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Did the Earth Shift? Or is it just Chicago?

observed change in distances for city searches in Y!Local

9:47 pm on Sep 6, 2005 (gmt 0)

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Noticed a shift in what serves as the "City Center" that Y!Local uses to measure relative distances. The change was approximately 2.40 miles, not altogether insignificant.

It begs to question, what determines the city center? Is it population density (one of those scattered points graphs)? Is it geometric center? How did it change?

Unfortunately, I don't watch other geographies quite so closely, so I can't tell for certain if it was a global shift. Has anyone noticed a change in their distance rating when searching a business in a city?

As distance is still a factor, albeit a seemingly lesser factor than what it was in previous iterations of Y!Local's algorithm, this does have the potential to dramatically change search results... at least in Chicago.

10:25 pm on Sept 6, 2005 (gmt 0)

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Another local search person in Chicago! (I told you it is the Local Search capital)

I'm sure you'll get an answer from someone on here, sorry I don't have the answer.

4:17 pm on Sept 7, 2005 (gmt 0)

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Good question arnick!

Speaking as TL, for our stuff, we use a proprietary algorithm to compute the city center based on population density, number of businesses, etc. Geometric center is "close enough" about 60% of the time, but fails miserably when you get cities near a major body of water.

Zip code centroids are computed as one would expect.

So, our distance estimate will be different if you input a zip vs. a city.

One thing I've noticed is that some local search engines will pick a zip code as the center of the city, and use the centroid of that zip code for all of its distance calculations when a user inputs a city. Often, they'll pick the zip code of the main post office. This fails miserably in a town like Cleveland, where the main post office is about 4 miles from what would be considered the "city center".

FWIW: Y! Local is using the same centroid for Chicago that we are. We made a significant change about a year ago to how we compute those centroids, and they're probably using similar data.

Oddly enough, the centroids that are being computed by Y! and us are not what I would pick as the manual centroid for Chicago, and probably throws off downtown queries quite a bit. This is a hard question, though - what do you pick as the center? Magnificent mile, which is where most tourists would consider the center of the city? Or the loop, which is where most residents would consider the center? Neither of those are the geometric or density center, BTW.

One idea I've been throwing around is that perhaps the city center should not be a point, but an area. That makes distance calculations harder, but for downtown-type queries, it makes a lot of sense.

Ex: Embassy Suites in Chicago [beta.truelocal.com] - I would not say that the first two Embassy Suites are 3 miles from the city center - they're both very much in the city center for someone who may be performing that query, but that search result doesn't give enough information for you to know that.

The whole city center thing is a tough problem.

2:08 pm on Sept 9, 2005 (gmt 0)

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I would imagine the old setting would have been 0n/0e in chicago, which is Madison and State.

It looks like the new setting is Lumber Street and Halsted(according to yahoo maps) ( [maps.yahoo.com...] ). Which seems to be as close as one can get to the intersection of all the highways leading into the city. I am not sure why you would make this center, other then the highways intersecting there.

Ari, how did you determine the change was 2.4 miles, and does that intersection seem to be the same one that is being reflected in your distance calculation?

3:11 pm on Sept 9, 2005 (gmt 0)

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Thanks for all of the good insight. I suspected that population density was a factor, using some sort of scatter graph, which resulted in the Chicago center occuring where it did (in the middle of nowhere, as far as Chicago businesses go). But, I was surprised to hear that business density is also figured. You guys sure have access to good data. Any explanation for the recent move? Did your calculated center also recently move? Or, was that were it had always been?


I "happened" to have a listing that occured right at the previous center point (you are right, it used to be 0N and 0E downtown). Now, that same listing is reported as 2.4 miles from the center.

I'd hate to have to move my business :D