How does the "Metafilter users near me" thing work?

February 25, 2006 3:12 PM

Question about the "Metafilter Users Near..." thing.

I show a user named mdevore within 4 miles of me. He in turn shows a huge slug of users at the 23 mile mark from him.

Given our relatively close proximity, shouldn't more of those being showing up on my listing as well? What's the outer threshold of that listing?
posted by hwestiii to MetaFilter-Related at 3:12 PM (10 comments total)

Yeah, according to my amazing trigonometric skills, that crowd cannot be more than 27 miles (or so) away from hwestlii, yet they do not show up. I don't think it's a cap on the number of "nearest users," either, because I've got about 130 more "nearest users" listed than hwestlii does. Intriguing?
posted by jenovus at 3:23 PM on February 25, 2006

What jenovus said. This is actually better that it has been. Before I figured that my correct longitude was actually negative, I found myself all alone, well there was one other guy, somewhere in rural China.
posted by hwestiii at 3:29 PM on February 25, 2006

Regardless, I wouldn't let that mdevore guy closer than four miles. Shifty character.
posted by mdevore at 3:59 PM on February 25, 2006

A user setting for the radius would be cool. Considering my rural China location it would still be interesting to note who is within a thousand miles or so.

Once the user tracking chips, (UTC's), are in place this will all be automated.
posted by geekyguy at 4:15 PM on February 25, 2006

Yeah, according to my amazing trigonometric skills, that crowd cannot be more than 27 miles (or so) away from hwestlii, yet they do not show up. I don't think it's a cap on the number of "nearest users," either, because I've got about 130 more "nearest users" listed than hwestlii does. Intriguing?

That's assuming the lat./long. are being exactly entered instead of approximated with a zipcode. I think there's something underlying happening with the zipcode-distance algorithm that we can't see.
posted by onalark at 4:32 PM on February 25, 2006

It just does a +/- 0.5 degree check. It's possible someone is within that range for one person and not another, even if they are nearby. It's a rough calculation and a hack.
posted by mathowie (staff) at 4:57 PM on February 25, 2006

And since you were wondering, at 41 degreees latitude, +/- 0.5 degree equals +/- 35 miles N/S and +/- 25 miles E/W.
posted by smackfu at 6:24 PM on February 25, 2006

Don Knuth once observed that a lot of US maps didn't observe the triangle inequality and tried to create an algorithm for a map that did. I believe it worked well everywhere except for Wisconsin.
posted by transona5 at 6:57 AM on February 26, 2006

I already posted this thing once before, but just in case it looks more helpful with age:

SELECT *
FROM usertable
WHERE lat IS NOT NULL AND lon IS NOT NULL
AND DEGREES(ACOS(SIN(RADIANS(slat)) * SIN(RADIANS(lat)) + COS(RADIANS(slat)) * COS(RADIANS(lat)) * COS(RADIANS(slon-lon)))) * 69.04 < limit in statute milesbr> LIMIT absolute max

Where lat, lon are your column names and slat and slon are the coordinates of the user you're searching for.
posted by moift at 9:21 PM on February 26, 2006

well, it didn't come out too bad. It should be '<= limit...' and without the half break tag.
posted by moift at 9:23 PM on February 26, 2006