Overview

Information is a fundamental commodity, as discussed in Chapter 1, that it has become difficult or impossible for government agencies, businesses, other organizations, as well as individuals, to do without. Many of the problems and opportunities faced by organizations of all types are so complex, and involve so many locations, that the organizations need assistance in creating useful and timely information. That's what information systems are for. Information systems are computer-based tools that help people transform data into information.

Suppose that you've launched a new business that manufactures solar-powered lawn mowers. You're planning a mail campaign to bring this revolutionary new product to the attention of prospective buyers. But, since it's a small business, you can't afford to sponsor coast-to-coast television commercials, or to send brochures by mail to more than 100 million U.S. households. Instead, you plan to target the most likely customers - those who are environmentally conscious, have higher than average family incomes, and who live in areas where there is enough water and sunshine to support lawns and solar power.

Fortunately, lots of data are available to help you define your mailing list. Household incomes are routinely reported to banks and other financial institutions when families apply for mortgages, loans, and credit cards. Personal tastes related to issues like the environment are reflected in behaviors such as magazine subscriptions and credit card purchases. Firms like Claritas collect such data and transform it into information by creating "lifestyle segments" - categories of households that have similar incomes and tastes. Your solar lawnmower company can purchase lifestyle segment information by 5-digit ZIP code, or even by ZIP+4 codes, which designate individual households.

It's astonishing how companies like Claritas can create valuable information from the millions upon millions of transactions that are recorded every day. Their products are made possible by the fact that the original data exist in digital form and because the companies have developed information systems that enable them to transform the data into information that companies like yours value. The fact that lifestyle information products are often delivered by geographic areas, such as ZIP codes, speaks to the appeal of geographic information systems (GIS). The scale of these data and their potential applications are increasing continually with the advent of new mechanisms for sharing information and making purchases that are linked to our GPS-enabled smartphones (more on those in Chapter 5). Here, we focus on how all the geographically-referenced data is organized, stored, and accessed in systems that turn the data into information.

A Geographical Information System (GIS) is a computer-based tool used to help people transform geographic data into geographic information.

The definition implies that a GIS is somehow different from other information systems, and that geographic data are different from non-geographic data. Let's consider these differences.

GIS arose out of the need to perform spatial queries on geographic data (questions addressed to a database such as wanting to know a distance or the location where two objects intersect). A spatial query requires knowledge of locations as well as attributes about that location. For example, an environmental analyst might want to know which public drinking water sources are located within one mile of a known toxic chemical spill. Or, a planner might be called upon to identify property parcels located in areas that are subject to flooding. To accommodate geographic data and spatial queries and help users understand the answer to their queries, the system for managing your data (i.e., a database management system) needs to be integrated with a mapping system. Until about 1990, most maps were printed from handmade drawings or engravings or at least had multiple manual processing steps between data collection and map generation. Geographic data produced by draftspersons consisted of graphic marks inscribed on paper or film. To this day, most of the lines that appear on topographic maps published by the U.S. Geological Survey were originally engraved by hand. The place names shown on the maps were affixed with tweezers, one word at a time. Needless to say, such maps were expensive to create and to keep up to date. Computerization of the mapmaking process had obvious appeal.

As stated earlier, information systems assist decision makers by enabling them to transform data into useful information. GIS specializes in helping users transform geographic data into geographic information. In particular, GIS enables decision makers to identify locations or routes whose attributes match multiple criteria, even though entities and attributes may be encoded in many different data files. A geographic information system uses a data model to incorporate geographic features from the real world into digital data representations. The geographic data are stored in a database and later displayed on a map. Users commonly manipulate and create new data within a database in order to solve a problem. For instance, a city planner may want to enhance public transportation by adding new bus lines. One important issue for the planner is to make sure new bus lines serve a large population. If the planner already has a geographic database with information on population and area for every city block, population density can be computed (density = population/area) into the existing database (Table 4.1).

Example of a portion of a table stored in a geographical database. This fictional table depicts data by Census Block (a geographical unit discussed in Chapter 3). In this database, this table will be dynamically linked to another that has coordinate information to define where the Census blocks referred to are in the world.

Credit: Jennifer M. Smith, Department of Geography, The Pennsylvania State University.

The hypothetical database above reveals that for block 3, there are on average 1.08 people per square meter. Based on the database computations, the city planner should make the bus line stop on along block 3 where the most people are located per square meter.

This chapter will explore the characteristics of digital data and how it is represented in a GIS by discussing how it is stored, managed, and manipulated.

Objectives

Students who successfully complete Chapter 4 should be able to:

1. distinguish the difference between features and attributes;
2. identify the different attribute measurement scales and basic operations for each type;
3. understand what a database management system is and identify what it is used for;
4. understand what metadata is and why it is used;
5. identify the difference between vector and raster data.

Chapter lead author: Jennifer M. Smith.
See citation to the full text and its precursor below; portions drawn directly from that text.