatial operation to overlay an input coverage that may be a line, point, or polygon with an intersect coverage, which is always a polygon, to produce an output coverage that carries information of only that portion of the input coverage that is common to the intersect coverage (Figure 3.10).
Figure 3.9 Union
Identity
Identity overlay carries all attributional data of the input coverage into the output coverage. An input coverage may be a point, line, or polygon.
For example, in the selection of a construction site layer, one is interested in lands suitable for construction. So he or she groups all other land use types into non-constructional lands. Similarly, one can identify agricultural and non-agricultural land in land layers. Figure 3.11 illustrates the selection of different combinations of land types might be useful for the analysis phase of this problem.
Overlay operators can be used for polygon overlay, line in polygon overlay, point in polygon overlay, and to identify overlapping lines.
Primarily, there are three basic types of overlays for vector data—point in polygon overlay, line in polygon overlay, and polygon in polygon overlay (Figure 3.12).
Figure 3.10 Intersection
Figure 3.11 Union and intersection overlay operations
• Point in polygon overlay: Point in polygon is a technique that overlays a point feature class into a polygon feature class. The topology of a point in polygon is an “is contained in” relationship.
Point topology is a new attribute of the polygon for each point.
• Line in polygon overlay: Line in polygon is a technique that overlays line coverage into polygon coverage. The topology of the Figure 3.12 Types of vector data overlay
line in polygon is an “is contained in” relationship. Line topology is the attribute of the old line ID and containing area ID.
• Polygon in polygon overlay: Polygon in polygon overlay is a spatial relationship that overlays one polygon coverage into another polygon coverage to generate a new polygon coverage. The spatial data and attribute data of both the polygon coverage are joined to reproduce new data relationships.
Raster-based Overlay Techniques
Raster overlay or map algebra is a technique to generate a new raster layer by applying mathematical operations on cell values of input raster layers of similar sizes. Map algebra is a simple and well-defined set-based algebra for manipulating geographic data, which was proposed by Dr Dana Tomlin in the early 1980s. It is an interdisciplinary approach that uses primitive operations of set theory in a GIS to produce a new raster layer (map) using algebraic operations such as addition and subtraction from two or more existing raster layers (maps) of similar dimensions. Map algebra provides an easy-to-use and powerful way to define geographic analyses as algebraic expressions. This allows users to take their real-world data and apply algebraic functions to derive new results.
In raster overlay, cell values of the resultant layer are obtained by some mathematical operations applied on cell values of input layers.
Raster-based overlay operations may be classified as follows.
• Arithmetic functions (+, – , ×, /): These are major operations related to the main branch of pure mathematics that studies operations and functions.
• Relational functions (<, >, = ): The relational operators compare two operands and determine the validity of a relationship.
• Logical operation (AND, OR, XOR, NOT): An instruction in which the quantity being operated on and the results of the operation can both have two values. Logical operations include AND, OR, NAND, XOR, and NOR.
• Conditional overlay (if, else, then): These all evaluate whether an ascertained condition has been met.
Examples of some overlay operations are as follows.