Geographic Information Systems
Introduction[edit | edit source]
What Is In This Book[edit | edit source]
What is GIS?[edit | edit source]
Geographic Information Systems provide a method for integrating and analyzing spatial (digital map based) information such as "where is the nearest movie theater?" alongside related non-spatial information (what movies are playing there?). GIS have three major capabilities (computer mapping, spatial analysis and spatial database) and can operate on a range of platforms (desktop/laptop computer, Internet, PDA, etc.). Many people are becoming far more familiar with seeing the results both textually - for example when their phone shows them the nearest pub - and on open map systems such as Google Maps. Where in the past people had to literally use pencils and string on a paper map to find their nearest school, a computer can do this now extremely quickly an accurately, as long as all the information has been entered correctly in the first place.
In a broader context, GIS involves people and often brings a philosophy of change. For example, in 1994, the New York Police Department introduced GIS to locate crime 'hot-spots', analyze underlying problems and devise strategies and solutions to deal with the problems. Since 1993, violent crime has dropped by two-thirds in New York City. This strategy, known as COMPSTAT, has expanded to cities and jurisdictions across the United States and around the world.
GIS Software[edit | edit source]
One leading GIS software vendor is ESRI, based in Redlands, California, which offers ArcGIS for the desktop, ArcGIS Server for Internet mapping, ArcPad for PDAs and a range of other products and services for developers. Other popular GIS software packages are available from Cadcorp, Intergraph, MapInfo, Manifold and Autodesk. ERDAS Imagine, ENVI, Idrisi, and PCI Geomatica are geared towards remote sensing i.e. analysis of satellite/aircraft images. There are many third-party extensions and utilities for ArcGIS and other GIS and raster software platforms. Currently, open source GIS software options can be chosen from the first OS GIS package GRASS, recent open source options are DIVA GIS, Quantum GIS, and uDig. There are efforts underway, through the Open GIS Consortium to provide interoperability among spatial data formats and software. The leading contender for spatial data storage is another open source package called PostGIS, which is a spatial extension to the open source database PostgreSQL.
What Is Topology?[edit | edit source]
Topology is the branch of mathematics concerned with spatial properties in special the continuous deformations of objects, such as deformations that involve stretching (but no tearing or gluing) and how they connect together. In geography this is an indispensable tool for map-making in expressing relevant geographic information, especially in topological maps.
A Brief History Of Topology[edit | edit source]
Topology In Mathematics[edit | edit source]
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Why Is Geospatial Topology Important In A GIS?[edit | edit source]
Geospatial analysis provides a unique perspective on the world. It is a tool with which to examine events, patterns, and processes that operate on or near the surface of our planet. To have this tool interact with the relevant topological information is a must; providing greater understanding of how rivers flow, weather is affected by terrain, or even how humans and other animals flow or choose a specific habitat. This kind of information can only be gathered and made useful if topological information is present in the GIS.
Spatial Data[edit | edit source]
Spatial data comes in two major formats, as vector or raster data. The main difference is that a raster is usually a static background picture used to illustrate, whereas a vector is an intelligent ladder of information that can be selected and searched.
Vector (points, lines, polygons)
Vector data often represents anthropogenic (human) features such as roads, buildings, political boundaries (counties, congressional districts, etc.), and other features such as lakes and rivers.
Vector data is scalable without loss of resolution and is generally represented by XYZ points in a Cartesian frame reference.
Raster data is pixellated data, and the more pixels that map the data, the better the resolution. However, if raster data is enlarged, it simply enlarges the pixels, which then leads to a loss of resolution. There are many moves being made to make raster data more usable/searchable as it is much faster to collect, unlike vectors where each piece of data usually has a manual input origin.
Raster data is usually derived from satellite imagery or aerial photography (known as remote sensing). Ordinary cameras are only sensitive to visible light. Satellite sensors can capture not only visible light, but also the thermal, microwave, infrared or other types of energy emanating from the Earth's surface. This extra data provides information about sea surface temperatures, vegetation, ozone, etc. Remote sensing is also used to study other planets and extraterrestrial bodies, such as Mars.
Data Integrity[edit | edit source]
Persistent Storage and Vendor Independence[edit | edit source]
Practical GIS[edit | edit source]
Data sources[edit | edit source]
The U.S. Federal Government offers a wealth of spatial data for free or at cost, with major offerings from the United States Geological Survey (USGS), NASA and the U.S. Census Bureau. State governments also serve spatial data through GIS portals such as MassGIS. As well, many local governments have detailed GIS data of tax parcels, roads, buildings, etc. Other governments around the world also offer spatial data, though not necessarily for free. Private industry also offers GIS data, with TeleAtlas/GDT offering a wealth of vector data, while Ikonos and DigitalGlobe (QuickBird) provide high-resolution satellite imagery. Some public-domain data sources are listed at http://visual.wiki.taoriver.net/moin.cgi/EarthMapTool .