Digital aerial photography in policing|
Chapter 6: Crime Mapping Futures
As noted throughout this guide, police departments employ GIS technology in various applications, including criminal intelligence and crime analysis, crime prevention, public information, and community policing. Typical GIS applications involve taking a georeferenced crime
database, filtering the data as needed, and mapping it over a street database to put the crime data in its spatial context. Other data layers may be used, such as census tracts, ZIP Codes, or council districts, but the most frequent underlying context is city streets.
Digital street maps typically provide extremely limited contextual information. Streets are represented as single lines and sometimes color coded to indicate their type (e.g., freeway, arterial, collector). They can be labeled, if necessary, to enhance the level of detail, but the information provided by the street database is quite limited. In recent years it has become clear that the visualization of crime data needs to be improved to provide opportunities for better communication within and between the internal and external constituencies of police departments.
We can portray the general context of visualization along a continuum from complete abstraction, at one extreme,
to reality at the other (figure 6.9). Most existing crime maps would be located left of center on the continuum. The crime mapping future will see increasing movement to the right, toward more realistic presentations.
Confidentiality and privacy considerations will probably set limits on how far to the right the process moves. Ultimately, these limits will be culturally conditioned, with more detail where public information and geographic precision are prized and less detail where values put more emphasis on privacy. This variation will be seen, and
is already seen, from city to city, State to State, and nation to nation.
Increasingly, police departments are using large-scale (representative fraction 1:2,400) digital aerial orthophotos, also known as DOQ, for digital orthophoto quadrangles. The "ortho" part means
perpendicular to the Earth's surface, just as orthodontics corrects teeth so that they are perpendicular to gums. Photos are produced in "tiles," or rectangles, such that each tile typically represents about a square mile (4,000 feet by 6,000 feet or 24 million square feet6). The tiles are
rectified for errors due to edge distortion (which increases away from the camera lens), other distortions due to the attitude (or position) of the airplane, and terrain. The edges of the tiles must match perfectly. The raster images (see chapter 4) are
registered to the center lines of the appropriate digital street-based maps so that geocoded crime data can be accurately portrayed in their "real" spatial context,
permitting the identification of land uses, landmarks, and virtually any relevant landscape features.
Furthermore, other coverages are typically digitized and made available as part of
the orthophoto package, including spot heights, building footprints, topography, street boundaries, water bodies, and open space. These can be quite useful with or without the associated orthophoto. The data within the photos and the associated digitized coverages are so rich that they could find applications in tactical team operations, for example, where land elevation, terrain, building heights, building footprints, fences, and water bodies can be used to plan where to place officers and determine what their sight lines will be.
While maps have progressed from two dimensions to three, so have aerial photos. Just as three-dimensional maps have limited specialized applications, the applications of three-dimensional photos are likewise limited at present, but potential uses are numerous and ultimately limited only by the user's imagination. As the example in figure 6.10 shows, images can be quite striking in their depth and realism and add to the crime analyst's arsenal of environmental data.
Potential applications of aerial photography are numerous. Any geocoded
information can be superimposed on the photos, including census data, liquor license locations, drug-market data, injury locations, probationer addresses, housing and zoning code violations, and other data that may be relevant to the needs
of community policing (see, for example, figure 4.4 in chapter 4). Eventually, we may see orthophotos supplanting "traditional" street-based maps in some, if not all, of the applications where such maps have typically been used.
Integration of orthophotos with conventional data is by no means the only possibility for enhanced visual display. For example, other raster images may be keyed to the orthophotos. One possibility explored in a pilot project7 in Baltimore County, Maryland, was the development of an archive of ground-level digital photos intended to characterize neighborhoods and landmarks, precinct by precinct. These ground-level images were linked to a land use map database, and symbols were placed on the land use map at all locations where a ground-level photo had been taken. In ArcView®, hot links were established between the symbols and the database; when the symbols were clicked on, the ground-level picture would pop up, as shown in figure 6.11 (for specific directions, see ESRI, Inc., 1997, chapter 16, "Creating Hot Links"). Yet another possibility is the use of virtual reality to enhance the visualization of scenes in a way that provides somewhat more flexibility than an analog videotape. A set of digital photos is taken in a circle using a special tripod, with the number of pictures calibrated to the focal length of the camera lens. Then a program is used to splice the pictures together at the edges. A
viewer with pan and zoom capabilities enables re-creation of the 360-degree panorama. Free viewers are available for downloading (see appendix). This technology could be used for training purposes, for documenting a crime scene that could serve as evidence in court, or for community meetings when a specific location may be the focal point of interest
The 360-degree view provides a sense of the local environment that may not be conveyed by either a ground-level still picture or an orthophoto. Virtual reality presentations could be linked to maps or orthos, too. To get a sense of what virtual reality is like, go the National Gallery of Art Web site (http://www.nga.gov/home.htm), where virtual reality tours are available (see appendix).
Until recently, the costs of storage and random access memory (RAM) limited
the use of memory-intensive applications, such as the display of orthophotos. Each tile, with its associated coverages, requires about 30 megabytes of hard drive space.
A city like Baltimore or Washington, D.C., each somewhat less than 100 square miles, would need more than 3 gigabytes (GB) dedicated to it. But with hard drives of 20+ GB readily available at modest cost, this limitation no longer poses a problem. Likewise, the relatively low cost of RAM permits the fast manipulation of large graphics files.
Anticipated applications of orthophotos in community policing include:
- Community residents being able to select their homes or apartment buildings and seeing how the locations of crime incidents or other illicit activities relate to their own locations even if they have no firsthand experience of the events.
- Officers more easily recognizing and contextualizing information, whether that information involves crime locations or various social or environmental problems.
- Police dispatchers (who are increasingly likely to be civilians relatively unfamiliar with the detailed community geography of the city) having the capability to enrich dispatching information with landmarks as reference points provided by photos that pop up on their computer screens. Dispatching technology could be enhanced dramatically if both dispatchers and officers in the field could simultaneously see orthophotographic images displaying the origins of calls.
- Crime prevention officers, analysts, commanders, and administrators being able to present more persuasive visual evidence of problems and communicate better with legislative bodies, community groups, and their professional colleagues.
- Planning being enhanced by realistic and accurate perceptions of the size and scope of proposed actions.