users, while classification schemes can have a significant impact on how people interpret data. Each of these issues is discussed in more detail below.
Map types
Maps can be classified into two broad categories: reference maps and thematic maps. Reference maps have a wide range of general information on them. For instance, US Geological Survey topographic maps have information on natural and cultural features such as elevation, roads, public buildings, water features, and political boundaries. Many online maps, such as Google Maps, also have general reference information on roads, businesses, public institutions, entertainment, and more. When you create a new map in ArcGIS Online, you are presented with a topographic reference map as a basemap (figure 1.8).
Figure 1.8.Reference map. The topographic basemap in ArcGIS Online includes basic reference information. Data sources: World Topo Map. HERE, DeLorme, Intermap, increment P Corp., GEBCO, USGS, FAO, NPS, NRCAN, GeoBase, IGN, Kadaster NL, Ordnance Survey, Esri Japan, METI, Esri China (Hong Kong), swisstopo, MapmyIndia, © OpenStreetMap contributors, and the GIS User Community.
Thematic maps, in contrast, focus on a single topic, or theme. This type of map may show population density, average income, dominant language, soil type, annual precipitation, or any number of other physical or cultural features. When you add layers to ArcGIS Online (excluding basemaps), such as Living Atlas of the World layers, you are adding thematic maps. Thematic maps can be represented in several different ways, including choropleth maps, graduated circle maps, isoline maps, dot density maps, flowline maps, and cartograms.
A common type of thematic map is the choropleth map. Choropleth maps use shades or colors to represent values of a variable within an area, such as census tracts, cities, counties, or states (figure 1.9).
Like choropleth maps, graduated circle maps also represent values of a variable within an area. However, instead of using shades or colors to distinguish values, circles of different sizes are used. A large circle represents a high value, while smaller circles represent lower values (figure 1.9).
Figure 1.9.Thematic maps: Choropleth and graduated circle. Choropleth maps use colors or shades within areal features to represent data. Graduated circle maps use circles of different sizes to represent data. Log in to your ArcGIS Online account to explore these maps. Choropleth map of median income: https://arcg.is/1WiuC4. Graduated circle map of market potential for regular exercise routines: https://arcg.is/1jjKHz. Maps by author. Data sources: 2016 USA Median Household Income, Esri, US Census Bureau. 2016 USA Adults That Exercise Regularly, Esri and GfK US, LLC, the GfK MRI division.
Isoline maps consist of lines that connect points of the same value. Typically, these are used to map continuous surfaces, where data values change often over the earth’s surface, such as with temperature or elevation (figure 1.10).
Dot density maps use dots to represent a specified value within a geographic feature (figure 1.10). If the population of a county is 10,000 people, then a dot density map where one dot equals 1,000 people would have ten dots randomly placed within the county borders.
Figure 1.10.Thematic maps: Isoline and dot density. Elevation contours on a topographic map are a type of isoline. Dot density maps use dots to represent values, such as number of households. Log in to your ArcGIS Online account to view these maps. USGS National Map with topographic isolines: https://arcg.is/91zf1. Dot density map of income extremes: https://arcg.is/m8DHL. Data sources: USGS National Map by Esri—USGS The National Map: National Boundaries Dataset, National Elevation Dataset, Geographic Names Information System, National Hydrography Dataset, National Land Cover Database, National Structures Dataset, and National Transportation Dataset; US Census Bureau—TIGER/Line; HERE Road Data. Income Extremes by Lisa Berry—Esri.
Flowline maps use lines of varying thickness to show the direction and quantity of spatial interaction between places. Thicker lines represent larger quantities, while thinner lines represent smaller quantities. These maps are often used to represent trade and migration flows between countries (figure 1.11).
Cartogram maps distort the area of features based on the value of a variable. A cartogram of population will show places with more people as larger and places with fewer people as smaller. In figure 1.11, state populations are shown for three time periods. The size of each state varies according to its population size. Note how western states, such as California, change in size in each time period.
Figure 1.11.Thematic maps: Flowline and cartogram. The flowline map shows Syrian refugee flows in 2014. View the Syrian refugee flow map at https://storymaps.esri.com/stories/2016/the-uprooted/index.html. Cartogram from US Census. Image sources: The Uprooted by Esri Story Maps Team; data sources: UNHCR, Airbus Defense and Space. Cartograms of State Populations in 1890, 1950, and 2010 by US Census Bureau; data sources: Census 2010 tables.
Map scale
Scale is another issue to be aware of when creating and interpreting maps. Real estate companies often produce maps with no scale or with distorted scales to make desirable places seem closer. For instance, a real estate map may include the location of a new housing development, with lines showing freeways, beaches, and parks, giving the impression that they are all nearby. However, with no given scale, these places are often drawn to appear much closer than they really are.
Properly produced maps include a clearly defined map scale that indicates the ratio of map distance to real-world distance. The scale allows map readers to measure the size of features and the distance between them. Map scale is represented verbally, graphically, or as a ratio or fraction.
Verbal scale: 1 inch equals 1 mile
Graphic scale:
Ratio scale: 1:24,000
Fraction scale: 1/24,000
In the case of ratio and fraction scales, the units remain the same on both sides of the scale. Using the examples noted. 1 inch on the map represents 24,000 inches in the real world.
Maps are often described as being large scale or small scale (figure 1.12). A large-scale map refers to a larger fraction or ratio, while a small-scale map refers to a smaller fraction or ratio. For instance, 1:24,000 is a larger ratio than 1:100,000, so it is a larger scale map.
Large-scale maps are more “zoomed in.” They cover a smaller area and include more detail. A city map is a larger-scale map than a country map. Small-scale maps are “zoomed out” and cover a larger area with less detail. A country map is a smaller-scale map than a city or neighborhood map.
Figure 1.12.Small-scale and large-scale maps. Large-scale maps are more zoomed in than small-scale maps. Explore this map at https://arcg.is/rjL8K.
