What are the most effective ways to display data in a choropleth map?

2 Color scheme and legend

The second step to create a choropleth map is to choose a color scheme and a legend that match your data type and classification. Color schemes can be either sequential, diverging, or qualitative, depending on the nature and range of your data. Sequential schemes use a single hue that varies in saturation or brightness, and are suitable for quantitative data that have a low to high order. Diverging schemes use two contrasting hues that meet at a midpoint, and are suitable for quantitative data that have a positive and negative or above and below average contrast. Qualitative schemes use multiple hues that are distinct and balanced, and are suitable for qualitative data that have no inherent order. The legend should clearly and accurately describe the meaning and range of each color or symbol, using labels, titles, and units.

3 Data scale and projection

The third step to create a choropleth map is to consider the scale and projection of your data and your map. Scale refers to the level of detail or aggregation of your data, such as country, state, county, or city. Projection refers to the method of transforming the spherical earth onto a flat map, such as Mercator, Robinson, or Albers. Both scale and projection can affect the appearance and interpretation of your choropleth map, as they can distort the shape, size, area, and distance of your geographic features. For example, using a large-scale data set (such as city) on a small-scale map (such as world) can result in overplotting or oversimplification of your data. Using a projection that is not appropriate for your region or purpose can result in misleading or inaccurate comparisons of your data. Therefore, you should choose a scale and projection that suit your data and map objectives, and avoid unnecessary or excessive distortion.

4 Data distribution and outliers

The fourth step to create a choropleth map is to analyze the distribution and outliers of your data. Distribution refers to the pattern or frequency of your data values, such as normal, skewed, or bimodal. Outliers are extreme or unusual values that deviate from the rest of your data, such as errors, anomalies, or exceptions. Both distribution and outliers can influence the visual and cognitive impact of your choropleth map, as they can affect the contrast, balance, and clarity of your colors and classes. For example, using a classification method that does not reflect the distribution of your data can result in uneven or misleading color intervals. Using a color scheme that does not account for outliers can result in overemphasis or underrepresentation of your data. Therefore, you should explore and understand the distribution and outliers of your data, and adjust your classification and color scheme accordingly.

5 Map purpose and audience

The final step to create a choropleth map is to define the purpose and audience of your map. Purpose refers to the main message or goal of your map, such as to inform, persuade, or educate. Audience refers to the intended or potential users or viewers of your map, such as experts, students, or general public. Both purpose and audience can determine the style and content of your choropleth map, as they can affect the complexity, simplicity, or suitability of your data and design choices. For example, using a data set that is too technical or a color scheme that is too subtle for your audience can result in confusion or misunderstanding of your map. Using a data set that is too simple or a color scheme that is too flashy for your purpose can result in boredom or distraction of your map. Therefore, you should consider the purpose and audience of your map, and tailor your data and design choices to meet their needs and expectations.