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    Gis Gis Document Transcript

    • Zimbabwe School of Mines Higher National Diploma Geographic Information System and Remote Sensing Chapter 1 Introduction1.0 What is G.I.S?A GIS is an information system designed to work with data referenced by spatial / geographicalcoordinates. In other words, GIS is both a database system with specific capabilities for spatiallyreferenced data as well as a set of operations for working with the data. It may also be consideredas a higher order map.GIS technology integrates common database operations such as query and statistical analysiswith the unique visualization and geographic analysis benefits offered by maps. These abilitiesdistinguish GIS from other information systems and make it valuable to a wide range of publicand private enterprises for explaining events, predicting outcomes, and planning strategies.(ESRI)A Geographic Information System is a computer based system which is used to digitallyreproduce and analyse the feature present on earth surface and the events that take place on it. Inthe light of the fact that almost 70% of the data has geographical reference as its denominator, itbecomes imperative to underline the importance of a system which can represent the given datageographically.A typical GIS can be understood by the help of various definitions given below:- A geographic information system (GIS) is a computer-based tool for mapping and analyzing things that exist and events that happen on Earth Burrough in 1986 defined GIS as, "Set of tools for collecting, storing, retrieving at will, transforming and displaying spatial data from the real world for a particular set of purposes" Arnoff in 1989 defines GIS as, "a computer based system that provides four sets of capabilities to handle geo-referenced data : data input data management (data storage and retrieval)1
    • Zimbabwe School of Mines Higher National Diploma Geographic Information System and Remote Sensing manipulation and analysis data output. "Hence GIS is looked upon as a tool to assist in decision-making and management of attributesthat needs to be analysed spatially.1.1 Objectives and Potentials of G.I.SGIS Objectives  Maximise the efficiency of planning and decision making  Provide efficiency means of data distribution and handling  Elimination of redundant database-minimize duplication  Capacity to integrate information from many sources  Complex analysis/query involving geographical referenced data to generate new informationPotentials of G.I.SOnce G.I.S has been implemented the following benefits are expected  Geospatial data are better maintained in a standard format  Revision and updating easier  Geospatial data and information are easier to search, analyze and represent.  More value added product  Geospatial data can be exchanged and shared freely1.2 COMPONENTS OF A GEOGRAPHIC INFORMATION SYSTEMA working Geographic Information System seamlessly integrates five key components:hardware, software, data, people, and methods.HARDWAREHardware includes the computer on which a GIS operates, the monitor on which resultsare displayed, and a printer for making hard copies of the results. Today, GIS softwareruns on a wide range of hardware types, from centralized computer servers to desktopcomputers used in stand-alone or networked configurations. The data files used in GISare relatively large, so the computer must have a fast processing speed and a largehard drive capable of saving many files. Because a GIS outputs visual results, a large,2
    • Zimbabwe School of Mines Higher National Diploma Geographic Information System and Remote Sensinghigh-resolution monitor and a high-quality printer are recommended.SOFTWAREGIS software provides the functions and tools needed to store, analyze, and display geographicinformation. Key software components include tools for the input and manipulation ofgeographic information, a database management system (DBMS), toolsthat support geographic query, analysis, and visualization, and a graphical user interface (GUI)for easy access to tools. The industry leader is ARC/INFO, produced by Environmental SystemsResearch, Inc. The same company produces a more accessible product, ArcView, that is similarto ARCINFO in many ways.GIS D A T A Base Maps- includes streets, highways, boundaries for census, postal and political areas, rivers and lakes, parks and landmarks; place names. Environmental maps – include data related to the environment, weather, environmental risk, satellite imagery, topography, and natural resources Socio-economic data- include data related to census/demography. Healthcare, real state, telecommunications, emergency preparedness, crime, business establishments and transportation.PEOPLEGIS users range from technical specialists who design and maintain the system to thosewho use it to help them perform their everyday work. The basic techniques of GIS aresimple enough to master that even students in elementary schools are learning to useGIS. Because the technology is used in so many ways, experienced GIS users have atremendous advantage in today’s job market.METHODSA successful GIS operates according to:  a well-designed plan  business rules  models and operating practices unique to each organization.3
    • Zimbabwe School of Mines Higher National Diploma Geographic Information System and Remote Sensing1.3 CONTRIBUTING DISCIPLINES AND TECHNOLOGIES GIS is a convergence of technological fields and traditional disciplines GIS has been called an "enabling technology" because of the potential it offers for the wide variety of disciplines which must deal with spatial data each related field provides some of the techniques which make up GIS o many of these related fields emphasize data collection - GIS brings them together by emphasizing integration, modeling and analysis as the integrating field, GIS often claims to be the science of spatial informationGeography broadly concerned with understanding the world and mans place in it long tradition in spatial analysis provides techniques for conducting spatial analysis and a spatial perspective on researchCartography concerned with the display of spatial information currently the main source of input data for GIS is maps provides long tradition in the design of maps which is an important form of output from GIS computer cartography (also called "digital cartography", "automated cartography") provides methods for digitalrepresentation and manipulation of cartographic features and methods of visualizationRemote Sensing images from space and the air are major source of geographical data remote sensing includes techniques for data acquisition and processing anywhere on the globe at low cost, consistent update potential many image analysis systems contain sophisticated analytical functions interpreted data from a remote sensing system can be merged with other data layers in a GIS4
    • Zimbabwe School of Mines Higher National Diploma Geographic Information System and Remote SensingPhotogrammetry using aerial photographs and techniques for making accurate measurements from them, photogrammetry is the source of most data on topography (ground surface elevations) used for input to GISSurveying provides high quality data on positions of land boundaries, buildings, etc.Geodesy source of high accuracy positional control for GISStatistics many models built using GIS are statistical in nature, many statistical techniques used for analysis statistics is important in understanding issues of error and uncertainty in GIS dataOperations Research many applications of GIS require use of optimizing techniques for decision-makingComputer Science computer-aided design (CAD) provides software, techniques for data input, display and visualization, representation, particularly in 3 dimensions advances in computer graphics provide hardware, software for handling and displaying graphic objects, techniques of visualization database management systems (DBMS) contribute methods for representing data in digital form, procedures for system design and handling large volumes of data, particularly access and update artificial intelligence (AI) uses the computer to make choices based on available data in a way that is seen to emulate human intelligence and decision-making - computer can act as an "expert" in such functions as designing maps, generalizing map features o although GIS has yet to take full advantage of AI, AI already provides methods and techniques for system design5
    • Zimbabwe School of Mines Higher National Diploma Geographic Information System and Remote SensingMathematics several branches of mathematics, especially geometry and graph theory, are used in GIS system design and analysis of spatial dataCivil Engineering GIS has many applications in transportation, urban engineering6
    • Zimbabwe School of Mines Higher National Diploma Geographic Information System and Remote Sensing Chapter 2 GIS FUNCTIONS2.0 GIS TASKSGeneral purpose GIS’s perform seven tasks.  Input of data  Map making  Manipulation of data  File management  Query and analysis  Visualization of results An overview7
    • Zimbabwe School of Mines Higher National Diploma Geographic Information System and Remote Sensing2.1 Input of DataBefore geographic data can be used in a GIS, the data must be converted into a suitable digitalformat. The process of converting data from paper maps or aerial photographs into computerfiles is called digitizing. Modern GIS technology can automate this process fully for largeprojects using scanning technology; smaller jobs may require some manual digitizing whichrequires the use of a digitizing table.Today many types of geographic data already exist in GIS-compatible formats. These data canbe loaded directly into a GIS.Data input includes three major steps which are:  Data capture ( keyboard entry, manual digitizing, scanning)  Editing and cleaning  Geo-coding2.1.1 Data sources for GISMapsAerial photosSatellite imagesTechnical descriptionsGPS dataGeographically data contains four integrated components, namely, location, attribute, spatialrelationship and time.Geographic data include those which are spatially referencedA GIS operation which support spatial analysis2.1.2 The three types of GIS Data (spatial, attribute, meta) 1. spatial data A. vector data i. Point Data -- layers described by points (or "event") described by X, Y (lat, long; east, north) ii. Line/Polyline Data -- layers that are described by X, Y, points (nodes, events) and lines (arcs) between points (line segments and polylines)8
    • Zimbabwe School of Mines Higher National Diploma Geographic Information System and Remote Sensing iii. Polygon Data -- layers of closed line segments enclosing areas that are described by attributes Polygon data can be "multipart" like the islands of the state of Hawaii. B. raster data (grids of numbers describing e.g., elevation, population, herbicide use, etc) C. images or pictures such as remote sensing data or scans of maps or other photos. This is special "grid" where the number in each cell describes what color to paint or the spectral character of the image in that cell. (to be used, the "picture" must be placed on a coordinate system, or "rectified" or "georeferenced") D. TINs - Triangular Irregular Networks - used to discretize continuous data2 attribute data are non-spatial characteristics that are connected by tables to points, lines, events on lines, and polygons (and in some cases GRID cells). They give descriptive information about specified spatial objects. They don’t have direct information about the spatial location but can be linked to spatial objects they describe and usually organised in tables.  A point, vector or raster geologic map might describe a "rock unit" on a map with a single number, letter or name, but the associated attribute table might have  age  lithology  percent quartz  etc, for each rock type on the map.9
    • Zimbabwe School of Mines Higher National Diploma Geographic Information System and Remote Sensing Most GIS programs can either plot the polygon by the identifier or by one of the attributes The above examples from the following project show two ways to portray census data in Virginia. In the top image, each county/city gets a name and unique color, and in the bottom, the population density per square mile is read from the layers attribute table and plotted using a different color for each class of density.10
    • Zimbabwe School of Mines Higher National Diploma Geographic Information System and Remote Sensing3 Metadata  metadata are the most forgotten type  ArcView is very poor at it (writes some stuff to a log file, but thats it)  absolutely necessary if youre going to use data, or if someone is going to use your data later (or your information)  contains information about i. scale ii. accuracy iii. projection/datum iv. data source v. manipulations vi. how to acquire dataYou will be keeping metadata in ArcGIS using ArcCatalogs metadata feature.2.1.3 Data input identifiersThey enable both spatial and attribute data to be stored separately but accessed together.Identifiers are: - unique values- usually integers - stored as part of the spatial data structure- as numeric value( system generated) - stored as part of the attribute data structure- as a field in a table2.1.4 Data modelConversion of real world geographical variation into discrete objects is done through datamodels. It represents the linkage between the real world domain of geographic data andcomputer representation of these features2.1.5 Spatial Data Models GIS uses two basic data models to represent spatial features: vector and raster. The data model determines how the data are structured, stored, processed, and analyzed in a GIS. The vector data model uses points and their x, y coordinates to construct spatial features of points, lines, and areas. Vector-based features are treated as discrete objects over the space. The raster data model uses a grid to represent the spatial variation of a feature. Each cell in the grid has a value that corresponds to the characteristic of the spatial feature at that location. Raster data are well11
    • Zimbabwe School of Mines Higher National Diploma Geographic Information System and Remote Sensing suited to the representation of continuous spatial features like precipitation and elevation. Many GIS functions are either vector-based or raster-based. Qualitative or nominal data - discrete (1=basalt, 2=granite, etc for a geological map) Ordinal or rank data - discrete (low, medium, high; implies a quantity but is in "bins" or discrete categories) interval - continuous (example from Theobald, Temperature) ratio - continuous (hill slope angle, which could be measured/calculated to any precision and reported in floating point values or integer values) cyclic - continuous (with a break at one or more points, like compass direction or the "aspect" of a hill slope)Choosing the format for continuous vs. discrete data typesVector storage better for discrete and raster for continuousRaster Data - divides the entire study area into regular grid of cells - each cell contains a single value - easy space –filling since every location in the study area corresponds to a cell in the raster - raster data can be imagined as collection of cells organized like matrix12
    • Zimbabwe School of Mines Higher National Diploma Geographic Information System and Remote SensingVector Data Model - represented by lines, points and polygons - fundamental primitive is a point - points are stored as X, Y coordinates and represent features as having no dimension - objects are created by connecting points with straight lines (or arcs) - areas are defined by set of lines 2.1.6 Comparison of Raster and Vector Data Models Raster Model Vector Model advantages advantages It a simple data structure It provides a more compact data structure Overlay operations are easily and efficiently Provides efficient encoding of topology and implemented more efficient implementation of operations that require topological information like network analysis High spatial variability is efficiently Is better suited to supporting graphics that represented closely approximate hand drawn maps Is more or less required for efficient less data storage volume manipulation and enhancement of digital images Disadvantages Disadvantages Is less compact More complex data structure Topological relationships are more difficult "overlays" rapidly increase complexity and to represent data storage needs decreased boundary precision, Representation of spatial variability is inefficient higher data storage requirements (8-32 bytes Manipulation and enhancement of digital per cell* rows* columns), but compression images cannot be effectively done (run length encoding, quad trees) helps13
    • Zimbabwe School of Mines Higher National Diploma Geographic Information System and Remote Sensing RASTER vector Real world14
    • Zimbabwe School of Mines Higher National Diploma Geographic Information System and Remote Sensing 2.1.7 Representing Spatial Elements Raster Stores images as rows and columns of numbers with a Digital Value/Number (DN) for each cell. Units are usually represented as square grid cells that are uniform in size. Data is classified as “continuous” (such as in an image), or “thematic” (where each cell denotes a feature type. Numerous data formats (TIFF, GIF, ERDAS.img etc) Vector Allows user to specify specific spatial locations and assumes that geographic space is continuous, not broken up into discrete grid squares. We store features as sets of X,Y coordinate pairs15
    • Zimbabwe School of Mines Higher National Diploma Geographic Information System and Remote Sensing Entity Representations We typically represent objects in space as three distinct spatial elements: Points - simplest element Lines (arcs) - set of connected points Polygons - set of connected lines We use these three spatial elements to represent real world features and attach locational information to them Attributes  In the raster data model, the cell value (Digital Number) is the attribute. Examples: brightness, landcover code, SST, etc.  For vector data, attribute records are linked to point, line & polygon features. Can store multiple attributes per feature. Vector features are linked to attributes by a unique feature number.2.2 Map Making Maps have a special place in GIS. The process of making maps with GIS is much more flexible than are traditional manual or automated cartography approaches. It begins with database creation. Existing paper maps can be digitized and computer-compatible information can be translated into the GIS. The GIS-based cartographic database can be both continuous and scale free. Map products can then be created centered on any location, at any scale, and showing selected information symbolized effectively to16
    • Zimbabwe School of Mines Higher National Diploma Geographic Information System and Remote Sensing highlight specific characteristics. The characteristics of atlases and map series can be encoded in computer programs and compared with the database at final production time. Digital products for use in other GIS’s can also be derived by simply copying data from the database. In a large organization, topographic databases can be used as reference frameworks by other departments. 2.3 Data Management A collection of non-redundant data which can be shared by different application systems is known as a database. Several layers of geographic data covering the same location are considered as database. When data volumes become large, it is often best to use a database management system(DBMS) to help store, organize and manage data. A DBMS is nothing more than a computer software for managing a database. There are many different designs of DBMSs, but in GIS relational design has been the most useful. In the relational design, data are stored conceptually as a collection of tables. A DBMS contains: Data definition language Data dictionary Data-entry module Data update module Report generator17
    • Zimbabwe School of Mines Higher National Diploma Geographic Information System and Remote Sensing Query languageAdvantages of Database Approach: ● Reduction in data redundancy - the databases are shared rather than independent and this reduces problems of inconsistencies in stored information, e.g. different addresses in differentdepartments for the same customer.● Maintenance of data integrity and quality● Data are self-documented or self-descriptive - as information on the meaning or interpretationof the data can be stored in the database, e.g. names of items, metadata. ● Avoidance of inconsistencies - making the data follow prescribed models, rules andstandards.● Reduced cost of software development – as many fundamental operations are taken care of,however DBMS software can be expensive to install and maintain. ● Security restrictions - database includes security tools to control access, particularly forwriting.18
    • Zimbabwe School of Mines Higher National Diploma Geographic Information System and Remote Sensing 2.4 Data manipulation GIS data need to undergo transformation before they can be integrated, displayed or analyzed. - Same scale, coordinate system, format, etc A temporary transformation for display purposes or a permanent one required for analysis. 2.5 Spatial Analysis/Modeling Spatial Operation  Buffering  Overlay Spatial Statistics Spatial Data Mining Proximity Analysis Buffer: Delineation of a zone around the feature of interest within a given distance. For a point feature, it is simply a circle with its radius equal to the buffer distance19
    • Zimbabwe School of Mines Higher National Diploma Geographic Information System and Remote Sensing Buffer Example Variable Distance Buffer Buffer zone can be made variable according to certain attributes. Suppose we have a point pollution source, such as a power plant. We certainly want to keep our residential area away a distance from it. However, this distance can be made variable according to the amount of pollution that a power plant produces. For small power plant, the distance can be short, while for large power plant that generate lot of pollutant, we should keep a longer distance from it. As we is shown below20
    • Zimbabwe School of Mines Higher National Diploma Geographic Information System and Remote Sensing Buffers for lines and Polygons21
    • Zimbabwe School of Mines Higher National Diploma Geographic Information System and Remote Sensing 2.6 Spatial Analysis Overlay function creates new “layers” to solve spatial problems Spatial Operation with Multiple Vector Layers • Overlay analyses – Operate on spatial entities from two or more maps to determine spatial overlap, combination, containment, intersection…etc. – one of the most “fundamental” of GIS operations – formalized in 1960s by landscape architects who used acetate map overlays – now a basic part of the GIS toolbox • Vector overlays- – combine point, line, and polygon features – computationally complex • Raster overlays- – cell-by-cell comparison, combination, or operation – computationally less demanding • Basic idea: – spatially combine/compare two data layers to:22
    • Zimbabwe School of Mines Higher National Diploma Geographic Information System and Remote Sensing (a) generate new output data layer, or (b) assign attributes of one data layer to another – most cases: one of the data layers will contain polygon entities • Point-in-polygon overlay  – increasing conceptual and computational complexity • Point-in-polygon vector overlay • Overlay point layer (A) with polygon layer (B) – in which B polygon are A points spatially located? » assign polygon attributes from B to points in A Example: comparing soil mineral content at sample borehole locations (points) with landuse (ploys)23
    • Zimbabwe School of Mines Higher National Diploma Geographic Information System and Remote Sensing Line-in-polygon vector overlay • Overlay line layer (A) with polygon layer (B) – in which B polygons are A lines spatially located? » assign polygon attributes from B to lines in A Example: assign landuse attributes (polys) to streams (lines)... Polygon-polygon vector overlay • Overlay polygon layer (A) with polygon layer (B) – result: what are the spatial polygon combinations of A and B? » generate new data layer with combined polygons • attributes from both polygon layers are included in output • How are polygons combined? (i.e. what geometric rules are used for combination?) – UNION (Boolean OR) – INTERSECTION (Boolean AND)24
    • Zimbabwe School of Mines Higher National Diploma Geographic Information System and Remote Sensing – IDENTITY • Polygon overlay will generally result in a significant increase in the number of spatial entities in the output – can result in output that is too complex too interpret Boolean Operations Some of the fundamental overlay analysis for vector data are UNION, and INTERSECT corresponding to Boolean operations of OR, AND UNION overlay polygons and keep areas from both layers INTERSECTION overlay polygons and keep only areas in the input layer that fall within the intersection layer IDENTITY overlay polygons and keep areas from input layer25
    • Zimbabwe School of Mines Higher National Diploma Geographic Information System and Remote Sensing2.7 CONNECTIVITY FUNCTIONSContiguity Measures. Contiguity measures evaluate characteristics of spatial units that areconnected. These units share one or more characteristics with adjacent units and form a group.The term UNBROKEN is the key concept. Different adjacent features may have more than oneattribute but they must all have a COMMON attribute to be considered as reflecting contiguity.Contiguity is used to measure shortest and longest straight line distances across and area and toidentify areas of terrain with specified size and shape characteristics.Example. An area of continuous pastureland with an area of no more than 10 acres with no partof it wider than the sound of the Acme Pig Call can be heard.Proximity Functions. The simple distance between features, commonly units of length but can beother units such as how far away the ACME PIG CALL can be heard.Four parameters are used to measure proximity. 1. target locations. 2. unit of measurement. 3. afunction to calculate proximity. 4. and the area to be analyzed.A common type of proximity analysis is the buffer zone. Coverage can be quit complicatedinvolving many layers and mathematically complex such as the decreasing sound levels due tothe inverse proportion law of noise generated by various types of air traffic in the vicinity of ahousing area.2.8 Network FunctionsDefinition: A network is a set of interconnected linear features that form a pattern or framework.They are commonly used for moving resources from one location to another. City Streets, PowerTransmission Lines, and Airline Service Routes are examples.There are three principal types of GIS Analysis performed by Networking. 1. Prediction ofloading on the network itself (prediction of flood crests), rate optimentation (emergency routingof ambulances), and resource allocation (zones for servicing rescue areas).26
    • Zimbabwe School of Mines Higher National Diploma Geographic Information System and Remote SensingNetworks analysis entails four components. 1. set of resources (goods to be delivered). 2. one ormore locations where the resources are located (several warehouses where the goods arelocated). 3. an objective to deliver the resources to a set of destinations (customer locational database). 4. Set of constraints that places limits on how the objective can be met (is it economicallyfeasible to deliver pizzas to Lincoln from a store in Omaha ?).2.9 Spread FunctionsThe Spread Function is simply the "best" way to get from point A to point B. "Best" can befastest, it can be most the most economical, or a subjective measurement such as most scenic. Itis an evaluation of phenomena that accumulates with distance.Imagine a square and you are going to travel from the lower left corner to the upper right corner.The straight line distance is 1.414 times the side of the square, and the distance across the sides is2.0 times the length of a side. If this square represented a pasture containing angry buffaloes itwould probably beneficial to walk around the fenced perimeter and go the extra distance.Output of this particular GIS functions is sometimes referred to as ACCUMULATIONSURFACE or FRICTION SURFACE. These concepts refer to the "effort it takes to get from Ato B, such as the square traversed was knee deep mud (or a lake) across the diagonal but dry atthe perimeter. It would be farther, but easier to again go the extra distance.2.10 Seek or Stream FunctionsSeek and Stream are synonymous and refer to a function that is directed outward in a step by stepmanner using a specified decision rule. This procedure is initiated and proceeds until the anyfurther movement violates the decision rule.This GIS function, as an example, could be used to evaluate erosion potential. The decision rulein this case would be elevation. As the process proceeds outward from the source (rainfall), thedecision will always proceed downhill, never uphill. The path of least resistance best describesthis function, Sea level, interior drainage or the edge of the area analyzed causes the function toterminate.27
    • Zimbabwe School of Mines Higher National Diploma Geographic Information System and Remote Sensing2.11 Intervisibility FunctionsThis GIS function is typified by the phrase LINE OF SIGHT. It is a graphic depiction of the areathat can be seen from the specified target areas. Areas visible from a scenic lookout, or therequired overlap of microwave transmission towers can be mapped using this procedure.Intervisibility functions rely on digital elevation data to define the surrounding topography.Applications such as landscape layouts, military planning, and the obvious communicationutilization are best serviced.The output of this function is somewhat unique in that it is often displayed in a SIDE VIEWformat. The vertical field of view and maximum viable distance are the component parameters.It is powerful tool for trial and error analysis in which the placement of objects can constantly bere-evaluated. Offshoots of this type of procedure can produce graphics that exhibit threedimensional perspective. SHADED RELIEF IMAGES or SHADED RELIEF MODELS, alongwith PERSPECTIVE VIEWS are valuable presentational tools. The process called draping isused to apply another data set over this shaded depiction to further enhance presentability.2.12 OUTPUT FUNCTIONSMap AnnotationDefinition: Titles, Legends, Scale Bars, and North Arrows are the simplest forms of depictinginformation concerning the map.The various programs available usually handle this as user input and it is not generated by thesoftware. Flexibility as to location (position), fonts, symbology, and size are varied as to theindividual programs. Text labels are an important aspect of map viewing and are all different asto program. Sophistication is increasing and actual hard copy maps can be enhanced withsecondary software applications.28
    • Zimbabwe School of Mines Higher National Diploma Geographic Information System and Remote SensingTexture Patterns and Line StylesTexture patterns and line styles are difficult to alter from program guidelines so initial analysis ofthe output should be considered when choosing a software.Graphic SymbolsGraphic Symbols are used to portray the various entities depicted on the map. Some softwarepackages provide a simple standard symbol set, but do not allow user input, others store themwithin the GIS and they can be called to use as needed, others assign a symbol to an attribute andallow the symbology to be automatically plotted. As before the selection of the software and itsapplication should be carefully considered as to the output presentation needed.29
    • Zimbabwe School of Mines Higher National Diploma Geographic Information System and Remote Sensing Chapter 3 Map Projections and Coordinate Systems in GIS 3.1 Map Projections Map projection transforms the spatial relationship of map features on the Earth’s surface to a flat map. Map projection enables a map user to work with two-dimensional coordinates, rather than spherical or three-dimensional coordinates. But the transformation from the Earth’s surface to a flat surface always involves distortion and no map projection is perfect A projection is the translation of spherical coordinates onto a planar surface, while a datum is the ellipsoid, or “figure of the earth” that approximates the actual shape of the earth, and is used in the transformation equation A datum is the geometric, 3-D “figure of the earth” which is used as the basis for projecting onto a planar surface. The most common datums we run across are the North American Datum of 1927 (NAD27), which is tangent to a point on the surface of the earth (Mead’s Ranch, in Kansas), and the North American Datum of 1983 (NAD83), which is centered on the center of the earth. The World Geodetic System of 1984 (WGS84) is the common datum used by the GPS configuration, and is essentially identical to the NAD83 How the ellipsoid (datum) is projected onto a planar surface.30
    • Zimbabwe School of Mines Higher National Diploma Geographic Information System and Remote Sensing 3.2 Coordinate Systems A coordinate system is based on a map projection. Once an ellipsoid has been projected onto a planar surface, a coordinate system must be defined to specify locations on that surface. The familiar XY coordinate pairs of a typical graph is an example of a coordinate system. Universal Transverse Mercator (UTM), State Plane Coordinate System (SPCS), and Longitude and Latitude are commonly used coordinate systems in GIS Plane coordinate systems are typically used in large-scale mapping such as a scale of 1:24,000 or larger. Coordinate systems are designed for detailed calculations and positioning. Therefore, accuracy in a feature’s absolute position and its relative position to other features is more important than the preserved property of a map projection. Map projections come with names like Lambert conic conformal or Albers conic equal-area. Lambert and Albers are names of the cartographers who originally proposed the projections. The other parts of the name describe the map projection’s preserved property and projection surface. Map projections are grouped into four classes by their preserved properties: conformal, equal area or equivalent, equidistant, and azimuthal or true direction. A conformal projection preserves local shapes. An equivalent projection represents areas in correct relative size. An equidistant projection maintains consistency of scale for certain distances. An azimuthal projection retains certain accurate directions. Cartographers often use a geometric object to illustrate how a map projection can be constructed. For example, by placing a cylinder tangent to a lighted globe, a projection can be made by tracing the lines of longitude and latitude onto the cylinder. The cylinder in this case is the projection surface, and the globe is called the reference globe. Other common projection surfaces include a cone and a plane. A map projection is called a cylindrical projection if it can be constructed using a cylinder, a conic projection using a cone, and an azimuthal using a plane. Why the understanding of map projections is important in practical applications of GIS? A basic principle in GIS is that map layers to be used together must be based on the same coordinate system.31
    • Zimbabwe School of Mines Higher National Diploma Geographic Information System and Remote Sensing Otherwise, map features from different layers will not register spatially in a proper manner. Increasingly, GIS users download digital maps from the Internet, or acquire them from governmental agencies and private companies. Some digital maps are measured in longitude and latitude values, while others are in various coordinate systems different from the one intended for the GIS project. Invariably, these digital maps must be projected and re- projected before they can be used together. Typically, projection and re-projection are among the initial tasks performed in a GIS project. 3.3 Errors in GIS No map is perfect, even the most accurate maps created by a GIS have some deficiencies. These deficiencies occur due to “Errors” that may have taken place at different stages of GIS implementation. These errors reduce the accuracy of the map generated. However by use of well defined and controlled procedures these errors can be avoided. There are two types of errors in GIS: 1 Source Errors: They are the errors that are present in “Source Data” that is given to the GIS. They occur before the actual implementation of GIS Instrumental inaccuracies - Satellite/ air photo/ GPS/ surveying (spatial). - Inaccuracies in attribute measuring instruments Human Processing: - Misinterpretation (e.g. photos), spatial and attribute - Effects of scale change and generalization - Effects of classification (nominal / ordinal / interval). Actual Changes: - Catastrophic change: fires, floods, landslides - Gradual natural changes: river courses, glacier recession. - Seasonal and daily changes: lake/sea/ river levels. - Man-made: urban development, new roads. - Attribute change: forest growth (height etc.), discontinued trail / roads, road surfacing.32
    • Zimbabwe School of Mines Higher National Diploma Geographic Information System and Remote Sensing 2 Processing Errors: They are the errors that occur during the processing of the data i.e. during the implementation of GIS. Input: - Digitizing: human error, the width of a line, spikes, knots, also entering attribute data. Dangling nodes (connected to only one arc): permissible in arc themes (river headwaters etc.). Pseudo-nodes (connected to one or two arcs) - permissible in island arcs, and where attributes change, e.g. road becomes paved from dirt or vice versa. - Projection input error Manipulation - Interpolation of point data into lines and surfaces - Overlay of layers, digitized separately, e.g. soils and vegetation. - The compounding effects of processing and analysis of multiple. layers: for example, if two layers each have correctness of 90%, the accuracy of the resulting overlay is around 81%. - Density of observations - Inappropriate or inadequate inputs for models Output: - Scale changes - detail and scale bars. - Color palettes: intended colours dont match from screen to Printer33
    • Zimbabwe School of Mines Higher National Diploma Geographic Information System and Remote Sensing3.4 LIFECYCLE OF A GIS (PLANNING GIS)Successful implementation of GIS requires planning the project before its actual implementation.Planning leads to a better structured and organized system.Phase 1-PlanningA planning process is the first stage in the life cycle. This phase involves a systematic review ofusers, their data, and their information needs. Decision makers are told about the costs andbenefits of GIS and to include potential users in planning process so that they receive anoverview of the technology.Phase 2-System DesignThe design phase matches user needs to GIS functionality. Design includes not only selection ofhardware and software, but also the design of the GIS spatial and attribute database. A Relationaldatabase is generally used for the GIS. The Database design will include specifications for scale,projection, and coordinate systems. Data is be tracked using a "Data Dictionary." During thedesign phase an incremental plan is often used for implementation of the technology.Incremental implementation means that users will build a GIS piece-by-piece.34
    • Zimbabwe School of Mines Higher National Diploma Geographic Information System and Remote SensingIn some cases a Prototype’ is developed so that refinements can be made before finalizing thefully implemented system.Phase 3-Implementation During the implementation phase, attention to all user needs must be provided through trainingand education. Hands-on users must be trained to utilize and maintain the system and thedatabase. All types of users should be made cognizant of how the GIS will affect them and theirdata processing tasks. They must also be made aware of the changes that GIS will introduce inthe area of information generation and decision making.Phase 4-MaintenanceFinally, a GIS application must be maintained and kept current in terms of data and user support.In some cases, a GIS is designed to meet the needs of a specific, finite project. In other instances,GIS is used to support an on-going mission or program. In the former case, the GIS applicationwill terminate once the project is completed and maintenance will probably not be an issue.However, even if the initial GIS application is no longer being utilized, the data generated for theinitial project may be useful to other projects or users. In those instances, a current datadictionary will be vital for determining the utility of the existing digital data for other usesIn the case of an on-going GIS effort the system must be kept up-to-date in order to fulfil itsdesign goals. Maintenance includes updating hardware and software, adding new data andupdating existing data records, and keeping users current in terms of system functionality3.5 Examples of Applied GISUrban Planning, Management & Policy  Zoning, subdivision planning  Land acquisition  Economic development  Code enforcement  Housing renovation programs  Emergency response  Crime analysis  Tax assessmentEnvironmental Sciences  Monitoring environmental risk  Modeling stormwater runoff  Management of watersheds, floodplains, wetlands, forests and aquifers  Environmental Impact Analysis35
    • Zimbabwe School of Mines Higher National Diploma Geographic Information System and Remote Sensing  Hazardous or toxic facility siting  Groundwater modeling and contamination tracking Civil Engineering/Utility  Locating underground facilities  Designing alignment for freeways, transit  Coordination of infrastructure maintenance36