Elshayal Smart Web On Line Software© The First Arabian GIS software©

Elshayal Smart
The First Arabian GIS Software©

What is a Geographic Information System?

Definition

A Geographic Information System (GIS) is a computer program for storing, retrieving, analysing, and displaying cartographic data.

In a GIS, the Earth's features are not only represented in pictorial form, as in conventional paper maps, but as information or data. This data contains all the spatial information of conventional maps, but when stored in a computer, is much more flexible in the way in can be represented. Spatial data in a GIS can be displayed just like a paper map with roads, rivers, vegetation and other features represented as lines on a map complete with legend, border and titles, or it can be represented as a set of statistical tables, which can be converted to charts and graphs. The most important feature of GIS is that spatial data are stored in a structured format referred to as a spatial data base. The way spatial data are structured will determine the how easy it is for the user to store, retrieve and analyze the information.

Thematic Mapping

Maps in Geographic Information Systems are represented thematically. A standard topographic map will show roads, rivers, contour elevations, vegetation, human settlement patterns and other features on a single map sheet. In a GIS these features are categorized separately and stored in different map themes or overlays. For example, roads will be stored in a separate overlay. Likewise, rivers and streams will each be stored as a separate theme. This way of organizing data in the GIS makes maps much more flexible to use since these themes can be combined in any manner that is useful. The following illustration shows conceptually how maps are stored as themes in a GIS.

How information is stored in a GIS

Each different theme is stored on a separate overlay. The overlays on the left represent a vector based GIS, where the information is stored as a series of points, lines and polygons. The overlays on the right represent a raster based GIS, where the information is stored as a series of discrete units called cells.

Geometric classification of spatial information

In addition to organising spatial data by themes, mapped information is also structured as points, lines and polygons.

Point Data

Examples of point data include location of wells, post office, man holes, stream gauges, bird nesting sites or control points.

Line Data

Examples of line data include road networks, utility lines, stream drainages, and fault lines.

Polygon Data

Examples of polygon data include land use, vegetation cover, electoral districts, soil types, and zoning.

Textual data base

Besides the spatial information in a map, the GIS can usually store non-spatial information which is related to the spatial entities. For instance an urban GIS database may have a map theme of property boundaries. Attached to each parcel will be a textual database which might store the name of the owner, the address, the assessed value of the property, or the type of services and utilities on the site.

Querying the GIS

The GIS stores both spatial and non-spatial data in a database system which links the two types of data to provide flexible and powerful ways of querying or asking questions about the data. An example of a spatial query might look like this:

'Locate and display all playgrounds downstream of landfills within 100 year floodplain'

This type of query is answered by a set of commands to the GIS that then generates a map display of all sites meeting the criteria expressed in the query. The user may also query the GIS by the textual attributes in the tabular database and then display the map features which correspond to these attributes. An example of this type of query is as follows:

'Display all water mains installed before 1950 with a diameter less than 12 inches'

This query results in a map display of the water mains in the study area with the specific mains in the query highlighted. Alternatively a report could be generated which lists the complete information on each segment of watermains which meet the criteria in the query.