Geographic data are recordings of measurements taken at certain places and times. For example, land surveyors measure property boundaries and record them as the verbal descriptions and maps that accompany deeds. Aerial surveyors operate special cameras that record the light reflected from the Earth’s surface onto photographic film. And remote sensing satellites measure invisible reflected energy and record it in computer memory as sequences of digits. This course is about the characteristics of geographic data, and of the technologies used to produce them.
Data are valuable because we rely upon them to help us make decisions. Individuals rely on data about the characteristics of locations to decide where to live, and where to travel. Businesses need to keep track of where their customers live and work, so they can deliver products and services as efficiently as possible. Government agencies use data to formulate policies on resource use and environmental protection, to respond to emergencies, and to defend nation.
To make an informed decision, individuals and organizations need to know where to find the right data, or how to create it if it doesn’t exist. In the past, when geographic data were rare, it wasn’t very difficult to determine whether the data you sought were available. Usually they weren’t. These days, data are plentiful (although they are often unsuitable for their intended use). The geographic data available from public and private sources are so voluminous, in fact, that it has become a challenge for organizations to keep track of the data they have, and to find the data they need when they need it.
Information is data that has been selected or created to inform a particular decision. For example, the locations of a household, a highway, and a hospital are just data, until they are needed to dispatch an ambulance in response to an emergency. When used to inform those who need to know “where is the emergency, and what’s the fastest route between here and there?” the data are transformed into information. When time is of the essence, decision makers need to be able to find and retrieve the data they need—or to generate new data from the old—quickly and efficiently.
Information systems are technologies that assist decision makers in creating, maintaining, storing, and retrieving the data they need. The more complex the problem, and the more locations involved, the more valuable computerized information systems become. One common type of information technology is database management systems.
GIS (geographic information systems) arose out of the need to perform spatial queries on geographic data. A spatial query requires knowledge of locations as well as attributes. 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, database management systems need to be integrated with mapping systems.
CAD, desktop mapping, and GIS
Until about 1990, most maps were printed from handmade drawings or engravings. Geographic data produced by draftspersons consisted of graphic features inscribed on paper or film. (In this course, a feature is a representation of the location and extent of an entity.) To this day, most of the lines that appear on topographic maps published by the U.S. Geological Survey were 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.
Computer-aided design (CAD)
CAD systems were originally developed for engineers, architects, and other design professionals who needed more efficient means to create and revise precise drawings of machine parts, construction plans, and the like. In the 1980s, mapmakers began to adopt CAD in place of traditional map drafting. CAD operators encode the locations and extents of entities by tracing maps mounted on electronic drafting tables, or by key-entering location coordinates, angles, and distances. Instead of graphic features, CAD data consist of digital features, each of which is composed of a set of point locations. Calculations of distances, areas, and volumes can easily be automated once features are digitized. Unfortunately, CAD systems typically do not encode attribute data in forms that support spatial queries.
In 1988, a geographer named David Cowen illustrated the benefits and shortcomings of CAD for spatial decision making. He pointed out that a CAD system would be useful for depicting the streets, property parcel boundaries, and building footprints of a residential subdevelopment. A CAD operator could point to a particular parcel, and highlight it with a selected color or pattern. “A typical CAD system, Cowen observed, ‘could not automatically shade each parcel based on values in an assessor’s database containing information regarding ownership, usage, or value’, however.” A CAD system would be of limited use to someone who had to make decisions about land use policy or tax assessment.
Desktop mapping systems offer most of the capabilities of CAD systems, but also provide rudimentary linkages between location data and attribute data. A desktop mapping system user would be able to produce a map in which property parcels were automatically colored according to various categories of property values, for example. Furthermore, if property value categories were redefined, the map’s appearance could be updated automatically. Some desktop mapping systems even support simple queries that allow users to retrieve records from a single attribute file. Most real-world decisions require more sophisticated queries involving multiple data files. That’s where GIS comes in.
Geographic information systems (GIS)
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. David Cowen defined GIS as a decision support tool that combines the attribute data handling capabilities of relational database management systems with the spatial data handling capabilities of CAD and desktop mapping systems. 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.
Cowen cited an earlier study by William Carstensen (1986), who sought to establish criteria by which local governments might choose among competing GIS software products. Carstensen chose site selection as an example the kind of complex task that many organizations seek to accomplish with GIS. The related example illustrates the kind of functionality that distinguishes GIS from mapping systems or database systems alone.
A geographer named William Carstensen (1986) sought to establish criteria by which local governments might choose among competing GIS software products. Carstensen chose site selection as an example of the kind of complex tasks that many organizations need to accomplish with GIS. Site selection involves operations that go beyond mapping and require a GIS.
Given the necessary database, Carstensen advised local governments to expect that a fully functional GIS should be able to identify property parcels that are all of the following:
- At least five acres in size
- Vacant or for sale
- Zoned commercial
- Not subject to flooding
- Located not more than one mile from a heavy-duty road
- Situated on terrain whose maximum slope is less than 10 percent
The first criterion—identifying parcels five acres or more in size—might require two operations. As described earlier, a GIS ought to include the ability of a CAD system to calculate automatically the area of a parcel. Once the area is calculated and added as a new attribute into the database, a database query could produce a list of parcels that satisfy the size criterion. The parcels on the list might also be highlighted on a map, as in the example below.