Quantifying the Geographical (Un)reliabilityof Police Data

Abstract

Place-based policing has attracted a substantial amount of attention, not least in relation to hot spot policing. Such policing efforts depend on geographical analysis of where crime takes place. However, while it is well known that police crime data suffer from many limitations, less is known about the extent to which the geographical reliability of these data constitutes a problem. The present study attempts to quantify the extent of this problem by exploiting the fact that in Sweden there is an alternative, and more reliable, source of geographical data for incidents of arson. The study compares the locations for car arson incidents as recorded by the police and the rescue services, respectively. The resulting quantification of differences shows that the median error for the police data is 83 meters. This presents a potential pitfall for geographical analysis, both for researchers using police data and for the police themselves in their operational and strategic analysis of crime.

Testing the Spatial Accuracy of Address-Based Geocoding for Gunshot Locations

“…These findings replicate some of the findings of Gerell (2018), the most similar prior work assessing the positional accuracy using data on the exact locations of criminal events, although the errors here are a bit smaller. The median error found here is 52 feet (16 meters) for comparison to street centerline geocoded addresses, and 81 feet (25 meters), as compared to the 250 feet (83 meters) found from the Swedish study.…”

“…As mentioned in the introduction, a key element for geographical analysis is how to convert the location of an event (such as a crime) into a location on a map. Sometimes this is easier, for instance when a GPS device is used from the exact location of an event, an example of which is how the rescue services record the location of a torched car in Sweden (Gerell, 2018). In relation to crime however, the norm is that the event gets coded to an address, which then needs to be geocoded to a location.…”

“…In some cases incidents that occur outdoors are assigned to the nearest intersection (Levine & Kim, 1998;Wheeler, Worden, & McLean, 2016). This error will usually be fairly small, but can be larger if the location has no nearby buildings, for instance when it is by a park (Gerell, 2018). Block & Block (2011) describe examples of geocoding homicide locations (based on where the body was found) can result in locations that are too ambiguous to assign to an address (such as in a river), and as such cannot be reliably located using a standard address or street centerline file.…”

“…A potentially better alternative, when available, is address point geocoding, where each address has an exact location coded, thus not reliant on interpolation of addresses along a street segment (Ratcliffe, 2001;Zandbergen & Hart, 2009). Hart and Zandbergen (2013) mention that while this method is not used often in the US, it has been used in Australia, Canada and the UK, and it is also used by Swedish police, as with the data in Gerell (2018). Hart and Zandbergen (2013) used address point data compared with street geocoding to determine the positional accuracy of different street geocoding and noted that local street line reference data outperformed the national data such as TIGER.…”

“…The majority of prior analyses assessing geocoding accuracy of address level crime data have lacked a gold standard for where the crime actual occurred in evaluating positional error (Bichler & Balchak, 2007;Hart & Zandbergen, 2013;Ratcliffe, 2001), and thus the extent of error which such address based approximations generate is for the most part unknown. For a recent exception, Gerell (2018) used GPS coordinates obtained by the fire department at the exact scene of 114 car arsons in Sweden, and he finds that address based incidents had an average error of 83 meters. That study was for just one crime type, among a small sample of incidents, and in the country of Sweden.…”