City of Lincoln Police Department Crime Mapping Information

More Information about Geocoding

Geocoding is the process by which computer software determines where to draw events, such as crimes, on a digital map. Geocoding is inherently inaccurate. Computer software is making what amounts to an "educated guess" by comparing the address of the event to the expected range of addresses along a certain block. For instance, a crime occurs at 575 South 10th Street, the address of the Law Enforcement Center and Hall of Justice. The mapping software finds a segment of South 10th Street that contains an address range of 501 to 599 along the west side of the street, and calculates that the point representing that crime should be drawn 75% of the way along the block. In the real world, however, addresses are not nearly so linear. Also, the Law Enforcement Center and Hall of Justice is a large building complex, occupying more than a full city block. The actual crime could easily be over 200 feet away from the point drawn along the street by the computer.

Geocoded crime data, therefore, needs to be considered in light of these limitations. When viewed at a distant scale, geocoded data does a very good job of showing the relative location and density of events. When zoomed in at close range, however, the picture can be deceiving. Geocoded crime points are only approximate. The symbol is not placed exactly where the crime occurred. Rather, it is usually drawn near the street, perhaps offset to the left or right depending on whether the address was odd or even-numbered. Dozens of incidents at the same address will look like only one symbol if they are stacked one upon another. Even the size of the symbol can cause some visual distortion. For these reasons, LPD's crime data is best viewed at a scale of no less than a typical city block.

Complicating matters, for a variety of reasons not all crimes can be geocoded. Sometimes the exact location of a crime is unknown. Some crimes occur at locations that cannot be described with an address (for instance, Wilderness Park). Human errors can happen at several steps in the process. For example, an officer reverses two digits in an address, or a clerk inadvertently adds an extra zero during computer entry. Huge data files describing street segments, although highly accurate, also have their own errors and glitches. The computer software cannot recognize certain addresses that actually exist in the real world but are not present in the street database.

The percentage of all events in a file that the software is able to geocode is often referred to as the hit rate. Even when a crime is geocoded, however, it can still be incorrectly placed on the map. This occurs when the software draws the crime, but at the wrong address. For instance, a crime occurring on Huntington Avenue is drawn at the same house number on West Huntington Avenue--several miles away from its actual location. Although this is rare, such errors are occasionally encountered in large data files. The Lincoln Police Department's geocoding hit rate for crime reports typically exceeds 96%, with accuracy greater than 98%. Although these are unusually high for crime data, even if only 1% of crimes could not be geocoded or were inaccurately geocoded, this would amount to over 400 crimes in Lincoln in a single year either missing from a map or incorrectly placed. Despite our exceptional capabilities and efforts, the data is not complete, nor completely accurate.

Interpreting Crime Maps

Mapping crime provides a valuable picture of the location of crime. But maps, like any other data, are subject to interpretation. Maps can be misleading. On all of our crime maps for instance, a large area in northwest Lincoln appears to be crime-free. The area, however, is composed of the runways, taxiways, and aprons at the Lincoln Municipal Airport. Crime affects virtually all neighborhoods in Lincoln, but using maps alone to compare crime from one area to another is difficult. Are the geographic areas the same size? Is the population comparable? Are the characteristics such as land use, housing types, and zoning reasonably similar? These features of the physical and social environment and many more, all affect the meaning of the points on a map. The number or density of crimes, moreover, is only part of the story. Time of day is a critical variable: an area that has quite a bit of crime around midnight may be very quiet at 6:00 PM. What about the types of crime? Consider the following examples:

• Two geographical areas are about the same size. Neighborhood A appears to have many more crimes than another neighborhood B. Most of the homes in A are on small lots and there are many duplexes and multi-family residences. Neighborhood B is composed of single family homes, on large lots. The effect of these differences in housing types is reflected in the population density--high in neighborhood A and low in neighborhood B. Crime rates--expressed as a number of offenses per residents--are a much better indicator of the risk of victimization than the raw number of crimes. In this example the crime rate of A is significantly less than B, despite the larger number of crimes.
  

• An area includes several large retail stores and very little residential development. Dozens of shoplifting, forgery, and credit card offenses occur at the stores every year. As a result, the area shows high crime numbers and low population figures--culminating in a very high crime rate. The appearance of the area as crime-ridden is quite misleading. The absence of residential population belies the huge volume of visitors and shoppers, and the dominant types of crime are only marginally related to public safety. Very few violent crimes occur in the vicinity. The area is actually unusually safe when these factors are taken into account.
  

These scenarios illustrate some of the difficulties in interpreting maps. While viewing maps and data can help citizens visualize the location and extent of crime, an intimate familiarity with crime classification, geographic features, socio-demographic characteristics of neighborhoods, and many other considerations is important to a fuller understanding what the map means. The Lincoln Police Department recommends that citizens avoid attempts to compare areas, and instead focus on the important information about the nature of crime in their own neighborhood. The department also provides more detailed tablular data about significant police incidents within the past 60 days in smaller areas known as police reporting districts. This information can help citizens more precisely understand the types of crime in their neighborhood than the maps alone.

If you have questions about our mapping application, we will do our best to respond promptly to your email. The Lincoln Police Department's Education and Personnel Unit, 441-7261, can provide crime prevention information and services to individuals and groups. If you would like more information about crime trends in a specific area, or help in understanding these data, LPD's Team commanders are at your assistance.

Northwest Team Capt. Steve Imes 441-7655
Northeast Team Capt. Doug Srb 441-7752
Southwest Team Capt. Bob Wilhelm 441-7754
Southeast Team Capt. Kim Koluch 441-7555
Center Team Capt. Joy Citta 441-7751

Police Home Page