The GIS perspective|
Chapter 4: Mapping Crime and Geographic Information Systems
Other information gleaned from the computer mapping survey showed that 88 percent of respondents used off-the-shelf GIS software, such as MapInfo® (about 50 percent), ArcView® (about 40 percent), ArcInfo® (about 20 percent), and others (about 25 percent). Some departments used more than one package. Approximately 38 percent of departments that used mapping had done some kind of customizing, and 16 percent were using global positioning system (GPS) technology.
It would be expected that GIS computer mapping would follow the classic bell curve of innovation adoption (figure 4.3). In the lower left are the early adopters, followed by the early majority, the late majority, and the laggards. A wider bell means a longer process, or greater reluctance to adopt. GIS adoption will likely be a rapid process because the technology is simultaneously becoming cheaper and more powerful.
Vector? Raster? Say what?
You will probably hear the terms vector and raster in mapping conversations. How much do you need to know about them? Enough to understand the jargon and enough to make informed choices about formats.
Raster maps store data in the form of a matrix, or grid. A raster map represents information by assigning each pixel, or picture element, a data value and shading it accordingly. The size of a raster data matrix can be determined by multiplying the number of rows by the number of columns in the display. For example, if the display settings on the computer read 640 by 480 pixels, the matrix has a total of 307,200 pixelseach of which would have a data value on a raster-based map.
Vector-based maps are built from digitized points that may be joined to form lines, or vectors, and polygons, or closed shapes. (Digitizing means recording the exact coordinates of each point along x-y axes.) You will sometimes see the term topology used in connection with vector format, and this refers to the study of geometric forms. This type of analysis is integral to GIS but is largely transparent to users.
Each format has advantages and drawbacks. For example, lines in raster displays may appear jagged if resolution is not high enough. Rescanning an image
at a finer resolution greatly increases file size. For example, if the 640- by 480-pixel screen is doubled to 1,280 by 960 pixels, the number of pixels increases four times from 307,200 to 1,228,800 pixels. However, raster processing is quicker.
Vector files are good at showing lines but are labor intensive due to the need to clean and edit vector data (Faust, 1998). Applications that use the vector format include emergency personnel routing and determining whether a suspect could have traveled a particular route in a given amount of time. Most databases in urban areas use vector format; examples include street centerlines, precinct boundaries,
and census geography. Vector files are
not very good for managing continuous
distributions, such as temperature or land elevation.
Although crime is not continuously distributed (crimes occur at separate points in geographic space), we can estimate values between known points to construct a continuous surface representation. The triangulated irregular network (TIN) data structure is one frequently used way of doing this. In it, points are connected to form triangles, the attributes of which become the basis for surface construction.
Summing up raster-vector differences, Clarke (1997) cited Bosworth's analogy about the work of composers Mozart and Beethoven. Raster is Mozart (dainty little steps), and vector is Beethoven (big jumps from place to place). Another way of characterizing the difference is to say, "vector systems produce pretty maps, while raster systems are more amenable to geostatistical analysis" (LeBeau, 1995). However, an alternative view argues that whether a vector or raster format is most useful depends on the type of analysis and what it will be used for. For example, crime densities are often calculated using the raster format, even when the point data for crime locations are in vector format. The vector data simply will be converted to the raster format.
It bears repeating that the analyst should know what format is used, why it is used, and the limitations and possibilities of each. A frequently encountered problem involves conversion to another format. Conversion from vector to raster is the simplest. Going from raster to vector, however, means that each line must be converted on a pixel-by-pixel basis and
a vector equivalent produced. This is much more time consuming than vector
to raster. Users may also convert from one software system to the other (Clarke, 1997).
Because photographic and satellite images are raster products, there is no choice between formats unless conversion is undertaken. As imagery of both types is used more frequently in crime mapping, we will see mixing of vector and raster technologies. For example, a large-scale aerial photo (raster) might be used as an underlay for point crime data (vector) and patrol area line files (vector). In most cases, the crime analyst will not be aware that different data formats are being used because the importation and manipulation will be seamless (figure 4.4). Although photos and satellite images are in raster format, they can be used to digitize data into vector format. Specific features on an aerial photo, such as the footprints of buildings or physical barriers between neighborhoods, are linear features amenable to vector representation. Aerial photos are often the source of other important base data, such as street centerlines, with data being digitally traced from the photo into a vector system.