Thematic queries are a common way of representing geographical data. You would often for example in magazines find maps of the world showing the incidences of for example diseases per country, with countries with a high incidence shaded in darker colors, and countries with a low incidence shaded in a lighter color. The above is an example of a thematic query.

Thematic queries depend on a combination of geographical features and attribute data. The region to be shaded is the geographical feature (for example a country) and the values on the basis of which to shade the features, are found in an attribute table. For example, if your shapefile contains countries, its attribute table may contain population density values for each country.

There are two types of thematic queries. The first type is based on numeric data. Population density would be such an example. In this case, the data is divided into a number of ranges or classes, for example 0-10 people per square kilometer, 11-20 / square kilometer, 21 to 100, etc.

The second type of thematic query is one based on distinct text values. An example of this would be places of interest. Some are churches, some are museums and others libraries. The values 'Church', 'Museum' and 'Library' in the attribute data would be used as the basis for determining how to draw map features.

For both of the above cases, the gSpatial Toolbar can calculate the classes or ranges automatically. It is in fact required that you allow the gSpatial Toolbar to calculate the ranges on your behalf by clicking on the Calculate button. You can always modify the automatically calculate ranges by overtyping the values that it fills in the grid. In the Thematic query window, there is a calculate button which you have to click in order for the ranges to be populated in the grid at the bottom of the window.

Each range or class mentioned above has a style. A style is simply the way in which it is drawn - what color, text font and map symbol to use for example.