Computed Fields and Thematic Maps

Before We Start

Choropleth Maps

Download this shapefile, Oakland_P1_P2_P3. It contains Oakland census tracts along with population, race5, and Latino/Non-Latino data from the 2010 census.

Make a choropleth map that shows census tracts graduated by population. Create a print composition window and add a legend. Align and save as an image file.


Examine each of the numbered items and make sure you understand what they do.

  1. Choose different types of thematic maps
  2. What column/field will the map be based on?
  3. What color scheme will we use?
  4. How many categories/classes?
  5. How should the classes be divided up?
  6. Double click on the entries in the label column to change the format of the numbers.

STOP AND THINK. Is this a really good way to show this data?

Creating a computed column

Suppose I want to create a column for population density (people per square kilometer) when I have a population field and a land area field (square kilometers). Basically this means creating a new column into which will go the quotient of two other columns:

\begin{align} density = \frac {PopInt} {ALand10} \end{align}

But I have to do a little fiddling with the units since I want square kilometers rather than square meters. Since there are 1000 m in a km, we have 1,000,000 square meters in a square kilometer.

\begin{align} Density(\frac {people}{km^2}) = \frac {PopInt(people)} {ALand10(m^2) \times \frac {1(km^2)}{1,000,000(m^2)}}= 1,000,000 \times \frac {PopInt(people)} {ALand10} \end{align}

I create the new column with the field calculator like this:


And my first inclination is to use the following calculation:


But this may turn out to produce incorrect results - negative densities. The reason is that I am doing a calculation, POPULATION DIVIDED BY AREA, that can produce decimal numbers but I am putting these into an integer field.

I can remedy this problem by writing my update formula like this:


Equal Interval, Quantile, Natural Breaks, Standard Deviation, Pretty Breaks