A point, line, or polygon feature on the surface of the world is on a three-dimensional spheroid, whereas features on a paper map or computer screen are on a flat surface. The mathematical transformation of a world feature into a flat map is called a projection. There are many projections, some of which you will use in chapter 4. Each projection has its own rectangular coordinate system and a (0,0) origin conveniently located so that coordinates are positive and have distance units, usually in feet or meters.
All projections necessarily cause distortion of direction, shape, area, or length in some combination. So-called conformal projections preserve shape at the expense of distorting area. Some examples are the Mercator cylindrical and Lambert conic projections. Equal-area projections are the opposite of conformal projections: They preserve area while distorting shape. An example is the Albers equal-area projection (Clarke 2003, 42 – 44).
Map scale is often stated as a unitless, representative fraction; for example, 1:24,000 is a map scale where 1 in. on the map represents 24,000 in. on the ground, and any distance units can be substituted for inches. Small-scale maps have a vantage point far above the earth and large-scale maps are zoomed in on relatively small areas. Distortions are considerable for small-scale maps but negligible for large-scale maps relative to policy, planning, and research applications.
GIS maps are composites of overlying map layers. For large-scale maps such as figure 1.2, the bottom layer can be a raster map that has one or more vector layers on top, placed in order so that smaller or more important features are on top and not covered up by larger contextual features. Small-scale maps, such as figure 1.1, often consist solely of vector-map layers. Each vector layer consists of a homogeneous type of feature — points, lines, or polygons.
Digital map infrastructure
GIS is perhaps the only information technology that requires a major digital infrastructure. The map layers of the infrastructure are referred to as basemaps — namely, a collection of standards, codes, and data designed, built, and maintained by government. Vendors provide valuable enhancements to the digital map infrastructure, but for the most part, it is a public good financed by tax dollars. Without this infrastructure, GIS would not be a viable technology.
The National Spatial Data Infrastructure (NSDI), developed by the Federal Geographic Data Committee (FGDC at http://www.fgdc.gov), incorporates policies, standards, and procedures that allow organizations to produce and share geographic data. The Geospatial One-Stop website (http://geo.data.gov/geoportal), which is part of the NSDI, provides access to spatial data. Websites change often. The websites noted in this book could be named differently or redirected to different sites, but the information will still be available if you search for it. GIS provides a way to use this wealth of publicly available data in your own studies.
Perhaps the most useful spatial data for health applications comes from the US Census Bureau in the form of TIGER/Line maps. These maps are available by state and county for many classes of layers. These classes, and examples of each, include the following:
• Political: states, counties, county subdivisions (towns and cities), and voting districts
• Statistical: census tracts, block groups, and blocks
• Administrative: ZIP Codes and school districts
• Physical: highways, streets, rivers, streams, lakes, and railroads
You can download TIGER/Line map layers in GIS-ready formats at no cost from the US Census Bureau website (http://www.census.gov). Steps for doing so are included in chapter 5.
Census data that corresponds to TIGER/Line maps of statistical boundaries is tabulated by census-area codes and American National Standards Institute (ANSI) codes (http://www.census.gov/georeference/ansi.html). ANSI codes are “a standardized set of numeric or alphabetic codes issued by the American National Standards Institute (ANSI) to ensure uniform identification of geographic entities through all federal government agencies,” according to the ANSI website. Data from the decennial census is available at no cost from the Census Bureau. Steps for downloading census data and preparing it for GIS use with TIGER/Line maps are included in chapter 5.
The US Geological Survey (http://www.usgs.gov/aboutusgs/) is the “nation’s largest water, earth, and biological science and civilian mapping agency … [and it] collects, monitors, analyzes, and provides scientific understanding about natural resource conditions, issues, and problems,” according to the USGS website. Among its products that are useful for health applications are national map orthoimagery aerial photographs (such as the one used in figure 1.2). Full national coverage of the most recent national map orthoimagery is available from USGS via The National Map at http://viewer.nationalmap.gov/viewer/. Steps for downloading USGS maps are included in chapter 4.
Local governments provide many of the large-scale map layers in the United States, and most features in this data are smaller than a city block. This data includes deeded land parcels and corresponding real property data files on land parcels, structures, owners, building roof footprints, and pavement digitized from aerial photographs. You can often obtain such map layers and data for nominal prices from local governments. Some local health departments provide limited health data.
Unique capabilities of GIS
Maps were historically made for reference purposes. Street maps, atlases, and USGS topographic maps are all reference maps. It wasn’t until the advent of GIS, however, that analytic mapping became widely possible. For analytic mapping, an analyst collects and compiles related map layers, builds a database, and then uses GIS functionality to provide information for understanding or solving a problem. Before GIS, analytic mapping was limited to organizations such as city planning departments. Analysts did not have digital map layers, so they made hard-copy drawings on acetate sheets that could be overlaid and switched in and out to show before-and-after maps for a new facility such as a baseball stadium or a hospital. Using GIS, however, anyone can easily add, subtract, turn on and off, and modify map layers in an analytic map composition. This capacity has led to a revolution in geography and an entirely new tool for organizations of all kinds.
As figures 1.1 and 1.2 show, maps use symbols, which are defined in map legends. Graphical elements of symbols include fill color, pattern, and boundaries for polygons; width, color, and type (solid, dashed, curved) for lines; and shape, color, and outline for points. A GIS analyst does not individually apply symbols to features, but applies and renders a layer at a time based on attribute values associated with geographic features.
For example, given a code attribute for schools that has the values “public,” “private,” and “parochial,” a GIS analyst can choose a green, circular, 10-point marker for public schools; a blue, square, 8-point marker for private schools; and an orange, triangular, 8-point marker for parochial schools. These three steps render all schools in a map layer by the desired point markers.
Similarly, we created the color-shaded county map layer in figure 1.1 based on an attribute that provides the lung cancer mortality rate of white males by county. A map that uses fill color in polygons for coding is called a choropleth map. In this case, it shows an equal-interval numeric scale, rendered using a gray monochromatic