geographic database

A GEOGRAPHIC database can be defined as a catalog that stores data that are spatially referenced. These databases are collections of similar data that are related either through location, data structure or type, or common underlying purpose.

Geographic databases can be divided into two main data concepts, graphic and nongraphic. Nongraphic data is data that does not “describe” or “define” graphic images or map features. There are four types of nongraphic data: attributes, geographically referenced data, spatial relationships, and geographic indexes. Generally, nongraphic data attributes describe particular map features or are linked to graphic elements through identifiers or geocodes. An example of an identifier or geocodes would be used to describe items or actions at a geographic location such as building permits, accident reports, or tax records. Nongraphic data within a geographic database can take the form of a geographic index or be used to describe a spatial relationship.

Examples of geographic indexes include street addresses, mailing addresses, parcel numbers, or account numbers. Spatial relationship examples include various topological relationships such as connectivity, adjacency, and proximity. All of these elements of nongraphic geographic data are stored in geographic databases as alphanumeric characters.

Graphic data in geographic databases consists of points, lines, polygons, and other map or cartographic features such as projections, coordinate systems, and cartographic symbols. Graphic data is commonly stored in the database in the form of coordinates, symbols, rules, or pixels. There are two methods in which graphic data can be stored within a geographic database: vector or raster. Vector data are denoted by coordinates (example, x and y or latitude and longitude) of nodes and lines or rules for connecting the various lines into areas. Vector data defines objects, polygons, and other involved units so that they can be displayed or analyzed based upon their associated attributes. Raster data sets are stored as a set of uniform grid cells that represent a continuous surface.

There are many different forms that data models or bases can take. Some of the more basic and widely usedones are hierarchical, sequential, and relational. Hierarchical data models can take the form of parent-child relationships or one-to-many relationships where, for the database to be successful, direct linkages between entries are necessary. Relational data models consist of relational tables where rows are records, columns define the attribute relation, and each cell has a particular attribute value.

Management of geographic databases is very important to help ensure the accuracy of the data, the integrity of the database, and the reliability of the analysis done using a particular database. Included in this management of the database is establishing data standards, definitions, quality, and maintenance schedules. Administrators of digital geographic databases are also charged with controlling access and use of the database, backing up the database and creating and maintaining a data directory or dictionary. The data directory, also referred to as metadata, is essential to the management of a geographic database. Information included in a data directory is the definition of the particular entities, attributes and valid values. Data directories also describe the various entities and attributes when necessary, along with describing the accuracy, completeness, or other data characteristics that are important the integrity of the database.