3.1 GIS Subsystems
A GIS is composed of four basic subsystems: data input, data storage and retrieval,
data manipulation and analysis, and a reporting system [8]. The data input subsystem
involves the conversion of data from maps, field observations, processed satellite
images, and aerial photographs into compatible digital form. Data conversion
routines are available to scan or digitize analog maps for input to a GIS. The quality
of input data will influence the quality of GIS products regardless of the sophistication of its hardware and software. The main functions of the data storage and
retrieval subsystem are database organization, query, analysis, and reporting of the
attribute data linked to the features on the maps. This subsystem prepares data by
removing errors and analyzes the data to provide answers to the questions the user
puts to the GIS. Typical analyses include overlaying different thematic maps,
computing areas and distances, acquiring statistical information about the attributes,
projection of maps, and making three-dimensional perspective view plots using
elevation data. The data manipulation and analysis subsystem allows for data
conversion and to perform analytical and modeling functions on the map layers.
The reporting subsystem deals with the way the information is displayed to the user.
This can either be as a visual display or hard copy or as magnetically recorded or
printed information in digital form.
3.2 Data Models
Geographic data can be classified into two main classes: spatial data and attribute
data. Spatial data are represented in two basic ways, either as a raster or a vector. In a
raster or cell-based system, spatial data is represented by a geometric array of
rectangular or square cells, each with an assigned value. Raster data is typically
used for remote sensing data. In a vector-based system, spatial data is represented by
a set of straight-line segments called vectors. The coordinates at the end of each
vector segment are digitized and explicitly stored, and the connections are implied
through the organization of the points in the database. Relationships between
features are described with topological relationships that include area definition,
adjacency, connectivity, and nestedness. A newer, non-topological data structure is
the shapefile, which stores the geometry and attribute information for a geographic
feature in a dataset that includes a main file, an index file, and a database table
[8]. Time can also be considered a data element, since geographic information often
changes over time. For instance, a river course may meander over time, or river
dimensions may undergo sudden changes due to floods, and land use may change
due to agricultural or industrial use.
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A GIS is composed of four basic subsystems: data input, data storage and retrieval,
data manipulation and analysis, and a reporting system [8]. The data input subsystem
involves the conversion of data from maps, field observations, processed satellite
images, and aerial photographs into compatible digital form. Data conversion
routines are available to scan or digitize analog maps for input to a GIS. The quality
of input data will influence the quality of GIS products regardless of the sophistication of its hardware and software. The main functions of the data storage and
retrieval subsystem are database organization, query, analysis, and reporting of the
attribute data linked to the features on the maps. This subsystem prepares data by
removing errors and analyzes the data to provide answers to the questions the user
puts to the GIS. Typical analyses include overlaying different thematic maps,
computing areas and distances, acquiring statistical information about the attributes,
projection of maps, and making three-dimensional perspective view plots using
elevation data. The data manipulation and analysis subsystem allows for data
conversion and to perform analytical and modeling functions on the map layers.
The reporting subsystem deals with the way the information is displayed to the user.
This can either be as a visual display or hard copy or as magnetically recorded or
printed information in digital form.
3.2 Data Models
Geographic data can be classified into two main classes: spatial data and attribute
data. Spatial data are represented in two basic ways, either as a raster or a vector. In a
raster or cell-based system, spatial data is represented by a geometric array of
rectangular or square cells, each with an assigned value. Raster data is typically
used for remote sensing data. In a vector-based system, spatial data is represented by
a set of straight-line segments called vectors. The coordinates at the end of each
vector segment are digitized and explicitly stored, and the connections are implied
through the organization of the points in the database. Relationships between
features are described with topological relationships that include area definition,
adjacency, connectivity, and nestedness. A newer, non-topological data structure is
the shapefile, which stores the geometry and attribute information for a geographic
feature in a dataset that includes a main file, an index file, and a database table
[8]. Time can also be considered a data element, since geographic information often
changes over time. For instance, a river course may meander over time, or river
dimensions may undergo sudden changes due to floods, and land use may change
due to agricultural or industrial use.
204
S. O. Darkwah et al.