Finally, thanks to the entire team at Esri and Esri Press.
Part 1
GIS benefits and map basics
Chapter 1
Introducing GIS and health applications
Objectives
• Define GIS
• Define spatial data for graphic and image map layers
• Review the national infrastructure for spatial data
• Review the unique capabilities of GIS
• Demonstrate how GIS can be used for health applications
• Introduce ArcGIS and its user interface
• Introduce online GIS tools
Geographic information systems (GIS) is a technology that has unique and valuable applications for policy makers, planners, and managers in many fields, including public health and health care. GIS health applications include an academic organization’s use of GIS for medical research, a hospital or managed-care organization’s improved delivery of health-care services, and a public health department’s use of mapping and spatial statistics for disease surveillance and analysis. GIS software and applications allow visualization and processing of data in ways that were not possible in the past. The purpose of this book is to provide hands-on experience with the use of ArcGIS for Desktop software in the context of health applications. You need no previous experience using GIS.
This chapter describes GIS and its inputs and special capabilities and follows with a discussion of health issues and GIS applications. We also preview the upcoming tutorials in this book, and then use short tutorials in this chapter to introduce you to the use of ArcGIS software.
What is GIS?
GIS is computer technology that engages geographers, computer scientists, social scientists, planners, engineers, and others in spatially analyzing issues. Consequently, it has been defined from several different perspectives (see Clarke 2003). We prefer a definition that emphasizes GIS as an information system: GIS is a system for input, storage, processing, and retrieval of spatial data. Except for the additional word “spatial,” this is a standard definition of an information system. Spatial components include a digital map infrastructure, GIS software with unique functionality that focuses on location, and new mapping applications for organizations of all kinds. Definitions of these distinctive aspects of GIS follow.
Spatial data
Spatial data includes the locations and shapes of geographic features, in the form of either vector or raster data. Vector maps have features drawn using points, lines, and polygons to represent discrete geographic objects such as automobile accident locations, streets, and counties. (A polygon is a closed area that has a boundary consisting of connected straight lines.) Raster maps are generally aerial photographs, satellite images, or representations of surfaces such as elevation, which are used to represent continuous geographies.
For example, figure 1.1 is a vector map that has three polygon map layers (state and county boundaries and lakes), a line layer of rivers, and a point layer of cities that have populations of 250,000 or more. The state and county boundaries are coterminous — that is, they share boundaries and do not overlap each other. Color fill is used within the county polygons to show the mortality of lung cancer for white males. This map has some striking geographic patterns that are discussed later in this chapter.
Associated with individual point, line, or polygon features are data records that provide identifying and descriptive data attributes. For example, in figure 1.1, the labels for the names of states and cities come from tables of attribute records associated with each map layer. You will revisit this map in tutorials 1-3 and 1-4 in this chapter where you will use ArcGIS to explore map layers and spatial patterns of cancer mortality.
Figure 1.1 Lung cancer mortality per 100,000 white males, 2000 – 2004.
Sources: (a) Esri Data & Maps; (b) Cancer Mortality Maps website, National Cancer Institute.
Raster maps are stored in standard digital image formats, such as tagged image file format (TIFF) and Joint Photographic Experts Group (JPEG) files. An image file is a rectangular array, or raster, of very small, square pixels. Each pixel, or cell, has a single value and solid color and corresponds to a small, square area on the ground, from 6 in. to 3 ft on a side for high-resolution images. Accompanying the image files are world files that provide georeferencing data, including the upper-left pixel’s location coordinates and the width of each pixel in ground units. Using world file information, GIS software can assemble individual raster datasets into larger areas and overlay them with aligned vector datasets.
Viewed on a computer screen or on a paper map, a raster map can provide a detailed backdrop of physical features. In figure 1.2, an aerial photograph overlaid with vector map layers shows locations where serious injuries of child pedestrians occurred in relation to public parks that have playgrounds. The two boundaries surrounding the parks are 600 ft and 1,200 ft buffers used to study injury rates near parks. You will explore the GIS data behind this map in depth in chapters 4 and 7, where you will download similar orthoimagery and create and use buffers similar to those in figure 1.2.
Figure 1.2 Locations of serious injuries to child pedestrians in eastern Pittsburgh, Pennsylvania.
Sources: (a) Children’s Hospital of Pittsburgh; (b) US Geological Survey; (c) City of Pittsburgh, Department of City Planning.
Map layers have geographic coordinates, projections, and scale. Geographic coordinates for the nearly spherical world are measured in polar coordinates, and the angles of rotation are measured in degrees, minutes, and seconds, or decimal degrees. The (0,0) origin of the coordinate system is generally taken as the intersection of the equator and the prime meridian (great circle), which passes through the poles and an observatory in Greenwich, England. Latitude is measured north and south for up to 90 degrees in each direction. Longitude is measured to the east and west of the origin for up to 180 degrees in each direction.
The world is not quite a sphere because the poles are slightly flattened and the equator is slightly bulged out. The world’s surface is better modeled by a spheroid, which has elliptical cross sections and two radii, instead of the one radius of a sphere. The mathematical representation of the world as a spheroid is called a datum; for example, a datum commonly used for North America is North American Datum of 1983 (NAD