Using a Fishnet to Catch Crime

Hot Spots

Paula F. Cutrone, Senior Crime Analyst – IT Coordinator

The John F. Finn Institute for Public Safety, Inc.

Onondaga Crime Analysis Center, Syracuse, New York

October 30, 2015

Why use a fishnet?

• Simple method for detecting areas with concentrations of crime

• Easier to explain than how kernal density is calculated

▫ This is a count of crimes within a grid, no magic or fuzzy math involved

What kind of fishnet?

This kind of fishnet

Where do you get a fishnet?

• X Tools Pro, trial version

• ArcMap: Data Management Tools, Feature Class, Create Fishnet

▫ Analysis Tools, Extract, Clip – to the extent of your area

• Program lays a grid over your area

• 500 feet X 500 feet, 250,000 square feet in area

▫ Use what works best for your area

Joining the fishnet with crimes

Right click your

fishnet

Choose Joins

and Relates then

Join

Join Spatially

• Choose your file

• Points (crime) to Polygons (grids)

• Choose output location

• A Count field will be added to your new feature class

Name it and Save it

Click OK and watch

the magic wheel spin

New layer!

It’s supposed

to look just like

your fishnet

(no worries)

Right click the new

joined feature class

Click Properties

Symbology

• Symbology tab

• Choose Quantities

Symbology

• Choose Graduated colors

Symbology

• Dropdown next to Value

• Choose Count

Symbology

• Automatically uses Natural Breaks (Jenks)

• Can be reclassified, you can choose your own breaks

• Click OK

Symbolized join

feature class

Fishnet is still on,

turn it off

Right click,

choose

Properties

Change Colors

• Choose your colors from ramp or by clicking individual symbol

• Click OK

Right click,

choose

Properties

Change Classes

• Click dropdown next to Classes

• Choose a smaller number

Change Classes

• Review the ranges and change if desired

• Click OK

Determine what

ranges work best

for your area

and your

crime

Copy and paste,

change symbology

and compare

Values might be

helpful, let’s add them

No surprise where

to go

Format Labeling

• Labels Tab

• Click the dropdown next to the label field

Format Labeling

• Select the Count field

Format Labeling

• Choose a smaller font to fit inside of the grid

• Check the Label features box

• Click OK

Count values will appear

on each grid

Analyzing the hot spots

• We execute the join weekly using the past 365 days of gun crime

• Depict locations of past 6 weeks of gun crime over the grids

Analyzing the hot spots

• GIVE (Gun Involved Violence Elimination) Grant relied heavily on fishnet analysis

• Created POP (Problem Oriented Policing) areas: areas of highest concentrations of long-term gun violence based on a spatial density of five years of gun crime

• Used the fishnet with the POP polygons to show trends in gun crime short term compared to long term

GIVE Grant Locations with a high amount of gun crime over 5 years were identified and designated as POP areas

GIVE Grant POP areas are depicted in conjunction with the grids.

High gun crime areas were almost always the locations of the homicides by gun and fairly consistent with the established gang territories or the area directly around them.

Fishnets can be used to aggregate any point layer. This example shows the locations where shell casings or guns were recovered as part of the OCAC comprehensive strategy of tracking Multiple Use Firearms.

Analyzing the hot spots

Where are the…..

• Gang territories?

• Vacant properties?

• Recent vehicle and pedestrian stops?

• Other crimes?

• Arrests?

• Supervised persons residences?

Analyzing the hot spots

What are the….

• Environmental factors (CPTED)?

▫ Broken street lights, overgrown shrubs, trash, dilapidated walkways or residences

• Citizen complaints?

Analyzing the hot spots

• What might be causing the issue?

▫ Specific people…

▫ Specific residences or businesses…

▫ Gang conflict…

Enforcement actions for hot spots

• Target problem addresses

• Target problem streets: road checks, drug buy busts

• Selected enforcement, not ‘scorched earth’

• Enforce city ordinances

• Develop intelligence

• Recommend locations for new stationary surveillance cameras

Conclusion

• You can create a fishnet and begin analyzing your data today

▫ Try various color ramps, class breaks

• There is no standard threshold for a hot spot

▫ Determine what works best for your area and your audience

Contact Info

Paula F. Cutrone

Onondaga Crime Analysis Center

511 S. State Street

Syracuse, NY 13202

Tel (315) 442-5645 ext. 5062

Email pcutrone@syracusepolice.org