Using an Inflection Point to Show Distinction - MapInfo_Pro - 2023

In thematic mapping, an inflection point is a color style you insert between ranges. It provides a second way to interpret your data.

The inflection point is placed where there is a distinct marker between the ranges of values above it and the ranges of values below it, or where a numeric shift in your data occurs. Most often, the inflection point is inserted as close to the zero value as possible, since zero marks the shift between positive and negative values, or at the mean of your data values.

The inflection point enables you to see this shift on your map. You see two different color spreads in your ranges. Instead of spreading toward each other, the top and bottom range colors both approach the color of the inflection point.

Another type of inflection point map is a grid surface thematic map. Here every color that displays in the map legend is an inflection point. Values that fall between these inflection points display with a blend of the two colors. This allows you to see a continuous gradation of color across the map. See Working with Grid Surface Maps for a discussion on this type of thematic map.

Example of an Inflection Point Map

To show positive and negative population growth in the United States, you can insert an inflection point close to where the zero value is in your ranges. The top and bottom range points will spread, or graduate, toward the inflection point: one for increased population, and one for decreased population.

For a detailed description of the figures involved in this process, see Updating Columns using Thematic Mapping. Since here we are focusing on the Inflection Point options, the process will cover that aspect of this map in more detail.

To enhance a map using an inflection point:

1. In the Create Thematic Map - Step 1 of 3 dialog box, click Ranges and a template type.
2. Click Next to open the Create Thematic Map - Step 2 of 3 wizard panel.
3. Create an expression to show population growth. For this example, we are using the expression (POP_1990 - POP_1980)/POP_1980 * 100.
4. Click Next to open the Create Thematic Map - Step 3 of 3 wizard panel.
5. Click Ranges to customize the ranges.

   For our example, choose Equal Ranges, and set the number of ranges to 14.

   When you use an inflection point, it is important that your data distribute well across the ranges. You may want to increase the number of ranges, or compare the distribution of the data between Equal Count, Equal Ranges, and Natural Break to see which gives the best representation of your data.

   You may also want to use Custom to adjust the range values so that zero is the minimum value in one range, and the maximum value of the range below it. This is optional.

6. Click OK to redisplay the Create Thematic Map - Step 3 of 3 wizard panel.
7. Click Styles to choose the range style.

   The Customize Range Styles dialog box opens.

8. Choose Options to expand the dialog box and display more options.
9. Determine where among the ranges the inflection point should lie and specify the range position for the inflection point.

   Examine the preview before you create the inflection point to locate where you want to place it. When you choose an inflection point, you must choose a number that identifies the position of a range in the map legend. Count the ranges in the preview map legend until you get to the range with a zero at one of its ends. If your Legend Label Order is Descending, count the ranges up from the bottom. If it is Ascending, count down from the top. You can keep the default color spread between the ranges (red to blue), or you can choose a different color spread. For this example, keep the top range at red, and change the bottom range to blue.

   In the Inflection at box, choose 2 for the second range, as you determined above. The default is None, meaning no inflection point.

10. Click on a Styles button to set the style for the inflection point.

    A style dialog box opens.

11. Select the color for the inflection point.

    The default style is a white fill with no pattern.

12. Click OK to return to the Create Thematic Map - Step 3 of 3 wizard panel.
13. In the Create Thematic Map - Step 3 of 3 wizard panel, preview your map legend.

    Your map legend does not show explicitly where the inflection point is, as your range colors only approach the inflection point. You can, however, add the inflection point to the map legend title or subtitle.

14. Click OK.

The Map window redraws with the inflection point you created.

Controlling the Number of Inflection Values

The Grid Appearance dialog box allows you to control the number of inflection values for grid themes.

To access the Grid Appearance dialog box:

1. Click on the Map window to make it active.
2. On the MAP tab, in the Content group, click Add Theme to open the Create Thematic Map - Step 1 of 3 wizard panel.
3. Proceed to the Create Thematic Map - Step 3 of 3 wizard panel.
4. Click Styles to open the Grid Appearance dialog box.
5. In the Number of Inflections field, select from a list of 2 to 16 inflections or type a number between 2 and 255.
6. Click OK to close the dialogs and wizard panel.