Lab 7: Mapping Census 2000 with ArcGIS

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The first map shows counties in the contiguous United States color-coded by the number of people living in them in year 2000.  The darkest counties have the most people, while the lightest counties have the least.  These values are calculated by counting how many people live in each county; then the counties are sorted into ranges of numbers of people to be represented by different shades of color.  The gradient color ramp, from dark purple to light blue, is great for this kind of data because it easily shows at first glance where the counties with the most people are compares to the ones with the least.

The second map (top right) shows counties in the contiguous United States color-coded by the difference from 1990 to 2000 in the number of people per county.  The difference in number of people per county is calculated by taking the number of people in each county in year 2000 and subtracting the number of people in that same county ten years ago in 1990.  If the number is positive, it means there was a net growth in that county's population; conversely, if it is negative it means there was a net loss.  The color ramp, from bright pink to dark green, is good for this kind of data because it shows the contrast very well between counties with the biggest gains or losses, which we are more interested than counties that did not change very much.  The colors help to show a clear migration away from the middle of the country towards the coasts and big cities.

The third map (bottom left) shows counties in the contiguous United States color-coded by the percent change from 1990 to 2000 in the total population of each county.  The percent change in the total population of each county is calculated by subtracting the total population in 1990 from the total population in 2000 in each county, then dividing that sum by the total population in 2000 in each county, and finally multiplying the resulting number by 100 to get your percent change.  The selected color ramp, from orange to purple but with most of the ranges a shade of purple, is great for this kind of data because the states with a negative percent change really stick out and there are far fewer counties that experienced a loss than a gain in population, so the gains are all in shades of purple while the negative percentages are orange.

The fourth and last map shows counties in the contiguous United States color-coded by their population densities in year 2000.  The population density is calculated by dividing the total population in each county in year 2000 by the total area of each county (in square miles).  This calculation gives you the population density in people per square mile for each county.  The selected color ramp from almost white to green to dark navy blue is ideal for this data because the lighter colors naturally indicate space and openness whereas the dark colors are more condensed and represent the counties with the highest population densities.