Before you create a thematic map, it is important to know about the elements that make up a thematic map and how to put them together. This section will discuss thematic variables, where you can obtain your data, using data from the same table or another table, and the arrangement and display of thematic layers.
What is a Thematic Variable
The data that you display on your thematic map is called the thematic variable. Depending on the type of thematic analysis you are performing, your map can show one or more thematic variables. Ranges of values, grid shading, graduated symbols, dot density, and individual values maps all examine one variable. With bar or pie charts, you can display more than one thematic variable at a time.
A thematic variable can also be an expression. Choose Expression from the field list in place of a data field to construct a statement that derives information from the data in your table(s). Although an expression can be made up of more than one variable (for example, POP_1990 - POP_1980), for purposes of thematic mapping, a complete expression is equivalent to one thematic variable. For more information, see Querying Your Data in MapInfo Pro, and Using Expressions in a Thematic Map.
You can also create bivariate thematic maps, where one map object, such as a symbol, represents two different pieces of data. The symbol color, for example, can represent one thematic variable, and the symbol size can represent another.
Where to Obtain the Data
Before you begin your thematic map, you need to decide what information you want to display and locate where that information resides. It can either be in the table on which you are basing the map, or it can be in another table.
If the data is in the same table on which you are basing the map, choose the desired field directly in the Field list box in the Create Thematic Map - Step 2 of 3 wizard panel.
If the data is in another table, you must first bring the data into the table on which you are basing the thematic map. This requires creating a temporary column using Update Column.
Each situation is described in the next few sections.
Using Data from the Same Table
If you are using data from the same table, choose the table and field on which you want to base your thematic map in the Create Thematic Map - Step 2 of 3 wizard panel.
For example, you have a table of parking meters that contains the location of the parking meter and the last time the parking meter was emptied. Using Individual Values you want to shade the parking meter symbols according to the last time each meter was emptied. MapInfo Pro will assign a color to each time. In the Create Thematic Map - Step 2 of 3 wizard panel, choose the parking meter table as your table, and choose the field that contains the time each meter was emptied.
Using Data from a Different Table
The Join feature within the Create Thematic Map - Step 2 of 3 wizard panel enables you to use data from other open tables to create a thematic map. Choose Join in the Field list box to display the Update Column dialog box where you can create a temporary column in the base table.
The temporary column can contain data taken directly from the other table, or you can aggregate the data to create derived information for the temporary column.
For example, you have two tables: a table of county boundaries and a table of police stations. You want to shade the table of county boundaries according to the number of police stations in each county.
To do this, all the information you want to use must be in the county table. Therefore, you must add police station data to this table.
Using Update Column, you create a temporary column in the county boundaries table that will store the police station information. To create this column, the two tables must have a link so that MapInfo Pro can access the data that goes into the temporary column. The link can either be a matching field (like county name), or you can make the link geographically (police stations contained within counties).
An example later in this chapter explains more about Update Column.