Total mileage of alleys in San Francisco

March 5, 2015 9:25 AM

How many miles of alleys are there in San Francisco? I don't have access to, or expertise with, GIS software, and this isn't turning out to be very Googleable. Anyone?
posted by sandettie light vessel automatic to Grab Bag (7 answers total)

Hey I did a photography project a few years ago documenting San Francisco alleys - and one of the hardest things I had to do, conceptually, was to decide what was and what wasn't an alley. I'm serious. Streets like Pond or Prosper in the Castro I didn't consider alleys, yet when I got South of Market and photographed Stevenson or Jessie (which I did consider alleys) - I had to stop and think about why I considered one one-lane narrow street an alley and another not. Even "alleys" like Ross Alley in Chinatown are pretty similar to "streets" like Colton St. near 12th and Otis. So I guess my question to you is: what do you consider an alley? I found that a harder question than I thought it would be.
posted by gyusan at 10:08 AM on March 5, 2015

I'm going to try using osmfilter, with a .osm of SF and the tag service=alley. That said, I have no idea what I'm doing, yet.
posted by sandettie light vessel automatic at 10:11 AM on March 5, 2015

gyusan, your question is valid. For the sake of sanity, I'm just going to go with objects called an alley by OSM.
posted by sandettie light vessel automatic at 10:12 AM on March 5, 2015

This is almost certainly not going to be answerable without some sort of GIS.

qGIS is free. Google Earth Engine has a free API that's well suited to this kind of thing.

But you just need a quick and dirty answer now, I'd do something akin to subsampling a few neighborhoods of varying densities, counting the alleys and streets by hand, and figuring out a ratio of street distance:alley distance. Then multiply by the streets in the city.
posted by zug at 10:21 AM on March 5, 2015

I'll make you a deal; if you can tell me what OSM properties describe "alley" to you in San Francisco, I'll do the GIS query. Or even better, teach you how to do it. I did something very similar for steps in SF a year or two ago. It's easy if you know how to import OSM data in to PostGIS and do some basic queries. Much harder if you don't and don't have time to learn.

So the hard part is defining "alley". I believe the official way to designate an alley in OSM is highway=service, service=alley. But I'm not certain anyone actually does tag things that way. Here's a map of all ways tagged service=alley in SF. (You can play around with this in the Overpass Turbo editor). That map doesn't really look all the alleys of SF to me though. You might want to look around in taginfo and find some other keys to identify alleys instead.

You can get an OSM extract just for San Francisco. That's a small enough dataset that it's pretty easy to manipulate with various tools. I use PostGIS myself, a database thing. QGIS is the right open source tool and can do the query you're asking. You can also just use grep or XML/JSON parsers to extract the data, although calculating the road length is a PITA without some geo-aware tool.
posted by Nelson at 11:20 AM on March 5, 2015

Nelson, still thinking on how to define alleys. More ASAP.
posted by sandettie light vessel automatic at 12:10 PM on March 5, 2015

It kind of depends on what you consider an alley. You'd be surprised how many narrow paths lined with dumpsters behind buildings, which most of us would call an alley, actually have street names and aren't, technically, considered alleys.
posted by Thorzdad at 1:56 PM on March 5, 2015