Some basic GIS functions

• integrating maps based on different scales, map projections, or legends
• changing of scale, projections, legend, annotations, etc.
• overlaying different types of maps of a particular area to make a new map that combines the attributes of the individual maps. For example, a vegetation map could be overlaid on a soil map, as shown in Figure 3. This in turn could be overlaid on a map showing length of growing season, thereby producing a land suitability map for a given crop
• generating buffer or proximity zones around lines or polygons on a map. This technique is used to find areas within a given distance from roads, rivers, etc., or from certain thematic conditions. These buffer zones can in turn be used as another layer in overlay operations
• querying spatial and attibute databases

Simple illustration of the overlay function. A map with three polygons (areas) and 3 classes, viz. 1, 2 and 3 is overlayed with another map again with 3 polygons and 3 classes A, B and C. The resulting "overlay" layer has 8 polygons and the class names are: A1, A2, A3, B1, B2, B3, C2 and C3.

The main components of GIS

a) GIS computer hardware

The GIS has, in addition, several specialized hardware components, including: a digitizer or scanner, which is used to convert the geographical information from maps into digital form and send it to the computer; a plotter, which prints out the maps and other graphic outputs of the system; and a visual colour graphics workstation on which spatial data editing and display can be performed by the user.

b) GIS software

• data input: digitizing or scanning the lines on the maps and entering the attribute information from a keyboard
• data base management
• data analysis and processing
• interaction with the user (map editing)
• data output and presentation (plotting)

Many GISs today utilize a manual digitizing approach to input maps. This means that someone must sit down with the map at a large, flat, digitizing table, and using a small cursor pad, follow the thousands of lines that make up the map, carefully keeping the cursor (cross hairs) on the lines, ensuring that lines are not double digitized or left out, and that intersections are accurately closed and no gaps are left in lines. 

However, large cartographic data inputs are generally made using automated digitizing systems such as scanners. These eliminate the manual work of following the lines and ensure consistent, repeatable results each time a map is scanned. Although scanning is quicker than digitizing, only good quality maps can be scanned, and even then the quality of the products is generally not as high. However, as in most areas of computerization, the technology is continually being improved. Furthermore, once a map has been digitized, it can be reproduced and transformed at will (much as a written document can be quickly edited or corrected once it has been entered into a word processor). 

The quality of input data will affect the quality of GIS products regardless of the sophistication of its hardware and software. In many cases, inventories of natural resources are often not completed or up to date and information in maps may have to be revised before digitizing. 

Data base management  operations mainly consist of the following functions: structure, query, analysis and reporting of the attribute data linked to the features on the maps. 

Data processing covers two types of operation: firstly, preparing data by removing errors or updating, and secondly, analyzing data to provide answers to the questions the user puts to the GIS. Processing can operate on the spatial and non-spatial aspects of the data, or on both. Typical operations include overlaying different thematic maps, computing areas and distances, acquiring statistical information about the attributes, changing the legend, scale and projection of maps, and making three-dimensional perspective view plots using elevation data, as shown in the figure at right.

Data output and presentation deals with the way the information is displayed to the user. This can either be as a visual display (soft copy) or hard copy drawn by a plotter, or as magnetically recorded or printed information in digital form. The plotter is to the GIS what a printer is to the standard word processor: it produces a copy of  map on paper.

c) Human resources and organization

It may seem that GIS is the resource planner's crystal ball, but - as with any computer system - the information produced is only as good as the information that is put in. Incorrect or inadequate information fed into the GIS will produce incorrect or inadequate answers, no matter how refined or "user-friendly" the computer technology may be. 

As in any map-making operation, data collection and data input operations require high standards of design and work, intensive training and frequent monitoring for quality control. In other words, in addition to having the right hardware and software to do the job, effective utilization of a GIS requires adequate staff training as well as planning, organization and supervision in order to maintain the quality of the data and the integrity of the final product. 

Another essential element of successful GIS operation is the need for data input and processing to be a joint effort involving the computer specialist and the subject matter specialist (e.g. crop production, forest management, aquaculture). This ensures that the necessary specialized subject matter expertise is applied in the interpretation and evaluation of data. Specialists in remote sensing and cartography may also be involved.

In many developing countries, resource information collection and processing systems are still relatively undeveloped. This means that application of GIS at the country and subcountry level will, in many cases, need to be accompanied by the improvement of existing information collection systems and the introduction of new ones. This provides an opportunity for international assistance, and imposes on FAO and other technical assistance agencies an added reason to develop their own capabilities in GIS and related technologies in order to provide technical expertise at the national level. 

Data formats or models: Vectors and rasters

In a vector-based system, the line work is represented by a set of straight-line segments called vectors. The X,Y coordinates at the end of each vector segment are digitized and explicitly stored, and the connections are implied through the organization of the points in the database	In a raster or cell-based system, the map is represented by a geometric array of rectangular or square cells, each with an assigned value.

Most GIS have the ability to transform the data from one format to the other. The following figures illustrate vector to raster conversion:

Each of format has its advantages and disadvantages. Some advantages of a raster-based system are: 

< Back	Home	Forward >