What is GIS

The graphical representation of spatial and attribute data in GIS software takes the form of either raster or vector graphics. The differences between raster and vector graphics, as detailed below, effect the level of detail, visual appeal, speed of manipulating graphics and data storage space required.

Aerial photographs and satellite images are generally in a raster format and are used in GIS to view a detailed map at a given extent or for the purpose of digitizing. Raster graphics are predominantly used to display spatial data and use a grid-type architecture in terms of storing spatial and graphic value data. Vector graphics are commonly used to represent features like roads, rivers, housing, and the like using points, lines and polygons. Based on scalable vector graphics, vector graphics provide a linear and detailed approach to manipulating attribute data. Raster and Vector graphics are frequently used together.

Differences between Raster & Vector Graphics

The key difference between Raster and Vector graphics is how they are structured. Raster graphics use pixels (“dots”) whereby a graphic is made up of a large number of pixels, each pixel having a location & colour value in a grid-like format. A vector graphic is rendered by a mathematical manipulation referenced by co-ordinates. Given the different structure of these graphic types, the following differences arise as a result:

Raster Structure

Vector Structure

1. Storage Space: Raster graphics require more storage space than vector graphics, as they store a location & colour value per pixel.

2. Detail: Raster images are more detailed within a given extent (“zoom”), however raster images become pixelated if too tight a zoom is applied. Vector images are less detailed, but maintain their original aesthetics regardless of extent or zoom.

3. Responsiveness: performance & responsiveness when manipulating vector image is faster than raster images, as the data structured used to render vectors is mathematically based whereas rasters requires the retrieval of individual pixel values and a manipulation of each pixel.

GIS Data is the key component of a GIS and has two general types: Spatial and Attribute data.

Spatial data are used to provide the visual representation of a geographic space and is stored as raster and vector types. Hence, this data is a combination of location data and a value data to render a map, for example.

Attribute data are descriptions, measurements, and/or classifications of geographic features in a map. Attribute data can be classified into 4 levels of measurement: nominal, ordinal, interval and ratio. The nominal level is the lowest level of measurement for distinguishing features quantitatively using type or class (e.g. tree species). Ordinal data are ranked into hierarchies but does not show any magnitude of difference (e.g. city hierarchy). The interval measurement indicates the distance between the ranks of measured elements, but a starting point is arbitrarily assigned (e.g. Celsius Temperature). Ratio measurements, the highest level of measurements, includes an absolute starting point. Data of this category include property value and distance.

Attribute data is the detailed data used in combination with spatial data to create a GIS. The more available and appropriate attribute data used with spatial data, the more complete a GIS is as a management reporting and analysis tool.

Sources of Spatial & Attribute Data

Spatial data can be obtained from satellite images or scanned maps and similar resources. This data can then be digitised into vector data or maintained as raster graphic data. Essentially, any format of a geographical image with location or co-ordinate points can be used as spatial data.

Attribute data can be obtained from a number of sources or data can be captured specifically for you application. Some popular sources of attribute data are from town planning and management departments, policing and fire departments, environmental groups, online media.

GIS Software works by combining a visual front-end, a database driven backend, incorporated with extensions that allow references between spatial and attribute data to support spatial analysis. But what doe sthat mean?

Considering the key component of spatial and attribute data first, the database portion of GIS software provides a store for spatial data to allow map rendering. In addition, further database or database table can be incorporated to provide attribute data, growing the scope of related data and allowing for more complex and original spatial analyses. Hence, the database back-end is a collection of spatial and attribute data using RDBMS like SQL Server or MySQL.

The visual front-end is like a reader that displays spatial and attribute data in layers or themes. Each theme represents a specific dataset, which can be toggled to customised what data is visual at any time. The front-end provides the user interface to perform data queries, directly manipulate the maps, perform spatial analysis and produce reports.
The GIS extension can be any collection of GIS specific tools or a GIS engine that provides the front- and back-end capability and GIS functionality. These extensions allow the wide scope of view, selection, editting and many more tools to manipulate your GIS data.

GIS Data & Software Structure

The structure of a GIS incorporating multiple data sources and GIS software would look be as follows:

The diagram could be an example of a GIS used to analyse the environmental impact of transport in an area relative to private economic activity. This is indicated by the sources of data for transport, environment, demographics, employment along with the spatial data.

The visual display of this data is via GIS software, the provides a layered approach to viewing data.

Most GIS take on this structure and it is mostly a question of what extent of data is appropriate for use.

View our comparison of some popular GIS software.

GIS software provides the functionality to create, manipulate, analyze and display maps and geographic data [learn more].

GIS software can be relatively simple or extremely complex and should be selected based on its appropriateness to your needs and cost-benefit analysis. Keep in mind your immediate as well as future GIS needs when making your GIS software decision.

Simple Mapping Options