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written by Nigel James
 
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MAPINFO How to...  
No. 7: Create a grid thematic map  
(References to tables, folders etc., are not applicable outside the Map Room, so if you are located elsewhere, you should substitute your own)  
   

This explains how to use MapInfo to produce a grid thematic map. Grid thematic maps plot data as continuous colour gradation across the map. The result is a 'contoured' map of your data, which effectively picks out hot spots. To produce a grid thematic map you need a mappable point data file with numeric values for each point. (See MapInfo How to no. 5 Creating a point data map for more information). The resulting map is a raster table, which can be saved and used independently of other layers.

If you change the data you will have to re-plot the thematic as it does not update automatically. Grid thematic maps seldom display well initially, so you will need to experiment with different settings.

 
   

1. Start MapInfo.

2. When you see the Quick Start dialog, select Open a Table and click Open...

3. Open your data table, which must be mappable.

4. Open the Map menu and click Create thematic map. In the Create thematic map - step 1 of 3 dialog, click the Grid button.

5. Choose Grid default (the colour option) then click Next...

6. In the step 2 of 3 dialog, select your data table in the Table listbox and the field containing the values in the Field listbox. If you data table contains blanks or zero values and you do not wish to include them, click the Ignore zeroes or blanks checkbox.

7. If you have displayed a boundary table (such as country or county boundaries) and you wish to limit the colour to within the boundary, choose it from the table list in the Grid options box.

8. Select the folder for the Grid file which will be created. This should be your working folder. Click Next...

9. You will now see a preview of the colours. Click OK to display the map.

10. The map may not look how you expect it to - depending on the number of points and the data range, the colour may not blend smoothly. To make changes, open the Map menu and click Modify thematic map.

11. In the Customise group of buttons, click Settings... Here you can modify the IDW Interpolator settings.

i) To produce a smoother colour, decrease the size of the grid cells. If you try to make it too small, MapInfo will tell you the minimum allowed. The new grid dimensions will be displayed.

ii) Changing the Exponent to a larger value decreases the influence of data points the further they are away from each other.

iii) The Search radius determines which points are considered when calculating the distance weighting.

iv) The grid border determines how far beyond the minimum bounding rectangle of the data table the grid extends. A value of zero means the grid only extends to the outermost points, so shading is cut off beyond them. You will need to experiment with this to produce the effect you need (such as to extend out to a surrounding border. For example). Click OK to apply the new settings.

12. Click the Styles button to display the Grid Color dialog. Here you can add and remove inflection points (values for a specific colour), change the brightness and contrast or change to a greyscale display.

i) To change an existing colour, double-click the setting to change and choose a new colour from the Colour dialog.

ii) To add a new inflection point, click the Add button. The maximum value is duplicated and added to the list. Select the duplicate value and enter your choice of value. Click the list to add the new value in its correct position, then double-click the new value and select a colour. You can also change an existing inflection point.

iii) Use the Contrast and Brightness sliders to change the overall intensity of the colour - it often improves the map if you reduce the stridency of the colour! Click OK to apply the changes.

13. Click the Legend button to create a legend for your map. Enter a title (and subtitle if required) and an edit the range labels (note this does not change the actual values, simply the way they are labelled in the legend).

14. The Template Save as... and Merge template buttons should not be used as these will change the default settings in MapInfo, which should be left as they are.

15. Try various settings until you are satisfied with the results. Grid Thematic maps allow a great deal of subjective input and you need to find which styles display your data in the way you wish.

 
This is a brief and specific guide to this topic, for more general information, see: MapInfo - an easy guide for new users  
Nigel James
Bodleian Library 2001
 
Bodleian Library Map Room