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Unformatted text preview: EDITORIAL BASICS OF GEOGRAPHIC INFORMATION SYSTEMS (GIS) Introduction History When geographic information systems (GIS) was intro- duced in the 1950s, its early use was limited to a small group of researchers. Botanists, meteorologists, and transportation planners began automating the process of thematic mapping. These researchers’ efforts represent the early attempts at com- puterized cartography. Today GIS is one of the fastest growing technologies. It is estimated that GIS will be a $4 billion industry this year. GIS has emerged as a powerful and sophisticated means to manage vast amounts of geographic data. This growth of GIS over the last 30 years can clearly be linked to technological advance- ments in the computer, digitizers, and plotters, coupled with an increasing demand by interested parties for geographic in- formation. The spatial organization, manipulation, and analysis of ge- ographic data are extending the analytical reach of organiza- tions around the world. The use of geographic information tools has received overwhelming acceptance by local, state, and federal government organizations as well as the private sector. This technology is enabling organizations to consider more effective ways of doing business, and in doing so, re- ducing costs and increasing productivity. GIS will soon be- come as widely used as spreadsheet software. Definition GIS has grown out of a number of technologies, including cartography, information management, computer science, pho- togrammetry, and remote sensing. Advancements made in these fields correspond to advancements in GIS. This tech- nology, therefore, consists of computer software and hardware designed to organize spatial data for analysis, assessment, and cartographic depiction. It provides a mechanism by which in- formation on a feature’s location, spatial interaction, and ge- ographic relationship can be assessed and viewed in moments. It provides an opportunity to efficiently view and access ge- ographic data to improve the decision-making process. Because of the very nature of GIS and the rapid growth of associated disciplines, many definitions of this technology ex- ist. The following is a useful definition because it addresses functionality as well as components: An organized collection of computer hardware, software, geographic data and personnel designed to efficiently cap- ture, store, update, manipulate, analyze, and display all forms of geographically referenced information. Others have attempted to use the name itself to better un- derstand the functions and components of GIS. GIS can be viewed in this way: 1. Geographic: The system is concerned with data relating to geography and geographic scales of measurement. This is referenced by some coordinate system to loca- tions on the surface of the earth. 2. Information: The system allows for the storage and ex- traction of specific and meaningful attribute information. These data are connected to some geography, and are organized around a model of the real world. Spatial and aspatial queries are made possible. 3. System: An automated system should include an inte- grated set of procedures for the input, storage, manipu- lation, and output of geographic information. GIS relies on the integration of three areas of computer technology. A relational database management system to store graphic and nongraphic data; cartographic capabilities to de- pict, graph, and plot geographic information; and spatial ana- lytical capabilities to facilitate manipulation and spatial anal— ysis. Three distinct areas: - Graphic capabilities - Relational database - Spatial analysis How to Conceptualize GIS One of the world’s leading GIS software vendors organizes data in such a way that they can be envisioned as digital layers or coverages of information. Each coverage is registered to the same common map base; each has a distinct type of feature, points, lines, or polygons. The GIS stores the spatial data (lo- cation information—where something exists on the earth’s surface) and attribute data (characteristics of the feature; e.g., pavement condition). A coverage represents a single theme, such as soils (polygon), streams (line), roads (line), land-use (polygon), and wells (point) (Fig. 1). ifli Land Use - polygon Waterbodles - polygon Streams - arc Roads - arc - Bulldlngs - point FIG. 1. Spatlal Data JOURNAL OF COMPUTING IN CIVIL ENGINEERING /JANUARY 1998/1 Maps and Map Data Handling Basic Map Concepts Maps are simplifications of the real world and are, therefore, models of reality. Cartographers develop maps using a set of rigid rules and guidelines. Absolute geographic locations are specified using a coordinate system such as latitude/longitude or universal transverse mercator (UTM) coordinates. The most familiar location reference system is that of the latitude and longitude. This system can be used to identify the locations of points anywhere on the earth’s surface. Latitude and longitude are angles measured from the earth’s center to a point on the earth’s surface. Latitude is measured north and south, while longitude is measured east and west. Both are traditionally measured in degrees, minutes, and sec- onds (DMS). Latitude/longitude, however, is a geographic ref- erence system, not a two-dimensional Cartesian coordinate system or Planer system. Flat maps with a Cartesian coordinate system are essential to a GIS. Therefore, because the early is a spheroid, a mathematical conversion is used to create a flat map. This mathematical conversion is commonly referred to as a map projection. Each coordinate system used is based on a particular map projection. These coordinate systems allow both the mapmaker and the map user to specify and define position for every location on both the earth and maps of the earth. Coordinate Systems are the x, y location in a Cartesian co- ordinate system. Coordinates are used to represent locations on the earth’s surface relative to other locations. A projection is a mathematical model that transforms the locations of features on the earth’s surface to locations on a two—dimensional surface. Some map projections preserve the integrity of shape; others preserve accuracy of area, distance, or direction. A scale is the mathematical relationship of real earth dis- tance to that same distance as it is shown on a map. The relationship is stated as the ratio of two distances. Maps are models of the real world and, therefore, the ratio of ground to map distance is normally much less than one. Maps stored in a GIS must be similar to scale if they are to be manipulated together. Maps with large differences in scale (e.g., 1:400 vs. 1:100,000) cannot be registered and overlaid without serious distortion. Map Features within a GIS The following is a list of the three features used in a vector database model: 1. Point feature: A single x, y coordinate that represents a geographic feature too small to be depicted as a line or an area. . 1 Pomts 0 Also referred to as nodes Represented by a single coordinate, no extent or area Attributes linked to point 1]) Examples include: Bus stop locations Bridges Signs Single residences Endangered Species Location 2. Line feature: A set of ordered coordinates that represent the shape of a geographic feature too narrow to be dis- played as an area. 2/JOURNAL OF COMPUTING IN CIVIL ENGINEERING /JANUAFIY 1998 Line / Arc Also referred to as arcs or segments Linear features. Length, no area Attribute linked to ARC ID Examples include: Roads Streams Power lines 3. Area feature: An area feature is a closed figure (series of arcs comprising its boundary) whose boundary encloses a homogenous area, such as a state or water body. Polygons 2 Also referred to as areas or complex shapes Features attached to polygon Area and perimeter Attributes linked to polygon ID Examples include: Political boundaries Zip codes land use Topology Spatial relationships between map features are represented on maps. The map user can interpret a map and estimate dis- tances and perceive relationships. GIS builds spatial informa— tion that facilitates an accurate data description of the rela- tionship of features. In some digital representation of maps spatial relations are depicted using topology. Topology is the spatial relationship between connecting or adjacent coverage features (e.g., arcs, nodes, polygons, and points). For example, the topology of an arc includes its from-and-to node and its left and right polygons. Topolog- ical relationships are built from simple elements into com- plex elements: points (simplest element), arcs (sets of connecting points), and areas of polygons (sets of connect- ing arcs). The three major topological concepts may include: 1. Connectivity (arc-node topology): The topological iden- tification of the set of arcs that connect at each node. Connectivity within a linear network is defined by re— cording the from-node and the to—node numbers for each arc. Arcs that share a common node are connected. 2. Area definition (polygon-arc topology): A list of arcs that make up each polygon. 3. Contiguity (left-right topology): The topological identi- fication of adjacent polygons by recording the left and right polygons of each arc. COMPONENTS OF GIS There are four integrated components of a GIS: (1) data and databases; (2) hardware; (3) software including database man- agement systems; and (4) users. 1. Data and databases: The data in a GIS are by definition “9- 4- Ed9° "WNW geographic. Spatial data being specifically location in- formation pertaining to where objects of interest are lo- cated, their distribution and extent, adjacency, proximity, and connectivity, verses, attribute data, or observations about features. 2. Hardware: A fully functional GIS must contain hardware to support data input, output, storage, retrieval, display, and analysis. Hardware essentials focus on the platform and the peripheral devices. Rapid improvements in com- puter technologies have allowed the advent of true desk- top GIS functionality. 3. Software: Many GIS software packages are on the market, each offering different levels of functionality. Turnkey systems (ready for use directly out of the box) and customized installations are all possible. Because GIS software packages are so numerous the Spatial Analysis trend is to perform a needs assessment or requ1rements analysis prior to committing to a purchase from one ven- dor. 4. Users: The true GIS professional needs to be well versed in many disciplines. Map reading, database management, spatial analysis, computer cartography, computer science, programming, and basic geography are disciplines in which a thorough grounding is required. A balanced ed- ucation in GIS theory and practical experience (familiar- ity with GIS software) are essential. However, three cat- egories of GIS users have developed: (1) system user, a technical person who has hands-on use of GIS; (2) end users, people who make use of the end product; and (3) V V Erodeble data generators, people who enter or capture data for a —. A — analysis in the GIS. a Non-erodable slop- soll erosion potential W by so" we FIG. 7. Classification FIG. 5. Windowlng I FOO'OI' -‘ FIG. 6. Aggregation FUNCTIONS OF GIS The following is a list and graphic depiction of GIS func- tionality: e v x Manipulation x simple complex FIG. 2. Coordinate Change @ E h FIG. 8. Measurement +I~ FIG. 3. Projection FIG. 9. Overlay JOURNAL OF COMPUTING IN CIVIL ENGINEERING / JANUARY 1998 / 3 constant wldth variable wldrh manhattan bufier euclidean buffer FIG. 10. Buffering O desdnatlon !-_l—I-— Oorlgin FIG. 11. Networks FIG. 12. MepAlgebra 4 / JOURNAL OF COMPUTING IN CIVIL ENGINEERING/JANUARY 1998 Query \65 A37. FIG. 13. Query Dlagram FIG. 14. Spatial Dlagram Aspatial queries—Attribute: What is the population of the city of Raleigh, NC? Spatial queries—Topological: How many people live within two miles of the Raleigh, NC city limits? or What is the shortest route between Raleigh and Cary, NC? Location: What exists at . . . ? Condition: Where is it? Trends: What has changed since . . . ? Patterns: How much growth has occurred? Modeling: What if? Proximity: What are the characteristics of an area sur- rounding a feature? Boundary operations: What exists within a specific re- gion? Logical operations: What is unique about a feature? David A. Holdstock Geographic Information Systems/Global Positioning System Program (GPS) Director Institute for Transportation Research and Education (ITRE), North Carolina State University, Centennial Campus, Box 8601, Raleigh, NC 27695-8601 919-515-8657 ...
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