The Power of Mapping

Caitlin Dempsey


The real value of GIS come from the ability to integrate spatial information with higher end statistical and analytical processes to derive spatial patterns not readily apparent to the observer.   As wonderful as this part of GIS may be, the power of straightforward mapping should not be overlooked.

Preliminary mapping of phenomenon can often help guide the way to the sort of variables needed to analyze spatial events. Indeed, the origins of GIS were in the overlay method. This method involved the mapping of spatial events and then overlaying the information to see where overlapping occurred. Before the widespread availability of computers this effect was first achieved through a base paper map and then physically overlaying transparent printouts on top. The government of Canada, which can lay claim to the birth of GIS, took this a step further by digitally overlaying spatial information.

The primary effect of mapping is informational. Mapping allows the viewer to get a spatial sense. Imagine how difficult it would be to navigate around a strange area without a handy map to provide direction. Mapping can also be used to understand the pattern of spatial phenomenon as well as provide clues to correlation with other events or features.

An early and oft cited example of the power of mapping comes from the famous Dr. Snow investigation. In September, 1854 during an outbreak of cholera in a section of the city of London, Dr. John Snow, a local physician, decided to test his hypotheses that the outbreak was a result of contaminated water supplies, a view contrary to the medical beliefs of the time. He accomplished this by interviewing residents and drawing conclusions about the likely water-borne source of the disease.  

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John Snow's 1854 Cholera Map.
John Snow’s 1854 Cholera Map.

Afterwards he commissioned the non infamous  map showing where the victims lived and where the local water pumps were located. The map plotted the location of the homes of 578 people that had died of the disease, as well as the position of 13 neighboring water pumps or wells, theorizing that cholera was a water borne disease.

 The map clearly showed a clustering of cholera cases around one of the pumps, and when that pump was shut down the outbreak stopped. Using the simple means of pen and paper Dr. Snow was able to  visually illustrate a cause and effect relationship between a contaminated supply of water and instances of disease by using a map to visualize the instances of disease and the location of water pumps.

This early example paved the way for a new, powerful tool in analyzing and addressing public health care on a spatial level to help understand phenomenon.

Mapping is also useful as a preliminary step towards higher end analysis. As one example the City of Santa Clarita mapped youth crime graffiti incidents in order to help Community Services understand which variables most affect the occurrence of this crime. Incident locations were mapped along with cultural features such as parks and schools.

A visual analysis indicated that graffiti predominately seems to be occurring near parks. Higher end statistical analysis was then utilized to determine statistical correlation between park locations and rates of graffiti. The end result was a more effective tool to resource patrols and outreach programs.

The strides that the field of GIS is making in its function as a research tool are incredible, but those of us in the field must never forget to the role that basic mapping plays.

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About the author
Caitlin Dempsey
Caitlin Dempsey is the editor of Geography Realm and holds a master's degree in Geography from UCLA as well as a Master of Library and Information Science (MLIS) from SJSU.