Chapter 5 GIS The Global Information System What is GIS? We have just discussed GPS a simple three letter acronym for a fairly sophisticated technique to locate a persons or objects position on the Earth s surface. Like GPS, GIS (which is short for Global Information System) is a simple three letter acronym but it is much more complex and complicated to define. In its simplest form a GIS combines layers of information about a place to give you a better understanding about that place. What layers of information you combine depends on your purpose finding the best location for a waste disposal site, analyzing environmental damage, viewing weather patterns in an area to predict rainfall, finding demographic data and so on. What do you need to use GIS? A full GIS, or geographic information system, requires: Hardware (computers and peripherals) Software Operators Figure 5.1 Combining layers in GIS (from GIS.com) and sound analysis methods for interpreting the results generated by the GIS. The GIS system links geographic information (where things are) with descriptive information (what things are). Unlike a flat paper map, where "what you see is what you get," a GIS can present many layers of different information. To use a paper map, all you do is unfold it. Spread out before you is a representation of cities and roads, mountains and rivers, railroads, and political boundaries. The cities are represented by dots or circles, the roads by black lines, the mountain peaks by tiny triangles, and the lakes by small blue areas similar to the real lakes. A digital map is not much more difficult to use than a paper map. As on the paper map, there are dots or points that represent features on the map such as cities, lines that represent features such as roads, and small areas that represent features such as lakes. All this information where the point is located, how long the road is, and even how many square miles a lake occupies is stored as layers in digital format as a pattern of ones and zeros in a computer. Think of this geographic data as layers of information underneath the computer screen. Each layer represents a particular theme or feature of the map. One theme could be made up of all the roads in an area. Another theme could represent all the lakes in the same Page 80
area. Yet another could represent all the cities. These themes can be laid on top of one another, creating a stack of information about the same geographic area. Each layer can be turned off and on, as if you were peeling a layer off the stack or placing it back on. You control the amount of information about an area that you want to see, at any time, on any specific map 1. Applications of GIS Geographic information systems are now used for land use planning, utilities management, ecosystems modeling, landscape assessment and planning, transportation and infrastructure planning, market analysis, visual impact analysis, facilities management, tax assessment, real estate analysis and many other applications. In this module we will focus on their environmental application but the environment is only a small area of the application of these types of systems. Some specific environmental examples are given below. Mapping Locations GIS can be used to map locations. GIS allows the creation of maps through automated mapping, data capture, and surveying analysis tools. Mapping Quantities People map quantities, like where the most and least are, to find places that meet their criteria and take action, or to see the relationships between places. This gives an additional level of information beyond simply mapping the locations of features. This type of information could be used to map the number of gravel roads in an area to estimate the amount of sediment that might be likely to be carried into a storm water drain Mapping Densities While you can see concentrations by simply mapping the locations of features, in areas with many features it may be difficult to see which areas have a higher concentration than others. A density map lets you measure the number of features using a uniform area unit, such as acres or square miles, so you can clearly see the distribution. Again this can be of great use in environmental planning as it allows quantitative estimates of how populations might affect environmental parameters. Finding Distances This is not the same function as using a GPS to find distances. Here the GIS might be used to find out what's occurring within a set distance of a feature. This could then be further used in environmental planning. Mapping and monitoring change GIS can be used to map the change in an area to anticipate future conditions, decide on a course of action, or to evaluate the results of an action or policy. The implications of this for the environment are fairly obvious. What Data is used for GIS applications This is again a very complex area as there are many different types of data that may be incorporated into a GIS database, but in general data for GIS applications includes: digitised and scanned data from many and diverse sources (examples might be digital photos) databases containing textual information (such as NPI or Census). These may be private data research organisations, of government authorities. Page 81
GPS field data and any associated information Internet data remote sensing and aerial photography information The Data used in GIS is said to be geospatial data in that it is linked to both a particular place and a specific theme. In other words the data has to be associated with both a location (special component almost always linked to a set of map co-ordinates and topographic information) and some form of variable attribute (sometimes called the thematic component). Computer assisted cartography packages can accurately manage the location of items (spatial analysis), statistical analysis can study the variable attributes (such as population growth etc.) a GIS is able to manage both. What are the advantages of using Geospatial Data? The advantages of digital versus analogue data are outlined in the table 5.1. Table 5.1 comparison of analogue and digital data (from Uni. Of Melbourne Introduction to GIS) Digital easy to update easy and quick transfer (e.g. via internet) storage space required is relatively small (digital devices) easy to maintain easy automated analysis Analogue whole map to be remade slow transfer (e.g. via post) large storage space required (e.g. traditional map libraries) paper maps disintegrate over time difficult and inaccurate to analyse (e.g. to measure areas and distances) Vector based GIS A vector based GIS represents geographic objects specifically (using spatial characteristics such as position and topography). The thematic aspects are then associated with these by the GIS. In other words it is a double database or a 3D database in that each point is associated with a whole set of information in another plane. There are different ways of organising this double data base (spatial and thematic). Usually, vectorial systems are composed of two components: the one that manages spatial data and the one that manages thematic data. A relational data base for the attributes is linked with a topological one for the spatial data. A key element in these kind of systems is the identifier of every object. This identifier is unique and different for each object and allows the system to connect both data bases. One advantage of this system, is the fact that points with no data are not counted saving space and speeding up information retrieval. Page 82
Raster based GIS Raster is a method for the storage, processing and display of spatial data. A map is divided into rows and columns, which form a regular grid structure. Each cell must be rectangular in shape, but not necessarily square. Each cell within this matrix contains location co-ordinates as well as an attribute value. The location of each cell defines its geographical position, unlike a vector structure which stores location data as well. Any data linked to a cell is in effect linked to a location. With the raster data model, spatial data is not continuous but divided into discrete units (like the cells of a spreadsheet). This makes raster data particularly suitable for certain types of spatial operation, for example overlays or area calculations. The disadvantage of raster structures may lead to increased storage in certain situations, since they store each cell in the matrix regardless of whether it is a feature or simply 'empty' space. The advantage of this system is that data entry may be easier and the database easier to set up. Raster GIS systems may also be compressed (just like you compress the images on your computer to transfer them over the internet). Grid Size and Resolution A pixel is the smallest sized dot that can be recognised in a display system. In raster GIS the pixel equivalent is usually referred to as a cell element or grid cell. Pixel/cell refers to the smallest unit of information available in an image or raster map. This is the smallest element of a display device that can be independently assigned attributes. The size of the pixel must be half of the smallest distance to be represented for the GIS to work properly. Table 5.2 lists the advantages and disadvantages of vector and raster GIS systems. Table 5.2 Comparison of advantages and disadvantages of raster and vector data models (from Uni. Of Melbourne Introduction to GIS) Raster Vector precision in graphics traditional cartography data volume topology computation update continuous space integration discontinuous Page 83
Rasterisation of Vector Data The process of converting vector data, which is a series of points, lines and polygons, into raster data, which is a series of cells each with a discrete value. This process is essentially easier than the reverse process, which is converting data from raster format to vector format. Raster to Vector Conversion The process of converting an image made up of raster cells into one described by vector data. This may or may not involve the encoding of topology. References 1. www.gis.com 2. www.sli.unimelb.edu.au/gisweb/ 3. Berhardsen, T. (1992) Geographic Information Systems. Viak IT/Norwegian Mapping Authority, Arendal, Norway. 4. Berhardsen, T. (1996) Geographic Information Systems. Halsted Press. 5. Bernhardsen, T. (1999) Geographic information systems : an introduction. Wiley, New York. 6. Chrisman, N.R. (1997) Exploring Geographic Information Systems. John Wiley and Sons. 7. demers, M.N. (1997) Fundamentals of Geographic Information Systems. John Wiley and Sons. 8. Huxhold, W.E. (1991) An Introduction to Urban Information Systems. New York, OUP. 9. Laurini, R. and Thompson, D. (1992) Fundamentals of Spatial Information Systems. London, Academy Press. 10. Maguire, D.J., Goodchild, M.F. and Rhind, D.W. (eds.) (1991) Geographical Information Systems: Principles and Applications. Avon, Longman Scientific and Technical. 11. Martin, D. (1991) Geographical Information Systems and their Socioeconomic Applications. London, Routledge. 12. Peuquet, D.J. and Marble, D.F. (eds.) (1990) Introductory Readings in Geographic Information Systems. London, Taylor and Francis. 13. Star, J. and Estes, J. (1990) Geographical Information Systems: An Introduction. Englewoods Cliffs, New Jersey, Prentice Hall. Page 84