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GIS
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GIS Data: A GIS Is a Spatial Database
A GIS is a unique kind of database of the world—a geographic database. It is an "information system for geography."
Fundamentally, a GIS is based on a
structured database that describes the world in geographic terms.
Geographic Representations
As part of a GIS geo-database design, users specify how certain features will be represented.
 For example, parcels will typically be represented as polygons; streets will be mapped as centerlines, wells as points, and so on. These features are collected into feature classes in which each collection has a common geographic representation.
Each GIS data set provides a geographic representation of some aspect of the world including
- Ordered collections of vector-based features(sets of points, lines, and polygons)
- Raster data sets such as digital elevation models and imagery
- Networks
- Terrains and other surfaces
- Survey data sets
- Other geographic information such as addresses, place names, geo-processing models, and cartographic information
Descriptive Attributes
In addition to geographic representations, GIS data sets include traditional tabular attributes that describe the geographic objects. Many tables can be linked to the geographic objects by a common thread of fields (often called keys). These tabular information sets and relationships play a key role in GIS data models, just as they do in traditional database applications.
Spatial Relationships: Topologies and Networks
Spatial relationships such as topologies and networks are also crucial parts of a GIS database. Topology is employed to manage common boundaries between features, define and enforce data integrity rules, and support topological queries and navigation (determining feature adjacency and connectivity). Topology also is used to support sophisticated editing and construct features from unstructured geometry (constructing polygons from lines).
- Geographic features share geometry. Feature geometry can be described using relationships between nodes, edges, and faces.
Networks describe a connected graph of GIS objects that can be traversed. This is important for modeling pathways and navigation for transportation, pipelines, utilities, hydrology, and many other network-based applications.
- In this example network, street features represent edges that connect at their endpoints (called junctions). Turns model the movement for traveling from one edge to another.
Thematic Layers and Data Sets
GIS organizes geographic data into a series of thematic layers and tables. Since geographic data sets in a GIS are geo-referenced, they have real-world locations and overlay one another.
In a GIS, homogenous collections of geographic objects are organized into layers such as parcels, wells, buildings, imagery, and raster-based digital elevation models. Precisely defined geographic data sets are critical for useful geographic information systems, and the layer-based concept of thematic collections of information is critical for GIS data sets.
- GIS integrates many types of spatial data.
Data sets can represent
- Raw measurements (such as satellite imagery)
- Compiled and interpreted information
- Data that is derived through processing operations for analysis and modeling
Many of the spatial relationships between layers can be easily derived through their common geographic location. GIS manages simple data layers as generic GIS object classes and utilizes a rich collection of tools to work with the data layers to derive many key relationships.
Therefore, it is important for GIS data sets to be
- Simple to use and easy to understand
- Easily used with other geographic data sets
- Effectively compiled and validated
- Clearly documented for content, intended uses, and purposes
Any GIS database or file base will adhere to these common principles and concepts. Each GIS requires a mechanism for describing geographic data in these terms, along with a comprehensive set of tools to use and manage this information.
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