This tutorial will show ArcGIS users how to perform a site selection analysis, including analysing travel times and accessibility to a location by different transport modes, using ArcGIS Pro and the TravelTime ArcGIS add-in.
For this tutorial, we will use the example use case of determining the best location for a new hospital in a city (London) by calculating the number of people that can reach each of our potential locations within 30 minutes by public transport and driving.
When to Use Selection Analysis in GIS
Site selection analysis is used in GIS to select the best location or site for a facility. When performing site selection analysis, travel time information can help us find the locations that are the most reachable, or those that optimise coverage of key services, such as healthcare.
Travel time information also helps us to identify blackspots where a population is currently underserved and choose the best location to improve coverage.
In addition to optimising coverage of healthcare services, site selection is used extensively in retail — for example, to identify where to open a new store. Another use case is office relocation, where the best new office location can be selected based on key factors such as employee commute times.
In this section, we’ll perform the site selection analysis in ArcGIS Pro using the TravelTime ArcGIS add-in. The add-in allows you to easily create and visualise site catchment areas based on travel times and different modes of transport.
Our goal is to identify the best location for a new hospital in London from a shortlist of three hypothetical locations (addresses) in London. This location must be accessible to as many people (or patients) as possible so that more of the local population can be served.
To support our site selection analysis, we’ll use travel time data from the TravelTime add-in to calculate how many people live within 30 minutes’ travel time to each of the locations by a) public transport and b) driving.
If you haven’t already installed the TravelTime add-in for Pro, you can get a free trial API key and access the extension here.
Creating these catchment areas for public transport and driving consists of the following steps:
- Draw the 30-minute public transport and driving times isochrone maps for the three geocoded locations
- Perform a spatial join between the different catchment areas and the postal code areas to calculate how many potential people (or patients) are served by each location.
Alongside the travel time data, we can also add population data as another data layer to our map. This will enable us to calculate the total number of people who live within a catchment area of each of our potential hospital locations once we begin our analysis. For the purpose of this tutorial, we’ve added population data from the UK 2011 census.
STEP 1: Draw the public transport and driving catchment areas for all three locations
Let’s use our three potential locations to draw 30-minute catchment areas for public transport and driving. This will be done using the TravelTime add-in.
We’ll first create the 30-minute travel time catchment areas using public transport. If this is the first time you’re using the TravelTime add-in, you’ll need to open the Settings and enter your API key details to activate it. You can access the settings by clicking the “Show Settings” icon on the far right in the image below:
Next, click the “Show the Toolbox” button on the TravelTime menu that is accessed on the ribbon interface and marked in the image below:
Next, find the TravelTime_platform toolbox in the Catalog window and double-click the Time Map Simple tool:
Use the following settings to calculate the catchment areas for all three address locations and click “Run”. The three catchment areas are drawn and added to the map as a separate feature layer, displayed in Figure 6.
We can repeat the same procedure for creating 30-minute driving time catchment areas, using the following parameters for the Time Map Simple tool:
The different catchment areas are now added to the map. On the map below, the driving time catchment areas are shown in blue, the public transport ones in red:
STEP 2: Calculate how many potential people are served by each location for different transport modes
With the different catchment areas drawn on a map, we can now calculate how many potential people are served by each location.
Using the Spatial Join tool, we can create an overlay of the postal code layer with the two different catchment area layers. The resulting layer will show the postal code areas within the different catchment areas, as well as the total population for each postcode area.
Next, we can calculate the populations for each catchment area by drawing a chart. We’ll start with the public transport layer (shown in red) and run the Spatial Join tool using the following parameters:
Run the tool, and select the newly added feature layer that looks just like the input layer, but has a different name (SJ1 in this case). Right-click the layer name in the Contents pane and select “Create Chart” – > “Bar Chart” and use the following settings for calculating the population totals for each catchment area based on the Spatial Join operation earlier:
The TARGET_FID field lists the individual catchment areas. We can calculate the population of the postcode areas inside each catchment area thanks to our Spatial Join operation earlier.
The resulting charts are displayed below, as well as the catchment areas on the map. If you hover the mouse over each chart, you will see the population sums for each catchment area.
The population counts for each public transport catchment area are as follows:
- Area 1: 269,671 people
- Area 2: 227,339 people
- Area 3: 311,901 people
For public transport, catchment area 3 has the highest population count and therefore would be the best candidate for opening a new hospital in terms of coverage.
We’ll now repeat the same procedure for the three 30-minute driving time catchment areas and show the results. The population counts for each catchment area are as follows:
- Area 1: 718,838 people
- Area 2: 488,965 people
- Area 3: 592,150 people
When it comes to reachability by driving, catchment area 1 is the best location, as it serves the highest number of people:
Based on our analysis, we can conclude that Area 1 is the best location when considering driving time catchment areas, whereas Area 3 is the best option when looking at public transport catchment areas.
However, looking at absolute numbers for both transport modalities, Area 1 serves the highest amount of people from the two best locations (718,838), so that would be the best location for a new hospital.
In this tutorial, we covered how to perform a site selection analysis in ArcGIS Pro, using the TravelTime add-in to calculate coverage areas based on travel times for different transport modalities. The different catchment areas show from where our three locations can be reached within 30 minutes travelling, either by public transport or by car.
Next, we overlaid the catchment areas with the UK 2011 census Postcode Headcounts and Households dataset and calculated the total population living in the different catchment areas, to see which location would be able to serve the most people based on these transport modes.
Finally, this tutorial is only one example of showing the benefits of using travel time data to perform site selection analysis. Other examples include forecasting service demand or planning sites based on transport modes most likely to be used by target demographic groups.