Basic Concepts in Geographic Information Systems (GIS)
A Geographic Information System (GIS) is a software package that is helping to digitize the world around us. Digital maps and location based information helps government agencies, businesses and even people like you and me keep up with the changing pace of the world around us. GIS applications improve efficiencies, reduce costs, and bring together data in ways that wasn't possible even 10 years ago. This article focuses on some of the basic components and aspects of Geographic Information Systems. If you'd like a little more information on what a GIS is and what it can do please check out another article I wrote called What are Geographic Information Systems (GIS) and What are They Use For? For further reading regarding how GIS applications can be used business please check out my article called How can Geographic Information Systems (GIS) Improve your Business?
Geographically Based Data Sets
A Geographic Information System (GIS) is a system of computer applications that can be used to display, manipulate, and analyze spatially varied information from multiple sources all in one place. Most often the datasets used in a GIS are categorize into multiple categories for easier storage and use. Each dataset that a GIS can support is divided into two main parts: graphical (spatial) information and tabular (attribute) information. Spatial data is data that is geo-referenced or location specific and is what is shown graphically on the computer screen. Each piece of graphic information is called a feature. Features can be points, lined or even polygons.
The attribute or tabular information is text based or numerical information that describe each of the features. The tabular information is linked to the graphical information and includes a unique ID number used to represent each point, line or polygon. Examples of tabular data can include such things as addresses, coordinates, area, length, sales information, road names, etc. The possibilities for data association between tabular and graphical information are endless.
Spatial data can often be further divided into two major types. The first type is raster data which is usually in the form of images such as aerial photographs or imported scans of old maps. The raster data stores the location and color value of each pixel that forms the image. The second type of data is vector based. Vectors can be a combination of linework, polygons and curvilinear data. This information is stored using a combination of location specific point, lines, and arcs. Raster imagery can lose quality and become blurred when scaled. However, vector data is scalable to any size without losing any integrity.
Another way to look at data types is understanding that some data is discrete while others are continuous. Discrete data is usually vector based and has specific information located at specific points with gaps in between. On the other hand continuous data is usually raster based and no gaps are present. Anywhere within the domain of a raster there will be information.
When each dataset is loaded and displayed in the GIS map window it is called a layer. When multiple layers are used in the same map window they can be stacked, color coded, and symbolized to represent an endless array of map compositions. Stacked datasets can also be manipulated by adjusting the colors ramps, hatching, shading and/or transparency levels to reveal new relationships that would not have otherwise been obvious with traditional maps. In essence, the main purpose of a GIS is to describe, analyze and display a variety of spatial data in a way that only a digital map can.
Components of a GIS Software Application
On the surface a GIS is simply a combination of software, hardware and data. However, more specifically, a GIS usually has a data hub (typically a collection of data on a server) and a graphical user interface for the people who will be using and manipulating the data. In this way all users of the GIS can be connected to the same data sets. When data sets are updated or augmented everyone who uses the data can see the changes. This ensures consistency of information and will also help to avoid duplication of work. Web-based applications can also be added so that remote users can see the same information that everyone else can.
Differences between CAD and GIS
In some ways CAD and GIS software are very similar. Both software tools can display data such as points, lines, and polygons to a specific scale. Both CAD and GIS tools can also be used to create maps and representative drawings. The similarities between the two types of software applications usually ends there. Typically CAD is used for design of intricately detailed objects such as manufactured products, buildings, commercial site layouts and roadways. The common theme with CAD work is that it is used for representing manmade projects. However GIS is used to analyze information on a much larger scale and can often encompass things that make up the natural world such as forests, soil strata and even rivers and floodplains. As mentioned previously, GIS is great for attaching tabular data to graphical information while most CAD tools lack that ability. CAD is a great engineering tool however its capabilities are not honed for most geographically based work that a GIS can perform.
Differences GIS and Spreadsheets/Databases
The main difference between spreadsheets/databases and a GIS is that a GIS uses geography and location information as the main what to display data and relate it with other data. Even a relational spreadsheet or complex database lacks the clear functionality of a graphical tool that bases everything on location. When it comes to location information spreadsheets limited to simply listing numerical information such as coordinates (lattitude, longitude, etc) or addresses in rows and columns. A spreadsheet with this kind of information can easily be converted into a form that can be imported to a GIS. In addition to this, data within a GIS can have spreadsheet like structures supporting them (see tabular data discussed above). For example, you could have a series of lines representing a roadway network with a spreadsheet joined to those lines with information such as number of lanes, roadway width, pavement surface, etcetera for each line segment.
Benefits of Geographically Based Data Integration
By far the most obvious benefit of creating a GIS is the fact that it eliminates all manual forms of geography based analysis. There is no longer a need to print large maps on transparency sheets nor does one need a light table to get the effect of layering multiple printed sheets on top of one another. The affects and problems of differently scaled maps can also be addressed as a GIS can convert and project all data on the same coordinate system and scale. The second benefit of a GIS is that it centralizes and organizes the data for use in one comprehensive system. No longer will there be lost records and all of the data created and collected by the user can be stored and used again in the future with ease. And yet a third benefit is the fact that there is an infinite variety of maps can be created with even just a few datasets. Cartographers are no longer limited to present geographic information in only one way. Most GIS programs can easily symbolize or represent data in any manner imaginable and at essentially any scale.
In order to represent spatially varied data everything needs to be placed on a common coordinate system. In the mapping world there are three main types of coordinate systems. The first system is called the Cartesian Coordinate system and can be represented by a grid with a numbering system that can locate information on a horizontal and vertical axis. The second systems is the polar coordinate system. This is an easy way to locate information about a central using only an angle and a distance (radius). In many junior high and high school math classes students learn about these basic coordinate systems. The third type of coordinate system is a global coordinate system. At its most basic level a global coordinate system is where two numbers (latitude and longitude) are used to reference a specific location on the earth.