Conceptual Model of Spatial Information:

Introduction:

GIS does not store a map in any conventional sense, nor it stores a particular image or view of geographic area. Instead, a GIS stores the data from which we can draw a desired view to suit a particular purpose known as geographic data. There are two types of data in GIS; spatial data and non-spatial data (Attribute data). Non-spatial data include information about the features. For example, name of roads, schools, forests etc., population or census data for the region concerned etc. Non-spatial or attribute data is that qualifies the spatial data. It describes some aspects of the spatial data, not specified by its geometry alone. A geographical information system essentially integrates the above two types of data and allows user to derive new data for planning.

Spatial models are important in that way in which information is represented affects the type of analysis that can be performed and the type of graphic display that can be performed and the type of analysis that can be obtained. In GIS systems there is a major distinction between what are usually referred to as vector GIS and raster GIS. These two approaches to spatial data processing, often to be found in the same GIS package, reflect two different methods of spatial modeling: the former focusing on discrete objects that are to be described, and the latter concerned primarily with recording what is to be found at a predetermined set of locations that may be grid cells or points.

Spatial Information:

 Spatial characteristics of information can be broadly distinguished between those that describe where things are, using locations consisting of reference positions, spatial units and spatial relationships; those that describe the form of phenomena, using qualitative and quantitative descriptions of shape and structure; and those that describe associations and interactions between different phenomena.

All geographical data can be reduced to three basic geographical phenomenon can in principle be represented by a point, line or area plus a label saying what it is. So an oil well could be represented by a point entity consisting of a XY coordinate; a road could be represented by a series of XY coordinates; a floodplain could be represented by an area entity covering a set of XY co-ordinates plus the label ‘floodplain’. The labels could be the actual names as given here, or they could be special symbols.

The essential features of any data storage system are that they should be able to allow data to be accessed- and cross-referenced quickly. There are several ways of achieving this, some of which are more efficient than others. Unfortunately, there seems to be no one ‘best’ method that can be used for all situations. This explains in part the massive investment in labour and money in effective database management systems, which are the computer programs that control data input, output, storage, and retrieval from a digital database.

 

Figure 1: Real world phenomena represented as three basic entities (Point, Line & Polygon)

Layers and Coverages:

The Common requirement to access data on the basis of one or more classes has resulted in several GIS employing organizational schemes, in which all data of a particular level of classification, such as roads, rivers or vegetation types, are grouped into layers or coverages (refer Figure 3.). GIS organize spatial data into layers. Typical layers represent information belonging to particular classes. The layers can be com- binned with each other in various ways to create new layers that are a function of the individual ones. Any layer does not contain any areal regions that are overlapping, therefore it is possible for each region to have multiple attributes corresponding to multiple perspectives on the meaning of that region.

 

Figure 2: Layers and Coverages

 Data Model:

 In order to represent the spatial information and their attributes, a data model – a set of logical definitions or rules for characterizing the geographical data is adopted. The data model represents the linkages between the real world domain of geographical data and the computer and GIS representation of these features. As a result, the data model, not only helps in organizing the real-world geographical features into a systematic storage/retrieval mechanism, but also helps in capturing the user’s perception of these features.