Podaris:Insight Difference Accessibility Index

The Difference Accessibility Index compares the difference in the Accessibility Index scores of a base and comparison network.

What is Podaris:Insight?

Podaris:Insight is a toolkit for simplifying a number of types of accessibility analysis. It is designed to dramatically speed up the process of performing accessibility analysis on networks and datasets created or imported in Podaris. It provides a simple interface through which analysis projects can be created and shared, and the corresponding results exported. You can learn more about Podaris:Insight and the analysis types that it offers here.

What is the Difference Accessibility Index?

The Difference Accessibility Index allows you to specify both a baseline and comparison network, one dataset with zones to travel from or to and one dataset with target locations to travel from or to.

For each of the base and comparison networks separately, it assesses how long it takes to travel from one location to another, considering various modes of transportation. An index is then generated to categorise the accessibility of each zone to the target locations for both the baseline and comparison networks, and the difference between the two networks calculated.

If you would like to learn more about setting up an accessibility index analysis for just one network, rather than a comparative analysis between two networks, then check out our article on the Accessibility Index Type here.

Step by Step

1. Add basic information, such as a name and description, that will be visible to viewers of the project
2. Select a baseline network and a comparison network, along with the respective views you wish to use for each. The Difference Accessibility Index will compare the accessibility between zones and targets when using the baseline network to when using the comparison network to analyse how differences between the two networks impacts accessibility
3. Add a zones dataset. This represents the zones that will have an accessibility score calculated for them.
   
   This can be plotted manually or uploaded from a georeferenced dataset. When you choose to plot the dataset manually, you can set your zones to be H3 hexagons, squares, points or hexes, and adjust the resolution to make the zone size smaller or larger.
4. Add a target dataset. This represents the locations for which you want to understand how accessible they are to each zone in your zones dataset.
   
   The targets dataset needs to have been created previously as a separate Dataset in Podaris. For more information on this see our article here.
5. The baseline routing options enable you to set how individuals can access your targets from each zone. Within these settings you can establish whether travel is to or from the target destinations, the transport mode used to make the journey, and the maximum duration of travel.
   
   Selecting transit as the mode unlocks a number of additional transit options; multiple arrival/departure times can be specified, for a specific day, and Podaris will calculate the best accessibility from those times based on the calendars and trip times associated with your networks. Max access distance and transfer limits enable you to set caps on the maximum walking/cycling access distance and the maximum number of public transport legs respectively (these settings are optional).
6. The comparison routing options can quickly be set to use the same routing options as for the baseline network by toggling the "Use same settings as above?" option to "Yes". Alternatively, these can be adjusted so that the accessibility index generated for the comparison network uses a different set of routing options to that which the baseline network uses. To do this, set the option to "No" and fill out the comparison routing option settings.
7. When happy with your set up click Save and Run Analysis. You will be notified when the analysis is complete.

Interpreting results

Accessibility is measured by the number of targets, or the time of travel to the targets, that can be reached from each zone within a given time.

You can generate the score based on either travel time (in minutes), number of targets accessible, or in a binary display of areas that are accessible or not accessible in the analysis.

These scores can be generated either for the baseline network, for the comparison network, or for the difference in scores between the two.

For example, the below graphic illustrates the difference in binary accessibility between the networks. The pink hexes represent zones which can access a target location in both the baseline and comparison network scenarios; the green hexes represent zones which can access a target location only in the baseline network scenario, and the grey hexes represent zones which cannot access a target location in either network scenario.

A second example below shows results presented using the number of targets accessible scoring index, and illustrates the zonal scoring outputted both on a baseline, comparison and difference basis.

Exporting results

Your analysis can be exported by clicking the 3 vertical dots in the top right of the results page. 

Here you can choose to export the analysis as a CSV, GIS file, or as a Podaris file (for easy importing back into Podaris as a new project).

You can also choose to export the analysis as an image (in PNG format), with or without the Accessibility Index legend.