WHAT
Heatmap is a technique to analyze data based on their magnitude.
WHY
Identify areas of interest that can or need to be acted upon. Popular usages include analyses of crime, infrastructure, weather events, win and loss scenarios.
WHEN
Use heatmap to show clusters of activity, i.e. the average influence of the data in relation to their location or density patterns of intersecting features.
HOW
Render a continuous data variable into a raster visualization that emphasizes areas of higher density or weighted values. A graduated color scheme represents variations in intensity, typically red spots (hot) signify highly frequent spots and blue ones (cold) are less frequent areas. The heatmap is usually a semi-transparent layer on top of a basemap showing the underlying geography.
Avoid creating heatmaps based on population data (e.g. income) or data that is tied to population (e.g. insurance policies) because they will result in hotspots around major cities and are therefore less useful.