Thematic maps are everywhere. Open a newspaper, browse the web, or turn on your TV, and there’s a good chance you will encounter a map that is intended to convey the spatial distribution of some sort of data. Examples might include election results, average temperatures, unemployment rates, or crime statistics. In the business world, thematic maps are used to analyze patterns in data or to quickly discern the location and nature of corporate assets as a means to enable quicker and more effective decision making. Thematic mapping is fundamental to GIS, so it is no surprise that the ability to create thematic maps is a key component of XMap.
One of the most straightforward ways to create a thematic map is to assign a unique color or symbol to all objects that share a common value. In theory, this process could be achieved using a manual selection and symbolization procedure. In other words, each object in a layer could be individually selected and its color or symbol style altered to match objects which have the same characteristic. Not only is this process time consuming, but it is also prone to error and extremely difficult to manage. Furthermore, after creating a thematic map using this method, there is no easy way to convey what each color or symbol is intended to represent.
A more effective and efficient method for creating a thematic map is to automatically assign colors or symbols based on a component of the attribute data that is associated with a layer. In XMap, this process is called classification.
Before applying a classification scheme to a layer, it is important to scrutinize your layer to determine if the required data is present. Does the information that you want to portray on the map currently exist as a field in your layer? If, for instance, you want to create a map showing population by county, does your county layer have a population field 
in the attribute table? If the required data is not present, you will need to create a field and enter the values in the attribute datasheet view or import and link an external file containing the data.
After determining that the required variables are present, you are ready to assign all of the objects in the layer to the appropriate class. To accomplish this, either right-click the layer name in the GIS Workspace table or select it and click the Tools button. In either case, select Classify from the resulting menu. By default, all of the objects in a layer are assigned to a Single Value class, so in the Classify Layer dialog box you will initially see that the class called All Records contains the total number of objects in the Layer.
XMap offers two options for developing a classification; which method to use depends on the type of data and the intended method establishing the classes or groups.
- A Unique Value classification is usually assigned to a field containing text values or one in which all of the records assigned to each class share an identical value. In such cases, the number of classes is automatically determined by the number of unique values that are found within the selected field. For instance, if the chosen field contains repeated instances of the words Small, Medium, and Large, the resulting classification would have three classes and all of the records in the layer would be automatically added to their appropriate class or group.
- Range classification can be applied only to a field containing numeric values. Unlike the Unique Value method, the records assigned to each class will not necessarily share identical values but instead they will fall within a user-determined minimum and maximum range. Both the number of classes and the extent of the values within each class are manually established using one of several pre-formulated mathematical methods. By default, the layer will be divided into five classes and the method of classification will be based on Equal Interval. This means that the range within each class will be the same for all five classes.
To apply a classification to a layer in the Classify Layer dialog box, first select the type (Unique Value or Range) then select the field to which the classification will be applied. The accompanying table will immediately display the total number of classes and the number of records or rows in each class. A default color or symbol scheme will be applied, but, as we will discuss later, a specific visual characteristic can be assigned to each class using the Symbolize function.
Both of the classification types offer various customization options for fine-tuning the classification structure:
- Additional classes can be manually added. This is especially useful for unique value classifications in cases where it is likely that additional records will be added to the layer and the anticipated new values are not contained within the existing classification.
- Classes can be deleted. Records that were assigned to the deleted class will be represented as null objects on the map.
- The order in which the classes are displayed can be customized.
- Each class label can be edited. This text will reflect what is displayed in the legend for the layer.
- The heading of the legend can be edited by clicking the Manage button and selecting Rename.
- For range classifications, the method for calculating the extent of each class can be adjusted or custom classes can be created by manually typing the Min and Max values in the results table. Additionally, the number of decimal places can be set by selecting from the Round At drop-down list.
Note that it is possible to create a classification within a newly created layer that does not yet contain any objects. Obviously the automatic class determination process will not work in this case, but each anticipated class can be manually added and as objects are later created or added to the layer, they will be automatically assigned to the appropriate class and will inherit the symbology of that class.
XMap offers the option of creating multiple classifications within a single layer so that the map display can reflect two different variables from within the attribute table. 
To create a second classification, click the Manage button and select New. The previous classification is preserved and the new default temporarily reverts to Single Value. After a second classification has been created, you can switch between the two by double-clicking the current classification name in the Workspace table and selecting the required classification from the drop-down list.
The Manage button also reveals the Classification Template feature that allows the structure of an existing classification to be saved and applied to a different layer.
Before exiting the Classify Layer dialog box, it is a good idea to run a checklist to determine if all of the necessary steps have been performed:
- Make sure the classification has been applied to the correct field.
- Check the legend labels.
- If necessary, edit the legend heading.
- Create additional fields in anticipation of future data additions.
- Verify the order of the classes.
- Save the classification template if required.
After clicking OK, the appearance of the objects on the map will be updated to reflect the classification and the pattern or distribution of each data element should be apparent. To accentuate this pattern, it is usually necessary to manually alter the default colors or symbols that were applied to each class. To do this, select Symbolize from the Tools menu.
Depending on the type of data, the Symbolize Layer dialog box offers the option to adjust the color, style, symbol, line weight, and opacity of each class as well as for null values. The null symbology will be assigned to objects in the layer for which the value in the field used for the classification falls out of the range of all of the classes or if the field contains no data. If a null field is edited to adhere to the classification parameters, the symbology will automatically update on the map.
The final step in creating a thematic map is to display the legend that explains to the user what each of the colors or symbols on the map represents. When the map is viewed onscreen, the legend is displayed in a floating window that is accessed from Tools menu in the Workspace. If you need to edit the legend heading or class labels, open the Classify Layer dialog box as described above and make whatever changes are necessary.
If the map in question is to be printed or exported to an image file, the legend is added as a component of the Layout Tools in the Print tab. Click and hold the Layout Tools button to expose the list of available options and select the key icon. From the Layer drop-down list, select the layer for which you want to display the legend and click the location on the map. The legend can be resized or repositioned as needed before printing.
Displaying your data as a thematic map is one of the most straightforward ways to perform a simple spatial analysis of your data. Applying symbols or colors to objects that share a common characteristic allows you to quickly discern patterns and trends in the data and is an invaluable tool in the decision-making process.
If you have a question that you would like to have answered in the DeLorme Professional Newsletter XMap Q&A section, send it to newsletter@delorme.com with XMap Q&A in the subject line.
