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1 Geographical Data

GIS is characterized by four main components (1):

• Input
• Administration
• Analysis
• Presentation

The discussed data types of this four-component model are classified in three categories such as geometry data, attribute data and graphics data with the following properties:

Geometry data describe the spatial situation of objects concerning their form and their relative situation in space. Usually, the spatial relation of individual points, lines or areas is made via the integration in a coordinate system resulting in the relation to the real world and the metrics. Geometry data can be available as raster data (pixel) or vector data (polygons / areas, lines, sites)(see Fig. 2).

Raster data are data continuously spread in space, which are structured in a measured matrix of usually quadratic cells and cells with the same size. Each cell gets an attribute (property, attribute date), which represents an appropriate phenomenon (e.g. temperature or color value). The storage of the cells is carried out by their coordinates. They are adjusted in rows and columns. The geometrical data are accessed via the geographical coordinates or by specifying the row and/or column. Working with raster data allows the application and analysis of remote sensing data such as color infrared images of aerial operations, satellite photographs/images and more. The necessary memory requirements and the appropriate high demand on computing resources (cpu time), which rise exponentially with the increase of resolution, is to be seen as a disadvantage of raster data. Due to steady increasing CPU-power and larger storage-capacities this disadvantage has become less important over the last few years. Additionally, raster data are not associated with so called neighborhood relationships because each pixel is defined by its own situation in the coordinate system.

Figure 2: Geometry and attribute data within Geographical Information Systems

Vector data are used for the storage of line information and/or for the storage of homogeneous areas at closed lines (polygons). One line connects two end points (nodes) each, which also have coordinates. Each vector object can be assigned with none, one or several attributes (property). For administation in GRASS the dBase data-format is used by default. Interfaces to different external DBMSs (Database Management System) like data bases e.g. PostgreSQL, MySQL, Oracle and so on are also available. In comparison to raster data, vector data are characterized by their comparatively low memory requirements and short computing times for conducted analyses. Unlike raster data, vector data have a topology, which means that the lines and areas "know" which nodes they possess and/or on which areas they border.

Point data (Sites) can be considered as a special form of vector data. They are used for saving selectively spread spatial information. This data type can be saved as vector sites in GRASS 6.0not in version 5.4. Accordingly, it has the properties of vector data.

Attribute data (categories) are attributes, which are interconnected with the data types mentioned above. They are mostly saved within GIS or in a database system coupled with GIS via DBMI (Database Management Interface).

Graphics data eventually describe the method how a spatial object is displayed under a certain topic on a certain output device (monitor, plotter, etc.).

In GIS, they are qualitatively and quantitatively set in relation as specimen via the combination of the described data types. These phenomenons and objects (entities) to be saved in a GIS occur in two basic structures:

• continuous appearances -> laminar and unlimited in space
• discrete appearances -> definable areas and objects like lines

When designing a GIS "correct" selection of the data structures to be used depends on the standard, the spatial resolution, the data quantity, the original data, the planned analysis and more.

Within GRASS GIS different modules allow conversion between the individual data structures. Contour lines can be stored as vector lines, e.g. in a laminar terrain model (in raster format). Similarly, a map consisting of digital contour lines (in vector format) can be transformed into a closed terrain surface in raster format by interpolation. In this case the quality of the conversion will depend on the resolution of the original data (see Fig. 3).

Figure 3: Comparison of raster and vector data types in an identical area