PostGIS Raster : Extending PostgreSQL for The Support of ... - CoDE

2.4.2 Data Theme
Figure 2.14: Multiple layers geographic representation.
Based on the data themes concept, geographic information in GIS could be partitioned into a series
of logical information layers rather than a random collection of objects. Hence, users can organize
information in various data themes that described the distribution of a phenomenon and how each
one should be represented across a geographic extent.
Many data themes are best represented by a single layer such as soil types or well locations. Other
themes like a transportation framework, are represented by multiple layers, where each feature (such
as streets, intersections, bridges, highway ramps, railroads and so on) is represented by a separate
layer.
Figure 2.15: Themes for a area [30].
Each GIS can contain multiple themes for a common geographic area. The collection of themes
acts as a stack of layers. Each theme can be managed as an information set independent of other
themes. Each one has its own representation and could be a collection of points, lines, polygons,
surfaces, rasters and so on. Because layers are spatially referenced, maybe they overlap one another
and can be displayed in a common map display. So GIS analysis operations like polygon intersection
can combine information between layers to discover and work with the derived spatial relationships.
The concept of layer-based data themes implies that:
• All layers must be georeferenced to a place on the Earth so that they can be combined together.
The georeferencing process is accomplished by defining the geographic coordinate system for
each dataset.
• Layers can be combined in many ways such as following the ordered layers in a map or by
employing operators or commands. Geographic operators can work with the relationships both
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