GIS Geographical Information Systems

Transcription

1 Chapter Two GIS Geographical Information Systems

2 Chapter Two GIS Geographical Information Systems

3 Chapter Two GIS (Geographical Information Systems) 2.1 Introduction In most cases of our day life, information we use has geographic context as well as identity and value. For example, when we remember our school in earlier days, we generally remember its name and location. The homes of friends, our places of work also have similar links between geography and other information. These relationships are so common that we are rarely aware of them. With few places, it's fairly easy to keep track of the connections between place and information. But with a lot of information, and a lot of places, we can no longer keep track of it all. And this task become more complicated if we added changes over time, say, changes in locations, names, or what you find there. There is a limit to how large a scale to which one can increase this. At some point, the amount of information is overwhelming. It's not very "scalable." To manage this overload of information, the scientists invented a very special tool. That is THE MAP. Maps contain representations of locations, notations of identity, and values (road type, city size, topography, and so on). Maps, in general, work very well. But because they are static, they become obsolete in most 22

4 cases. In the past, scientists have gotten around this, to some extent, by having tables of information to go with maps. The information on these blotters, logs and spreadsheets can be updated frequently. A linkage is maintained by having a unique name on the map match a corresponding unique name in the records. Maps plus information listings is really quite good system. But there are a couple of limitations. The changes in either one must be reflected in the other. Linkages that misplaced or misread separate the map and table of records from each other, rendering that portion useless. For that reason, the information and linkages must be carefully maintained. At the same time, given their complexity, this system is subject to transcription errors. This can be controlled, to some extend, with proper quality control; but with the increase of the system's size. Here again, we run into the problem of scalability. A location is attached to many bits of information, or too much geography connected to a set of information, and it all becomes indecipherable. Thousands of areas, names and points on a set of maps along with hundreds of pages of data, cannot be easily translated into something meaningful. We can either no longer see patterns, or spend far too much time translating and collating what we are reading. One perfect solution is to automate the link between the map and the information In order to process and track much larger quantities of geographic information, and this what Geographical Information System (GIS) software do. 23

5 It is true that GIS software shares many similarities with Computer- Aided Design [& Drafting] (CADD), but it is important to understand how they differ. GIS can be described as a computer-based information system that inputs, stores, retrieves, displays and analyzes geo-referenced data. CADD, by contrast, was developed for engineering design functions. Even as CADD data may be spatially referenced, it cannot perform spatial analysis. Also CADD systems do not usually link their graphic data to databases. So while the CADD system may display the correct position of the utility pole and its connection to the next one, it will not connect to a pole database providing information on the pole's height, year of placement, material, maintenance records, or closest transformer. Dr. Michael McNerney, associate professor at the University of Texas Center for Transportation Research, further divides these systems into three categories: CADD, Smart CADD and GIS. (63) > CADD is an excellent tool for engineering and design, and can accurately place objects in their correct relative positions. > Smart CADD has all the functionality of CADD plus a link to a database. > GIS is distinguished from Smart CADD by its capability of performing complex spatial analyses. 2.2 Definition of GIS There have been so many attempts to define GIS that it is difficult to select any one definition (78). Maguire (1991) (68) offers a list of 11 24

6 different definitions. This variety can be explained, as Pickles (1995) (84) suggests, by the fact that any definition of GIS will depend on who is giving it, and their background and viewpoint.(48) > GIS is an information system that is designed to work with data referenced by spatial or geographic coordinates. (52) > GIS is a powerful set of tools for collecting, storing, retrieving at will, transforming and displaying spatial data from the real world for a particular set of purposes. (26) > GIS is automated systems for the capture, storage, retrieval, analysis, and display of spatial data. (30) > GIS is a system of hardware, software, and procedures designed to support the capture, management, manipulation, analysis, modeling and display of spatially referenced data for solving complex planning and management problems. (31) > GIS is an integrated package for the input, storage, analysis, and output of spatial information... analysis being the most significant. (40) > GIS is simultaneously the telescope, the microscope, the computer, and the Xerox machine of regional analysis and synthesis of spatial data. > GIS are decision support computer based systems for collecting, storing, presenting and analyzing geographical spatial information. (85). 25

7 2.3 How does a GIS work? GIS can use data collected from many different sources, and stored in different forms, because of this capability of GIS it can be very helpful to companying all those data as layers to create one huge database system which capable of performing complex analysis (see Figure 4). Source: Thailand Database Research Archive Figure 4: Layers of GIS With this layers structure, GIS can perform different analyses and queries to produce very valuable information that cannot be obtained in any other way. For example, while maps can answer questions like where a particular cricket stadium located? GIS, with proper data layers can answer questions like where this cricket stadium located? In which year was built? What its capacity? Is there a match going on now? What matches are scheduled? And so on. 26

8 2.3.1 Data Capture Data capturing process refers to preparing the data to be in a format that computers and GIS software can understand. For this data must be in a digital format. If the data to be used are not already in a digital format, there are various techniques and technologies available to capture the visible features. The simplest, technologically, is using the computer mouse to hand-tracing the paper map which so-called digitizing. We can also use electronic scanning devices to convert lines and points in a map to vectors of digits. As an attempt to minimize the cost of this process, an automatic ways are applied and proved to be effective and accurate only if the number and type of features are limited, and area contrasts are clear. Because data capturing - using digitizers and scanners - is time consuming. New ways that saves time and costs are widely used in recent years that is capturing data through remote sensing from satellites, and using Global Positioning Systems (GPS) Data Integration Integrating data from different sources and different formats is a very complex task, but GIS makes this task possible. Thus, a GIS can use combinations of mapped variables to build and analyze new variables. For example, while a CAD may represent a road as a line, GIS system may also recognize that road as the border between wetland and urban 27

9 development, or as the link between Main Street and Substreets. GIS technology can use the database of the water company billing information as a layer, and this make it possible to simulate the discharge of supplies in the septic systems in a region upstream from a wetland. The bills show how much water is used at each address. The amount of water a customer uses will approximately predict the amount of material that will be discharged into the septic systems, so that areas of heavy septic discharge can be located using a GIS Projection When the interest in using GIS is mapmaking, then projection process is needed to transfer information from the Earth's three-dimensional curved surface to a two-dimensional medium, paper or a computer screen. Different projections are used for different types of maps because each projection is particularly suitable for certain uses. For example, a projection that accurately represents the shapes of the continents will distort their relative sizes. Since much of the information in a GIS comes from existing maps, GIS uses the processing power of the computer to transform digital information, gathered from different sources with different projections to a common projection Data Formats in GIS There are two data formats in GIS, Vector format data and raster 28

10 format data. And since digital data are collected and stored in various ways, it is common to find two or more data sources may not be entirely compatible. Because of this GIS must be able to convert data from one format to another. Vector format data are more suitable for land use map and roads, where GIS reads this format as it is captured and stored as points, lines (a series of point coordinates), or areas (shapes bounded by lines), see Figure 5a. Typical example of vector format would be the property boundaries for a housing sector. On the other hand, raster format data are more suitable for land cover classification, because it is captured and stored as rows of uniform cells coded according to data values, see Figure 5b. While it is true to say that the computer can read and process raster data files faster, it is also important to note that raster files are often less detailed and may be less visually attractive than vector data files, which can be fairly accurate and have the appearance of more traditional handdrafted maps. :c ill o ^ a. Vector Representation b. Raster Representation Figure 5: Vector and Raster format data 29

11 A GIS can be used to convert one format to another. For example, GIS can be used to convert raster format to vector format by generating lines around all pixels that belongs to the same class Data Modeling GIS can be used to show two-dimensional and three-dimensional characteristics of the Earth's surface, subsurface, and atmosphere from information points. For example, generating a map for rainfall is fairly quick and easy when using GIS. Such a map can be thought of as a rainfall contour map. This contour map can be used as a layer that can be overly other layers for further analyses. Many complicated methods can estimate the characteristics of surfaces from a limited number of point measurements Unique Abilities of GIS GIS technology has unique abilities and characteristics such as: Queries and Retrieving Information By pointing to a location, object, or area on the screen, GIS can retrieve recorded information about it from off-screen files. Moreover, with overlaying aerial photographs as a visual guide, Boolean Logic in GIS (AND, OR, NOT) can be used to ask GIS about the geology or hydrology of the area or even about how close a location is to another. This kind of analytic function in GIS is very helpful for decision makers to 30

12 obtain the optimal decision Topological modeling For mapped phenomena, GIS can identify and investigate the spatial relationships among it. For example, GIS can give answers to queries like what is next to what? What is enclosed by what? And how close something is to something else? Networks In a linear network, A GIS can simulate the route of materials in this network so that it becomes possible to assign values such as direction and speed to the digital stream and move the contaminants through the stream system Topological Map Overlay Within a GIS it is possible to overlay two or more map layers, to create one or more new map layers or new information. Overlay operation can be achieved in a stepwise fashion by combining two or more layers to form an intermediate layer. This new layer can be combined with any other layers to form new intermediate layers. This overlay process can be repeated till we achieve the desired map Data output GIS technology has utilized the great developments and tremendous 31

13 improvements in computer graphics and attained high speeds in analyzing data and producing outputs that have very interesting appearances, and are reasonably easy to understand either as static or interactive maps, or simply as tabular and graphic reports. 2.4 GIS through history The simplest form for modern GIS can be described as follows: it is a graphic tool linked to a database. Based on this description, we can say that the history of GIS goes back to years ago. At that early time, on the walls of caves near Lascaux, France, Cro-Magnon hunters drew pictures of the animals they hunted, and those pictures are associated with track lines and tallies, as thought by researchers, to show migration routes and the numbers of animals (81). Today, scientists are using GIS technology to protect endangered species of animals and birds by tracking their migration routes through satellites in the form of satellite images. Modern GIS history can be summarized as having developed through three generations (81): First Generation: 1970's. Though the theory of modern GIS was first proposed in the 1960's, the real history of GIS started in the 1970's when computer speed increased, automated drawing improved, and Digital Terrain Modeling (DTM) was first presented. Second Generation: 1980's. Research and development about GIS much progressed, which created GIS venture businesses and software 32

14 vendors such as ESRI, Intergraph, Map Info etc. Geographic Information System (GIS) was upgraded to Geo-Information Science or Geoinformatics/Geomatics. Third Generation: 1990's to present. PC based GIS, particularly web GIS, was developed during this period. Standardization together with the concept of clearinghouse has become an important issue to promote GIS. ISO TC 211 has been established to goal the standard format GIS data or interoperability of GIS software. For more details refer to (81). 2.5 The Future of GIS To understand and predict the future of GIS, we must begin by finding out who can benefit from it. Here it is clear that many disciplines - especially areas where complex analyses are needed and critical decisions are to be taken - can benefit from GIS techniques. An active GIS market has resulted in lowered costs and continuous improvements in the hardware and software components of GIS. These developments, in turn, will result in a much wider application of the technology throughout governments, business sector, and industries. Some important horizons of GIS could be the following: Understanding the Process of Global Changes The continuous improvements in GIS technologies along with continuous improvements in computer hardware make it possible and perhaps easier than ever before to monitor and analyze the changes for the 33

15 whole planet of Earth. With this capability, humans can discover Earth resources and use them more efficiently. GIS technology, through its potential for monitoring global changes, can also help us recognize the environmental consequences of human developmental activities and, with proper actions, mankind can avoid deadly environmental and ecological consequences Adding the Time Factor Adding the time factor to the global mentoring enables scientists and researchers to examine the variations in Earth processes over days, months, and years. And the results of such an examination could be better knowledge of the condition of the Earth's surface and atmosphere (see Figure 6). Source: USGS Poster Figure 6: One time slice of the vegetation index 34

16 (a) for part of the globe from AVHRR data. The Advanced Very High Resolution Radiometer (AVHRR) sensor is capable to detect the amounts of energy that reflected from the Earth's surface through different channels (bands) of the spectrum for surface areas, see Figure 7 and Figure 8. AVHRR is only one of many sensor systems used for Earth surface analysis. More sensors, for sure, will follow, generating ever-greater amounts of data. With this large volume of data, only GIS technology is capable to deal with in terms of storing, retrieving, analyzing, and producing very valuable information, which allow for better understanding of global processes and better management of human activities across the world economics GIS over the Internet The World Wide Web (WWW) has witnessed, in the last few years, more and more web sites that provide access to GIS servers for specialists and for the common public as well. With those GIS web servers along with user-friendly Graphical User Interface (GUI) on the Net, users can submit GIS queries or commands and get the response from the server machines. This is resulting in an increase in the amount of use of GIS technology as well as an improvement in the quality of use of this tehnology over the last few years. Using a GIS service over the Internet is no more a fancy or academic activity, but it has become a down-to-earth routine governmental, professional, or corporate activity. 35