There are times when you need to create a thematic map that displays two different thematic variables at the same time. For example, if you are trying to show population growth and literacy at the same time, you might want to draw more than one theme on the same layer. Bivariate thematic mapping uses point or line objects to represent two thematic variables. For example, a star can represent one variable, such as the number of teenagers, while a blue fill for the star represents their annual purchase amounts. To create a bivariate map in MapInfo Pro, you create two thematic maps, and layer one over the other so that the objects display two variables.
The only types of thematic maps suitable for bivariate mapping are ranged and individual values maps. You can choose between two combinations for a bivariate map, depending on your data:
- two ranged maps
- one ranged map and one individual values map
Prism maps can also be used for bivariate theme mapping. For more information, see Working with Prism Maps. If you have a non-numeric variable, one of your maps must be an individual values map. You cannot create a bivariate map with two non-numeric variables.
This feature is found from the Create Thematic Map wizard or by clicking the Modify Thematic command on the LEGEND tab.
You can use transparent pattern styles over an original layer to make the underlying layer(s) visible. This allows the thematic variable to retain the pattern but display transparently over another thematic variable. For more information, see Creating a Transparent Bivariate Layer.
Example of Bivariate Thematic Mapping
To illustrate this process, suppose as a sales manager you want to know which businesses have generated the most sales in your territory. You have a table of customers with their total sales and sales representative.
First, you must decide what combination of thematic maps to use. Because you are working with sales representatives (non-numeric data) and total sales (numeric data), you must create both an individual values map and a ranged map.
Second, decide how you want to display the data. You can use a combination of color and size attributes to display your thematic variables. You want to assign a color to each sales representative and use a different size symbol to represent each range of sales figures. Note that this is different than a graduated symbols map, where the symbol is scaled to one value. Here, the size of the symbol represents a range of values.
Third, you must create a separate thematic map for each variable. For the Individual Values map, use the table of customers as the table and choose Sales_rep as the field to which to assign values. MapInfo Pro assigns each sales representative a different color.
Next, create a ranged thematic map that shows the customers' total sales. Choose the Customer table as the table, and choose Sales as the field. Divide the total sales into six ranges with approximately the same number of customers in each range. In the Thematic - Step 3 of 3 wizard panel choose the Ranges button, change the number of ranges to six, and verify that the type of range is set to Equal Count.
To show the ranges as sized symbols, choose Styles in the Thematic - Step 3 of 3 wizard panel. In the Autospread group choose Size. The symbols in the Styles group change to graduated size. However, the only variable you want to display is size, as the symbols' color is coming from the individual values map. Choose Options to display the Apply group and choose the Size button again.
The selection in the Apply group overrides any selection you make in the Autospread group. To specify or change which symbol attributes display, you must choose Options, and choose the Color or Size button in the Apply group.
The symbols become hollow circles that are graduated in size. Choose OK, and your ranged map draws over the individual values map. The symbol color from the individual values map indicates the sales representative, and the size of the symbol represents a range of total sales figures. The map legend lists the bivariate map as two separate thematic maps, in effect displaying two legends. The legend does not combine the two variables.
 |
 |