Make thematic maps
In this exercise you will make various thematic maps.
1. Start ArcMap and open a brand New map.
2. Click the AddData button (yellow one with plus sign) then navigate to the c:\apolicy\CalData folder.
Double-click Counties.shp to add that layer to the map.
Now you will label and color code the counties.
3. Right-click the Counties layer and click Label Features.
4. Right-click the counties layer and click Properties. Click the Symbology tab. Then in the Show box click
Categories.
5. For the Value field, pick Name. Then click the Add All Values button (not Add Values). When you
dialog matches the one below, click OK.
Have you created a Thematic Map yet? No this is still a reference map, you have color coded each county
and labeled it, by name. Next you will make a color coded population map.
6. Right-click the Counties layer and click Properties. In the Show box, click Quantities.
7. For the Value field, pick POP00_SQMI.
8. Click the Classify button. For the Classification Method, click Quantile.
Under the Classification method, notice that the number of classes is set to 5. That means, the map will
have five colors. California has 58 counties. The quantile method tries to put an equal number of counties
in each class. So each class will have about 11 counties. The top 11 counties will be drawn in the darkest
color. The least populated 11 counties will be drawn in the lightest color.
9. Click OK. And click OK again.
Now you have a thematic map.
Thematic maps attempt to show a quantitative attribute graphically. The reference map from earlier,
showed locations and the Name attribute, but name is qualitative. There are not quantities behind each
name. Next you will make a Dot map.
10. Right-click the Counties layer and click Label Features to turn the county names off. Right-click the
Counties layer and click Properties. Then in the Show box under Quantities, click Dot Density.
11. In the Field Selection list, scroll down and click on NO_FARMS97. (It’s the number of farms per
county in 1997.) Click the right arrow (>) button to add it to the list. In that list, Under Symbol, right-click
the dot and set the color to Black.
12. For Dot size type in 2. For Dot value type in 20. That means each dot will represent 20 farms.
13. For Background, click the left button. For the line’s Color, click the Color dropdown arrow set it to No
Color. Click OK. Click the right Background button dropdown arrow and click Sahara Sand for the color.
Click OK.
This thematic map shows the density of farms in California. You could really change the way this map is
read by a map reader, but altering the dot size and number of farms per dot. Bigger dots will make it look
like there are more farms. 1000 farms per dot will look like California has few farms.
Next you will make a graduated symbol map.
14. Right-click the Counties layer and click Properties. In the Show box under Quantities, click Graduated
Symbols.
15. For the Value field, pick AGE_65_UP.
16. Change the To: symbol size to 22. Click OK.
This thematic map attempts to show where the population over 65 is the greatest. You might look at the
map and say that makes sense lots of old people live in LA and San Francisco compared to Modoc County
in the northeast (top right one). But this map is a lie. Of course lots of older people live in those places, just
because lots of people live in those places.
What we should be looking at is, per county - what is the proportion or ratio of older people to all people in
a particular county. We could then see which counties have the most elderly people proportionately to all
the people in a county.
17. Right-click the counties layer and click Properties. Under the Value field, For Normalization, click
POP2003. Click OK.
This map now shows the ratio of the older population compared to total population. Most of the counties in
the north and east have the oldest population. There are lots more older people in those counties than
younger. San Bernardino County is very young.
Lastly you will create a chart map.
18. Right-click the counties layer and click Properties. In the Show box, click Charts and Pie.
19. In the Field Selection list, scroll WAY down and click on RecallYes. (It’s the number of Yes recall
votes in California’s 2003 recall election.) Click the right arrow (>) button to add it to the list. In the Field
Selection list, click on RecallNo. Click the right arrow (>) button to add it to the list.
20. Click the Size button. Change the size to 20 and click OK. Click OK again.
If needed, change the pie chart colors to get them to stand out. You can right-click on the yes or no color
boxes and click on a new color. For example, make the Yes’s Red.
It looks like the highest proportions of NO votes came from the San Francisco area counties.