Creating Thematic Maps in Global Mapper
As a powerful GIS program, Global Mapper contains an extensive collection of tools for data creation and editing, but arguably just as important as creating data is applying an appropriate style to reflect the characteristics and structure of the data. When working with vector data in Global Mapper, points, lines, and areas can all be styled based on an attribute to create a thematic map.
From the Layer Options for a layer of vector features, points, lines, and areas can be styled based on an attribute value. After selecting the option to Apply Styling Based on Attribute/Name Values, an attribute can be selected, or calculated, and classes can be created based on the values present in the attribute. Before the release of Global Mapper v23, classes of features could be manually created and assigned a unique style to create a thematic map. This is still the prominent method for styling text field attributes, but a new Classify button in the feature styling options allows the automatic creation of classes for numeric attribute values.
The new vector classification option helps generate classes of features based on numeric attribute values and common statistical methods such as equal interval, equal count, and natural breaks. In the Vector Classification dialog box, a histogram shows the values of the attributes and the classes based on the number of classes and selected method. In this example, the classification tool is used to visualize the distribution of population by county in the state of Montana.
The equal interval method creates classes of equal value ranges. This will not distribute the number of features in the layer evenly across the classes, but each class will cover an equal range of possible values. The calculation for the size of these classes is simple; the total range of values is calculated and divided by the desired number of classes to determine the range in each class.
The Equal Count, or Quantile, method of classification attempts to create classes that each contain the same number of features. The histogram shows that the classes are clustered toward the lower values since most of the counties in this dataset have a relatively small population. This process results in a map where each color represented in the legend colors approximately the same number of features. With this method, a legend is of utmost importance since the highest value class contains 9 out of the total 56 features but covers about 81% of the total range in population values.
The Natural Breaks or Jenks method creates classes that contain the least variation by analyzing natural breakpoints in the data. Using a more complex statistical analysis method, Natural Breaks calculates and minimizes the variance within each class to create classes that best fit the data.
Classifying the vector data is the analytic portion of the new functionality, but new styling options to enhance the visual representation of the classes as a choropleth map. This tool can be used with points, lines, and polygons, and the styling options vary for each geometry type.
For all feature types, the styling options include the selection of a color ramp. Several built-in color ramps, created with ColorBrewer, are available. These palettes are designed for clarity in choropleth maps and provide an easy way to color the classes of features. For finer control over the colors used in the map, custom color ramps can be designed. When creating a custom color ramp, add a color for each class, or only a few, and Global Mapper will interpolate between the color values to fit the designated number of classes.
With area features, the classification is communicated by the fill color of the polygon that is determined by the classification and color ramp. Additionally, users can customize the fill pattern and border style, color, and width.
Line and point features display the classification using the color of the feature as well as the size of the point or width of the line. The size of the feature on the map can be set and linked to the classification if desired to create a proportional symbol map.
While much of the class creation and styling options for vector features can be controlled through the Vector Classification tool, classes and styles can always be manually edited through the Layer Options providing users with complete control over the classification and styling of features on their thematic map.
If you would like to explore thematic mapping and vector classification and thematic mapping in Global Mapper, download a 14-day free trial today! If you have any questions, don’t hesitate to get in touch.