Nice first pass, but its clearly calculating these as the pigeon flies and not taking the various stitched together street grids into account. You should get diamond and square shapes, not circles, for the most part. I'm working on a redesigned subway map / nomenclature an indicators of walking time to nearest stations is a subtle feature I want to build in. Something between a watershed like this and a vornoi diagram depending on which edge comes first.
No, it is clearly not as the pigeon flies. That is incorrect.
When I click on my local subway stop it seems entirely accurate, and the stations don't form any kind of circle at all. In fact I can't find any subway stop origin that seems to form anything even close to a circle.
Also, according to the "about" it's calculating from actual subway schedules.
And it has nothing to do with street grids. It's subway trips, not walking.
The station-to-station part does indeed use subway travel times, but the website says "Hover over a station to see how much of the city is accessible within 40 minutes by subway and walking."
And the "walking" part clearly shows a perfect circle around each destination station, so it is 100% doing an as-the-pigeon-flies estimation for the walking.
"Isochrones are manually calculated using turf.js assuming 1.2m/s walking speed after the subway trip. These are simple buffers around each station/prior isochrone and do not take the street network into account."
Other people already corrected you, but I feel like being obtuse so here it goes. The circle goes from Jamaica, along Queens Blvd, down 8th Ave to W 4th St, along the F line to Jay St, down Fulton St to Broadway Junction, past Cypress hills on the Nassau line (J/Z), and finally returns to Jamaica. This loop is possible without adding any additional tracks or switches to the system. Thats how the pigeon flies. Checkmate.
It's using some hybrid of isochrones & walking time "as the crow flies." The subway travel is in isochrones, but once you exit the subway the walking time assumes walking in straight lines with no obstructions in any direction. So yes that's inaccurate but it's relevant that only the walking time is done so. I assume subway ride time is based on subway schedule times.
I did this exact thing in 1st year geography. Gotta use the network analysis tool, not spatial analysis. Distance along road segments, not the Euclidean distance tool.
You can take a shortcut, because NYC is several large grids stitched together. If your isochron line doesn't cross a border between two grids its just a diamond instead of a circle. If it does cross the border between two grids, then you just have to figure out where along the border the "diamond" of one grid system meets the tilted diamond of the other. But you don't need a general purpose road network analysis tool to do it. It may even be tractable analytically.
It's all bravado commenting on something you're working on in the comments of someone else's actual working site. Let's see the comments from a link to your site? Thought not
