Buffering: Meaning and applications
Chapter 4: Mapping Crime and Geographic Information Systems

A buffer is a zone around an object, such as a school or intersection, that has some investigative or analytical significance. For example, drug-free school zones may be defined using a 1,000-yard radius. Such buffers can be drawn around schools and overlaid on large-scale aerial photographs so that field officers can easily recognize the zone's boundaries, even without demarcating signs. Hardcopy maps can be given to patrol officers as an aid in recognizing the zones. Buffering tools in GIS programs make this a relatively simple task (figure 4.21).

Figure 4.21

Techniques for selecting objects can be used to identify certain types of events. For example, what are the characteristics of calls for service within 1 mile of high schools? Calls for service can be identified and separated into a new data set if they are within the 1-mile buffer.

Buffers are shown as circles if the location buffered is a point or street address, but buffers do not have to be circles. For large polygons like school campuses, parks, apartment complexes, or industrial plants, buffers can mirror the shape of the polygon. In figure 4.22, public housing properties were buffered to evaluate the relationship between public housing and the surrounding neighborhood. The underlying question was whether crime in public housing was committed mostly by residents or by persons from the surrounding community. Analysis of incidents in the buffer zones can help determine the answer, using data on the residential addresses of victims and offenders. The same areas could be represented either by a circle buffer (if it is represented as a point or address) or as a polygon buffer (if the area is mapped to match its actual footprint).

Figure 4.22

In a community policing example, questions may arise about the quality of street or neighborhood lighting. Analysts can consult with city engineers to learn about the illuminated radius of various streetlights and their coordinates in the community. Then, using the buffer tool in GIS, circles of the appropriate radii can be drawn around each light location to create a basis for assessing community concerns about lighting quality (figure 4.23).

Figure 4.23

Also in the context of community policing, an extremely disruptive phenomenon is house fires. This is especially true in older neighborhoods with substandard row houses, where the likelihood of fire spreading from one home to adjacent houses is great. In one study, buffers were drawn around residences where fire- related injuries occurred in the previous 2 years. This, along with census data indicating risk, helped establish zones that were appropriate for the distribution of smoke alarms. (This may seem like a fire department function, but economic and social disruptions can contribute to conditions in which crime flourishes, making both fire and police functions relevant, as with arson cases. This helps demonstrate the breadth of data that crime analysts need access to.)

In Tornado Alley, the high-frequency tornado region in the Plains States, a community safety concern is the audibility of warning sirens. Because the decibel output of sirens is known and varies with weather conditions, a GIS can be used to map the audible zone of each siren. Areas where sirens cannot be heard can be targeted by public safety agencies for special action during tornado warnings. This type of map could also serve as a blueprint for locating new sirens.

Chapter 4: Mapping Crime and Geographic Information Systems
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Mapping Crime: Principle and Practice, by Keith Harries, Ph.D., December 1999