Building network of streets for generate service areas

Question

* I have loaded the GTFS data into ArcGIS, and performed GTFS shapes to features and stops to features.
* I then created a Service Areas layer
* Then I added the GTFS shapes to features layer into the Facilities.

The only layers I have in my map are:

* Service Area
* Trip origins
* Trip destinations
* GTFSStops_to_features
* GTFSShapes_to_features

My goal is to see if transit can service the trips - so the origin and destination must lie within 0.25 miles walking distance of transit stops.

Can I execute Generate Service Areas with just the facilities loaded? I don't understand how it can calculate a service area with a walking distance impedance with just facilities? Do I need to import a street network in so it knows what paths a pedestrian can walk? Or is there already a pre-loaded universal street network in ArcGIS that it uses?

Melinda Morang · Answer

73.2 million is a lot of trips.  Yeah, good idea to narrow it down...

I don't recommend you go back to ArcMap.  For your problem, there is nothing in ArcMap that Pro can't do, and Pro is much better at handling large amounts of data.

Yes, you can use the Generate Service Areas tool to calculate walking-only Service Areas around the transit stops and then use a spatial join (or just Select By Location) to identify the origins and destinations that fall within the Service Area polygons.  This works, although keep in mind that Service Area polygons are inexact "artistic" representations of the area covered, and the polygons themselves can look quite different depending on the settings used.  A more precise answer can be achieved using the Generate Origin Destination Cost Matrix tool, which also uses the ArcGIS Online Services.  You can put a cutoff of 0.25 miles in this tool, and it will calculate the walk distance between each transit stop and each of your origin/destination points.  The results will only include those origins/destinations that fall within the 0.25-mile limit.

However, with 73.2 million points, the problem becomes very huge for Generate Origin Destination Cost Matrix (and could burn lots of credits).  If you go this route, I would recommend narrowing it down first by doing Select By Location to select only those origin/destination points that are within a 0.25-mile straight line distance of the bus stops.  Straight line distance is guaranteed to be >= network distance, so anything outside of 0.25-mile straight line distance definitely won't be within a 0.25-mile network distance.  Then, you can see how many points you have left over and calculate the credit usage for using Generate Origin Destination Cost Matrix vs Generate Service Areas.

If you still have a very large number of origin/destination points, you might need to chunk up the problem when solving it.  This video and sample script from a DevSummit presentation might help.

HelenaBreuer · Answer

Thanks for your response Melinda - I only took an introductory class last semester where I learned ArcGIS pro basics, and I am now working with Network Analysis and GTFS, so I am not familiar with the terminology used and implications of specific things.

I am working in Pro only because that is what I learned from, although I have ArcMap downloaded - do you think it would be more useful in my case? I still don't know the difference between ArcMap and Pro.

My goal is to calculate the walking distance towards and away from transit stops because I first want to see which origins and destinations are within 0.25 mile walking distance - from there I would eliminate all the trips that do not meet that criterion - I have roughly 73.2 million trips so I wanted to use the walking distance only first.

Regarding service areas, I thought Generate Service Areas was analogous to solving a service area. My plan is to use the Generate Service Areas tool to create the polygons representing the service areas within 0.25-mile walking distance, then I would perform a spatial join between the trips feature layer and the polygons layer. I would chose within as the parameter, so the output are the trips within 0.25 miles from at least one transit stop.

I apologize for any confusion.

Melinda Morang · Answer

Right, those videos are getting a bit old by this point, but keep in mind that they're using ArcMap and an older workflow for a different purpose than what it sounds like you're doing from this post.

Here are a couple of distinctions that I think you'll find helpful:

1) There is a difference between calculating a walking Service Area around transit stops and a transit+walking Service Area for some facility.

A walking Service Area around a transit stop shows you the area within a short walk time or distance of the transit stop.  To calculate this, you just need a road network.  The network should not include the transit lines because you are not calculating travel by transit.  You are simply calculating travel by walking.  In this case the transit stops are the origins/destinations in the analysis, not part of the network.  You can use the ArcGIS Online Services to calculate this.

A transit+walking Service Area answers the question "how far can I travel by transit and walking within a certain time limit?"  It uses the transit lines as part of the network.  The transit stops in this case are not the origins/destinations of the analysis but are instead part of the network.  You cannot use the ArcGIS Online Services to calculate this.  You will have to create your own network dataset that includes your GTFS data.  The videos you're referring to are older and talk about the now-deprecated downloadable Add GTFS to a Network Dataset toolbox.  You can now do this using core tools in ArcGIS Pro.

So, you need to be clear which of these things you're calculating, as that determines what kind of network you need to use.

2) "Generate Service Areas" refers to a particular tool, but this is not quite the same as solving a Service Area analysis generally.  It's not always easy for me to tell which one you're referring to in a post, so that's why it's sometimes hard to answer your questions.

As explained in my earlier post, the Generate Service Areas tool in ArcGIS Pro is one of the Ready-to-Use tools that uses the ArcGIS Online Services to solve an analysis problem.

To make matters confusing, ArcMap has the same Generate Service Areas Ready-to-Use tool, but it also has a Generate Service Areas tool in the Services Tools toolset of the Network Analyst Tools toolbox, which allowed you to use your own network dataset.

However, both ArcMap and Pro allow you to solve a Service Area analysis using a multi-step workflow of

a) Make your Service Area layer

b) Add Facilities to the layer

c) Solve the layer

The video you're referring to does this procedure in ArcMap using a transit-enabled network dataset (the one made from OSM data for Cincinnati).

You can follow this tutorial to learn how to do this procedure in Pro.  If you want to calculate a walking+transit Service Area in Pro, you will need to create a transit-enabled network dataset and then do this multi-step Service Area workflow.  This tutorial can help you out with all of that.