During the cholera outbreak, there were two competing water companies that had water pipes in the same streets. He wanted to compare them and find out exactly which one was the cause of the deaths. For his maps he used black bars to indicate the deaths. He went door to door, investigating where exactly the deaths had occurred to trace it back to a pump.
The following map shows us the deaths within a metropolitan neighborhood. The map includes the houses marked by dark black bars and the bars behind them indicate the amount of deaths within each house. He and another investigator, Cooper made several maps along the way depicting the neighborhoods in different ways trying to figure out the outbreak. As the investigations went on, the government began to use this map and fixed it up a bit, making the markings clearer to ultimately find the breakout of the epidemic and outbreak.