CartoDB is an extensive tool to use if a user has a large database of material with a geographic component to it. The site allows for multiple large datasets to be uploaded and plotted onto a map. The map can then be modified to highlight different aspects of the data. As an example, I will utilize the WPA Slave Narrative dataset available from the Library of Congress.
This dataset holds information from a study undertaken to record the experiences of slaves in the South. The dataset contains information regarding the interviewer, interviewee, where the interview took place, where the slave was from, and whether the slave worked in the fields or the house. CartoDB is able to take this information (uploaded as a .CSV file) and plot on a map each person. There are a number of different styles that the site can run to change the look of the map (here they are called Wizards). Multiple datasets can be layered on top of each other to add to the complexity of the map. Each layer can be adjusted independently from each other. The following are some examples of maps that can be created within CartoDB.
One useful type of map is a cluster map, as it counts how many plots are in a specific location and shows the number within the plot. This type of map allows the viewer to see where the interviewer tended to be when they were interviewing former slaves.
A related map type is called a heat map. Instead of showing numbers, the dots are replaced with colors, with the center of each blob representing more hits.
One final type of map that can be useful is a category map. This takes the column header from the CSV file and plots one column onto the map. Here is one that plots male and female slaves. These types of maps are useful for comparing different types of information within a particular dataset.