What is GIS?
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What is GIS?
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A Geographic Information System (GIS) is not one thing, nor a single analysis; but rather a collection of hardware, software, data, organizations, and professionals that together help people represent and analyze geographic data.

A GIS references real-world spatial data elements (also known as graphic or feature data elements) to a coordinate system. These features can be separated into different layers (a.k.a. map themes or coverages). A GIS can also store attribute data, which is descriptive information of the map features. This attribute information is placed in a database separate from the graphics data but is linked to them. A GIS allows the examination of both spatial and attribute data at the same time.

Also, a GIS lets users search the attribute data and relate it to the spatial data. Therefore, a GIS can combine geographic and other types of data to generate maps and reports, enabling users to collect, manage, and interpret location-based information in a planned and systematic way.

In short, a GIS can be defined as a computer system capable of assembling, storing, manipulating, and displaying geographically referenced information.


Data Elements  
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As mentioned before, geographic data can be classified into 2 main classes: spatial data and attribute data.

The two most popular types of spatial data are raster and vector. Raster references spatial data according to a grid of cells (or pixels), whereas vector data references spatial data to a series of coordinates. Raster data consist of different numerical values assigned to individual pixels. Raster data are more suitable for representing features without discrete boundaries such as forest cover type and precipitation. Vector data, on the other hand, consist of points, lines (or arcs), or polygons (or areas). These features are recorded in a series of coordinates. Denoting a point requires assigning one coordinate pair, a line requires 2 or more coordinate pairs, and a polygon requires 3 or more coordinate pairs. Vector data are more suitable for features that have discrete boundaries such as roads and houses.

Attribute data are descriptions, measurements, and/or classifications of the geographic features. Attribute data can be classified into 4 levels of measurement: nominal, ordinal, interval and ratio. The nominal level is the lowest level of measurement in which the data can only be distinguished qualitatively, such as vegetation type or soil type. Data at the ordinal level can be ranked into hierarchies, but such differentiation does not show any magnitude of difference. Examples of this ordinal data include stream order and city hierarchies. The interval level of measurement indicates the distance between the ranks of measured elements, but a starting point is arbitrarily assigned. The best example of interval measurement is Celsius temperature, in which 0 degrees Celsius is an arbitrary value, and 20 degrees Celsius is not twice as hot as 10 degree Celsius. Ratio measurements, the highest level of measurements, includes an absolute starting point. Data of this category include property value and distance.

Time can also be considered a data element, since geographic information often changes over time. For instance, a river course may meander over time, or river dimensions may undergo sudden changes due to floods, living pattern of a certain kind of wildlife may change, and landuse may change due to agricultural to industrial use.


Collection of Geographic Data

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Obtaining data to insert into a GIS is a large subject in itself that includes a number of different approaches. One of the most common ways to collect spatial geographic data is to perform a physical survey. This includes surveying the land, underwater areas, and underground features of the earth (which are referred to as field survey, hydrographic survey and mining survey respectively).

Aerial photography/remote sensing is an increasingly popular way to gather spatial data. Aerial photographs are taken from an aircraft, after which they are measured and interpreted. Similarly, satellite remote sensing can be interpreted for physical features and attributes.

Censuses conducted by the U.S. Census Bureau gathers a variety of demographic data such as population, age structure, sex ratio, race composition, employment rates.

Statistics are a set of mathematical methods used to collect and analyze data. These methods include the collection and study of data at different time intervals and at a fixed location, providing information for yearbooks, weather station reports, etc. This information often has a spatial component and can thus be incorporated into a GIS.

Finally, tracking is a process of collecting attribute data on changes that occur at a location over a period of time. Examples of tracking include: monitoring the change of an ecosystem, and real-time monitoring of a moving objects such as vehicles.


Components of a GIS

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The three main components of a GIS are hardware, software and human resources.

GIS hardware includes: computers, computer configuration/networks, input devices, printers, and storage systems. Computers for GIS usage can be PCs at the low end, or supercomputers and X-Terminals at the high end. These computers can be stand-alone units or can be hooked into a network environment. Input devices include digitizers and scanners. A digitizer is the device used for selecting features from a hard copy map, which are then registered to a coordinate system. Currently digitizing is the most common method for converting existing maps and images into digital form. However, this process can be tedious, especially when converting high-density maps. Scanners sometimes can replace digitizing by automatically converting hard-copy maps to a digital raster file. Once in a GIS, the raster image can be converted to a vector format through a "raster-to-vector" conversion. The third hardware component is the printer/plotter. These devices are used to produce a hardcopy map. There are several types of printers including: matrix, inkjet/bubblejet and laser. Plotter types include: laser, electrostatic, direct thermal and pen plotter. Finally, GIS storage systems include: optical disks, magnetic disks (such as a hard drive), floppy disks or magnetic tapes.

GIS related software includes both the GIS program and special application packages, such as digital terrain modeling and network analysis. The main difference between GIS software programs and desktop mapping programs is the ability of GIS programs to perform spatial analysis. Examples of GIS software packages include: [1] Modular GIS Environment by Intergraph Corp., [2] Geo/SQL by Generation 5 Tech. Inc., [3] ARC/INFO by Environmental Systems Research Institute Inc., [4] SPANS by Tydac Technologies, [5] FMS/AC by Facility Mapping Systems Inc. Desktop mapping programs offer many of the same features as a GIS, but their ability to support spatial analyses are limited. They are developed to satisfy individual user needs for mapping presentations. Some examples of popular desktop mapping programs include: [1] MapInfo developed by MapInfo Corp., [2] Atlas GIS by Strategic Mapping Inc., [3] MapGrafix by ComGrafix Inc., [4] QUIKMAP by AXYS Software Ltd. etc. Public domain GIS software packages are GIS programs developed by government and universities, available free or for a nominal cost. Examples of such GIS programs include [1] IDRISI by Clark University, [2] GRASS by the GRASS Information Center and [3] MOSS by Autometric Inc.

Human resources used to operate a GIS typically include: operational staff, technical professional staff, and management personnel. Operational staff are people such as [1] cartographers, who monitor the design of map displays, the standards for map symbols and standard map series [2] data capturers, who converts map into digital form and [3] potential users of a GIS. Technical professional staff include [1] information analysts who solve particular user problems and satisfy their information needs, [2] system administrators, who are responsible for keeping the system (hardware/software) operational, [3] programmers, who translate the application specifications prepared by the analyst into programs and [4] the database administrator, who assists the analysts, programmers and users to organize geographic features into layers, identify sources of data, develop coding structures for non-graphics data, and document information about the contents of the databases. Management personnel include [1] the manager, who monitors the daily performance of the GIS project implementation team and manages the output production as required by the organization and [2] the Quality Assurance coordinator who manages the output of the final product to ensure that it meets the conversion specification and data acceptance plan.


Analysis of GIS Data

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Attribute Data Queries

A GIS stores spatial and attribute data in two separate files. Corresponding records in the two files (for example, a map of property parcel boundaries and the corresponding data such as the name of the owner, structure built on the property parcel, and the value of the structure) are linked by an identification number (in this case, probably the parcel number). This allows a GIS to search and display attribute data based on spatial criteria, and vice versa. For example, a GIS user can point to a particular property and ask the GIS to retrieve and display the attribute data of that parcel, but can also ask the GIS to locate the corresponding property parcel on the map by supplying a record in the attribute database.

A GIS is also able to handle much more complex queries. GIS users can define search parameters involving arithmetic and logical expressions by building a "query statement." Operators such as +, -, =, not =, <, <=, >, >=, between and others specify the value to be found. Several query statements can be combined to perform a single query.

For example, a GIS user may write a query asking, "How many structures are for sale, have 4 or more bedrooms, with a price of $300,000 or less and in a specific location." The GIS would first build and send a query statement to the DBMS (database management systems), which would search the attribute database for those parcels meeting the criteria. The DBMS then would return the parcel numbers to the GIS. The GIS would then search its spatial data file to find these parcels and their coordinates, and finally it would highlight those parcels on the display of the property parcel map.

On the other hand, GIS users can also describe an area on the parcel map and ask the GIS to retrieve the attribute data of all parcels that are within the specified area. This area could be in any shape. Logical operators could be employed to narrow the search or even to specify which attributes to retrieve. The resulting query set could be used to generate reports, graphic displays, and new map files.

Spatial Queries

Spatial queries are performed by creating a query set based on the spatial relationship of map features. Spatial operators in the queries define the spatial relationships that exist between map features. Most spatial operators (overlaps, entirely contains, entirely contained by, contains, contained by, terminates in, terminus of, passes through, passed through by, on boundary of, has on boundary, touches, meets, and spatially equal) can be combined to answer complex spatial queries. Proximity operators (between, within, beyond, entirely between, entirely within, and entirely beyond) are those that cannot be combined with each other.

For example, a city's emergency management agency might compile a database of all property addresses and classify them into flood-prone and non-flood-prone areas by performing a spatial query. It could use GIS to compare a map of the 100-year flood plain to the tax parcel map. First of all, the user would query for all parcels that are within the flood plain area (using the "entirely_contains" operator). These addresses would be classified as "flood-prone." The GIS would then be used to find all parcels crossed by the flood plain boundary (using the "passes-through" operator). Address in this category would be classified as "intermediate" and would receive further inspection by the staff so as to determine if the structure itself was flood prone or just a portion of the property. The remaining parcels would be entirely outside the flood plain and put into the "not flood prone" category. We can use the "disjoint" operator to check that parcels have been identified and labeled.

Set Queries

GIS can also compare or combine query sets in several logical ways. Queries that involve two or more query sets are called "set queries." Set query operators often include union, intersect, minus and difference, which are commonly known as "Boolean Operators." Their definitions are as follows:

Union: uniquely combines the contents of both operands (query sets).
Intersect: keeps only items present in both operands.
Minus: keeps only items from the first operand that are not in the second.
Difference: keeps only items present in one, but not both, operands.

For example, when conducting a study to find possible sites for a new factory, a consultant might use a GIS to create several query sets to find locations that are not suitable. The first query set might identify buffer areas surrounding endangered habitats, another might identify all forested areas, another might show all flat areas, and another might indicate locations of competing industries. The consultant could then combine these query sets (using the "union" operator) and all areas on the map that are not suitable would be identified. The remaining areas would then be examined for potential sites.

Network Analysis

Network analysis is usually used by organizations/agencies that manage or use networked facilities, such as utility, communication and transportation systems. Utilities use network models to monitor and analyze their distribution systems and meter-reading routes. Municipal public works departments use networks to analyze bus and trash routes, and businesses use them to find the optimal routes for the delivery of goods and services. Retail stores use network analysis to analyze driving times so as to determine retail store trade areas.

The 3 main types of network analyses are: network tracing, network routing and network allocation. Network tracing finds a particular path through the network based on criteria provided by the user. Network routing determines the optimal path along a linear network. Some possible criteria to select the path include shortest distance, fastest distance and minimum cost. It can also be "point-to-point" (source to destination) or "multiple-point" (with multiple intermediate stops). Network allocation deals with the designation of portions of the network to "supply centers" or "destination points." Such points can be, for example, school (to determine its capacity) or bus stops (to determine its coverage and service distance).


GIS Applications

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Environmental fields have long used GIS for a variety of applications that range from simple inventory and query, to map analysis and overlay, to complex spatial decision-making systems.

Examples include: forest modeling, air/water quality modeling and monitoring, environmentally-sensitive zone mapping, analysis of interaction between economic, meteorological, and hydrological & geological change. Typical data input into an environmental GIS include: elevation, forest cover, soil quality and hydro-geology coverages.

In many cases environmental GIS are used so that environmental considerations can be better incorporated into socioeconomic development enabling a balance between the two.

Infrastructure and Utilities

GIS technologies are also widely applied to the planning and management of public utilities. Organizations dealing with infrastructure and public utilities find GIS a powerful tool in handling aspects such as planning, decision support, customer service, regulatory requests, standardization of methods, and graphics display.

Typical uses include management of the following services: electric, gas, water, roads, telecommunication, storm sewers, TV/FM transmitting facilities, hazards analysis, and dispatch and emergency services. Typical data input includes street network, topographic data, demographic data and local government administration boundary.

Business Marketing and Sales

The use of GIS in business has greatly enhanced the efficiency in a number of areas, especially marketing research. Examples of the use of GIS in business include: locating potential competitors, mapping market thresholds for retailers, providing computerized hazard information classifications, aiding risk management decisions in insurance companies, and enabling real estate agents to handle property data more efficiently.

Delivery services also utilize GIS in aspects such as navigation & monitoring of their fleets, routing optimization for shipping and deliveries, geocoding address matching, and location searches. Typical data input into this category include road networks, street addresses, business profiles, and socioeconomic profiles.

Computer Cartography

The growth of computer-assisted-cartography (CAC) has been largely dependent on the development of vector-based GIS. With the help of GIS, cartographic tasks such as thematic overlays of information, map projections, and map sheet layouts can be performed much more conveniently.

Continually updated geographic databases provide an easy way to produce new map editions. Automated mapmaking and virtual map images have replaced traditional paper maps in many applications. Web-based maps have made general-purpose navigation far more accessible to the public.

However, manually digitized paper maps remain the primary form of data input in an automated cartography GIS. Scanned maps are also often used.

Land Information

GIS has aided management of land information by enabling easy creation and maintenance of data for land records, land planning and land use. In particular, a flourishing number of municipal governments have started to implement GIS to help manage their land information. GIS makes input, updates, and retrieval of data such as tax records, land-use plan, and zoning codes much easier then during the paper-map era.

Typical uses of GIS in land information management include managing land registry for recording titles to land holdings, preparing land-use plan and zoning maps, cadastral mapping etc. Input of data into a land information GIS includes: political and administrative boundaries, transportation, and soil cover.

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