### 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.

- Choose different types of thematic maps
- What column/field will the map be based on?
- What color scheme will we use?
- How many categories/classes?
- How should the classes be divided up?
- 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:

(1)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.

(2)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: