In the previous lesson, you changed the symbology of your Lincoln Crime Analysis map to better distinguish between its various features. In this lesson, you'll analyze the proximity of crimes to police stations and the density of crimes throughout the city. For the Lincoln Police Department to determine future resource allocation, they need to know how prepared their current resources are to fight crime. If most crime occurs near police stations, few changes may be necessary. But if a significant amount of crime occurs far away from police stations, the Lincoln Police Department will need more officers in the field.
Create drive-time areas around stations
First, you'll create a five-minute drive-time area around Lincoln's police stations. A drive-time area uses information about road networks and travel speed to determine how far someone could drive from a location within a specified time. Your analysis will determine the percentage of crime that the Lincoln Police Department can respond to within five minutes.
- If necessary, open your Lincoln Crime Analysis map.
- In the Contents pane, point to the Stations layer, and click the Perform Analysis button.
The Perform Analysis pane opens. In this pane, you can choose from a variety of analysis functions to derive new information from your data. Drive-time areas are a type of proximity analysis, which determines how close features are to one another.
- In the Perform Analysis pane, click Use Proximity.
A list of proximity analysis tools expands.
To learn more about any tool, click the blue information button to the right of the tool's name.
- Click Create Drive-Time Areas.
The Create Drive-Time Areas tool opens. Before you run the tool, you must input several parameters, including the drive-time distance and traffic conditions.
- For Choose point layer to calculate drive-time areas around, confirm that Stations is chosen.
Next, you'll choose the time for the drive-time area. Because rapid response is critical to combat crime, you'll choose a short drive time of five minutes.
- For Measure, confirm that the driving time is 5 Minutes.
You can also choose whether to incorporate typical traffic conditions into the drive-time analysis. Emergency response vehicles can usually bypass traffic, so you'll leave traffic conditions unchecked. The next parameter specifies how drive-time areas from different points will be combined. When using a point layer with multiple points, such as the Stations layer, the Create Drive-Time Areas tool will calculate a drive-time from each point. If the points are close enough, these drive-time areas may overlap. You can choose whether to keep the drive-time areas from each station as individual features or to dissolve them into a single feature for every station. You're interested in total drive-time coverage within Lincoln, so you'll dissolve the drive-time areas.
- For Areas from different points, click Dissolve.
- For Result layer name, change the name to Five-Minute Drive-Time from Stations. Add your name or initials to make the layer name unique in your organization.
- Click Run Analysis.
The tool runs and the drive-time layer is added to the map. The drive-time layer can be somewhat difficult to see due to the Districts layer. Additionally, the Stations layer is now below the drive-time layer.
- In the Contents pane, uncheck the Districts layer to turn it off.
- Point to the Stations layer. Click the vertical ellipsis next to
the layer name and drag the layer above the drive-time layer.
The Stations layer now appears above the drive-time layer on the map.
The purple drive-time area indicates all places within five minutes of a police stations. Although the police stations only seem to have coverage of about half of the total area of Lincoln, the majority of crimes seem to occur within the drive-time area. However, the exact amount of crime that occurs outside the drive-time area is unknown.
Calculate the percentage of crimes within drive-time areas
Next, you'll calculate exactly what percentage of crime occurs close to police stations (and at the same time, how much occurs far away from them). You'll calculate this percentage using the Aggregate Points tool. This tool summarizes a set of points that falls within a specified area.
- In the Contents pane, point to the Crime layer, and click the Perform Analysis button.
- In the Perform Analysis pane, click Summarize Data.
- Click Aggregate Points.
The Aggregate Points tool opens. First, you'll choose the layer of points and the layer of areas. You want to know the total number of crimes that occurred within the drive-time area.
- For Choose layer containing points to aggregate into areas, choose Crime. For Choose layer containing aggregation areas, confirm that Five-Minute Drive-Time from Stations is chosen.
Optionally, you can choose to calculate statistics for a specific attribute field. However, when you looked at the Crime layer's table in the previous lesson, you learned that none of its attributes contained numeric data, so you'll skip this optional setting.
You can also choose a field by which to group statistics. For instance, you could find the total amount of each type of crime (larceny, assault, and so on) within the drive-time area. You only want to determine the total percentage of crime close to stations, so you'll leave the Choose field to group by parameter unchanged.
- Change the result layer name to Crime Near Police Stations and add your name or initials.
- Click Run Analysis.
The tool runs and a new point layer is added to the center of the map. The layer's appearance on the map isn't important, but its attribute table contains the summary information about the crime points.
- In the Contents pane, point to the Crime Near Police Stations layer and click the Show Table button to open the table.
The first attribute field in the table is Count of Points, which contains the total number of crime points within the drive-time area: 846. When you looked at the Crime layer's table in the previous lesson, you learned that the total number of crimes was 1,025. 837 out of 1,025 is 82.53 percent.
Approximately four-fifths of crime in Lincoln occurs within five minutes of a police station. This coverage rate is pretty good, but a significant amount of crime occurs too far away from police stations for a fast response time. To cover these crimes, the Lincoln Police Department will have to allocate resources to officers in the field, who can patrol farther away from stations and respond quickly if a crime occurs. The amount of resources allocated must be enough to handle a fifth of all crime in the city.
- Close the table.
- In the Contents pane, uncheck the Crime Near Police Stations and Five-Minute Drive-Time from Stations layers to turn them off.
Create a heat map
A significant amount of Lincoln's total area is not within five minutes of a station, so the Lincoln police department must make strategic decisions about where to send their field officers. To help them, you'll create a heat map to visualize crime density and highlight areas where resources should be concentrated. A heat map is a continuous surface map that maps how a phenomena or occurrence changes across an area. Previously, you used discrete data, which depicts objects or areas with unchanging spatial characteristics (such as the location of police stations). A heat map, in particular, displays density, or where features are clustered together. You can create a heat map by changing the symbology of the Crime layer.
- If necessary, open your Lincoln Crime Analysis map.
- In the Contents pane, point to the Crime layer and click the Change Style button.
- For Select a drawing style, under Heat Map, click Select.
The crime points on the map change into a heat map.
The symbology of the heat map indicates the density of crime in a given area. Blue areas are places where crime occurs but at a low density. Red and yellow areas are places where crime occurs at a particularly high density. Based on the heat map, the highest density of crime occurs in the center of the city, with smaller pockets of crime scattered throughout. Next, you'll compare the heat map to the drive-time area to see the densest areas of crime more than five minutes away from police stations.
- At the bottom of the Change Style pane, click Done.
- In the Contents pane, turn on the Five-Minute Drive-Time from Stations layer.
The police stations cover most of the dense areas of crime in the city. However, there are a few dense areas on the edge of the drive-time area where the Lincoln police department may want to deploy more field officers. Additionally, while the area to the left of the Lincoln Airport does not have very dense amounts of crime, its far distance from any stations may necessitate resources being sent there.
- Save the map.
The map you made can provide the Lincoln Police Department with valuable information about where crime occurs in relation to where police stations are located. You first performed a proximity analysis to find crimes that happened more than five minutes away from a police station. Then, you created a heat map of crime density to identify areas where more resources should be allocated. The information in your map can be used for other purposes, too. For instance, you could use the Districts layer in conjunction with the crime heat map to determine resource allocation for a specific district. Or, you could filter the Crime layer to only focus on crime of a specific type, such as assault or larceny. For the purposes of this project, however, your analysis is complete. You can learn more about GIS analysis and the capabilities of ArcGIS by trying some of the other projects in the Learn ArcGIS Gallery.