Ellipse, choropleth, and isoline maps.
When hot spots cover broader areas and
coincide with neighborhoods, they need to
be depicted in another way. Ellipse and
choropleth maps imply that the areas within
the designated hot spots share the
same risk level, so a specific street or location
within the area is irrelevant. Isoline
maps imply a continuous gradient of risk
within a hot spot, so a particular place has
risks similar to but not the same as an
adjacent place or street. A gang-related
robbery problem can be an example. If
gang members commit robberies throughout
specific neighborhoods (i.e., do not
focus on specific streets or around specific
sites), but refuse to commit robberies
outside their territories, and their territorial
boundaries are streets, then a choropleth
map might be useful. One could create a
map of the gang areas and shade the
areas according to the robbery frequency
within each. If the likelihood of a gangrelated
robbery diminishes the farther one
goes from the center of gang activity, then
an isoline map depicting gradients of robbery
frequency does a better job of showing
the problem.