CRIME MAPPING

The use of Geographic Information Systems (GIS) in the criminal justice field has its roots in the earlier generation of police crime maps. Historically, law enforcement agencies and other organizations used hardcopy pin maps to chart criminal activity; but these maps were static and, as crime rates increased, difficult to maintain. In recent years, with advances in quick and user-friendly software, manual pin mapping has given way to computerized crime mapping.

GIS is an application that links database¹ software to graphics software to create visual images of various types of data in map format. It is a unique tool for analyzing physical space and conveying perspective. Presenting data in the form of a map helps agencies understand the significance of where, when, and by whom crimes are committed.

Technological advancements have enabled agencies to collect enormous amounts of data. In the law enforcement field, a rise in crime and an increase in the number of calls for service have led to a greater need to sort, organize, analyze, and disseminate data. As a result, criminal justice agencies are turning to GIS software and the latest crime-mapping techniques to deliver data in a more efficient and instructive manner. In addition, using GIS to map crime and criminal behavior eliminates the rampant duplication of efforts among agencies.

This has prompted the introduction and application of GIS technologies in the victim service field. Consequently, there is a growing need to educate and guide agencies that rely on other means of recording their data in the direction of implementing GIS. For example, if a VOCA compensation administrator wants to create a visual depiction of the location of applicants, street addresses and other relevant information can be converted into a database format and linked to a graphics software application. In this example, the link is based on the street address. Other spatial or geographic identifiers² that could be used include ZIP Codes and census tracts.³ This particular example would allow administrators to analyze locations that generate victim compensation claims. Areas that lack applications could be examined to determine whether additional applications can be generated and if administrators need to plan for outreach to those areas.

GIS software represents data on a map using points, lines, and polygons.⁴ Features that can be represented as points include streetlight poles, crime events, and bus stops. Bus routes, streets, and rivers are usually represented using lines; counties, states, and ZIP Codes are depicted using polygons. GIS software is designed to capture, store, manage, integrate, and manipulate various layers of data, allowing the user to visualize and analyze the data in a spatial environment (exhibit 1).

Most GIS applications contain base information that orients the map to the reader. Some examples of base information include roads and state and county boundaries. One easy way to visualize base layers is to think about the information found in a road atlas.

In GIS, a database can represent a layer of information and that can be expanded to create additional layers. For example, the OVC Subgrant Award Report System (SARS) could be one layer, with the location of all subgrantees defined as individual points on that layer. Another layer of data could be added by querying the SARS database for a particular type of service provider, such as programs for survivors of homicide victims. This additional layer would be mapped using a different color or graphic symbol.

The real power of GIS is that it gives users the ability to analyze multiple layers of information. Not only can users create additional layers from a single database, they can also integrate disparate datasets⁵ from other sources such as police departments, planning and housing agencies, and the tax assessor’s office (exhibit 2). Each agency’s data would become another layer of information in GIS. With this layering of information, users can discern spatial relationships among previously disassociated data. For example, the layer of SARS information could be overlaid with incidents of domestic violence data from local police departments about locations of courts handling domestic violence cases and locations of public transportation systems. With this displayed information, users could examine how accessible services and the criminal justice system are for domestic violence victims.

GIS can pinpoint the physical location of features⁶ in every layer. It allows an administrator to conduct spatial searches or queries in addition to tabular database queries. For example, a tabular database query can retrieve information about the increase or decrease in the number of crime victim compensation claims submitted in a particular region. What a tabular query cannot show is whether there has been a spatial displacement or diffusion of claims. In other words, has the number of claims remained the same but shifted from one neighborhood to another, or has the number of claims been diffused due to additional resources in an area?

Another example of a spatial search would be to determine the proximity of one location to another. For instance, one dataset or layer shows school locations, while another indicates crime locations involving juveniles. Overlaying this data in GIS, users can identify crimes that occurred within 1,000 feet of a school. This information can be used to determine where services could be located most effectively (exhibit 3) and could lead to an understanding of the spatial relationship between crimes and school locations.

Although GIS software packages can be purchased containing base information such as streets and census data, most GIS also require agency-specific data. A VOCA administrator could tailor a GIS to fit his or her requirements by populating it with other data, such as VOCA subgrantee service areas, which could be used to examine statewide coverage or types of crime to plan for general or specialized services. In summary, GIS is a powerful mapping tool that allows agencies to identify their data spatially to better analyze data relationships.

If an agency collects data via Excel, Access, Quattro Pro, Paradox, Oracle, SQL Server, or any other type of spreadsheet or database management system, the data have potential for use in GIS, but must have a geographic reference. Generally, ZIP Codes, street addresses, or x–y coordinates are used to link data to the map by geocoding, or plotting on a map, the data. For example (exhibit 4), a street address such as 1150 Main Street can be matched against a street centerline⁷ file to determine its location. The geocoding function will link an address to its approximate location on the street segment based on its number. For example, 1150 Main Street would be placed on the even side of the street about halfway between the 1100 and 1200 blocks.

To protect private and confidential information, sensitive data are geocoded to the street block, ZIP Code, or census-tract level rather than the street level to reduce the possibility of identifying an individual from the mapped data.