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- In the Contents pane, right-click the US_tract_1980_Harris_County layer name and select Symbology.
- In the Symbology pane, click the primary drop-down menu and select Graduated Colors.
- Use the 'Field' drop-down menu to select the AV0AA1980 field. NHGIS data downloads include a read-me file that defines the codes for each field (column). AV0AA1980 stores the number of people within each census tract for the year 1980.
The Map View view now displays a choropleth map, where the darker colors represent higher numbers of people. In studying the map, it appears as if there is no discernible pattern to population numbers. While this is true according to raw counts per census tract, there could be differences in the census tracts that are unaccounted for in this symbology. Now you will try normalizing by the area of the census tracts. - Use the 'Normalization' drop-down menu to scroll to the bottom and select the last Shape_Area field.
Note: the projection of the census layer is GCS WGS 1984. Therefore, the layer is unprojected and the coordinates are stored in angular units of decimal degrees. Therefore, the Shape_Area field is displaying square decimal degrees and the map is displaying number of people per square decimal degree. This is a somewhat incomprehensible unit, however, the values are still proportional to how they would be in a different unit and the relative coloring on the map remains correct. Notice that according to the density of population count, the greatest amount of people appear to be inside the major loops. - On the lower half of the Symbology pane, click the Histogram tab.
- Use the 'Method" drop-down menu to select Equal Interval.
- Use the 'Method" drop-down menu to select Quantile.
- Use the 'Classes' drop-down menu to select 20.
- Use the 'Classes' drop-down menu to select 2.
- Use the 'Color Scheme' drop-down menu to select a different color scheme of your choice.
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