… 82% of the time.

One of the cooler features of the new site design is the weather “widget” in the top left hand corner of the page. Some people might have noticed that the weather report being displayed is for the major city that is closest to you. How did our website know what city you are in?

It’s All About The IP Address
Every computer on the Internet has an IP address. These set of numbers allow networks to send the data you requested to your computer and not the one down the street. The IANA is the organization that keeps track of IP addresses on the Internet. They assign a block of addresses to 5 different portions of the world: Africa, Asia/Pacific, North America, Latin America and the Caribbean, and Europe, Middle East, and China. The fact that this division is made is the first step in figuring out your physical location based on your IP address. At this point, we can narrow down which region of the world you are located. Obviously not accurate enough to use in our application.

In each one of these regions there is a organization that then redistributes IP blocks allocated to them by the IANA to various people or companies in that region. For our discussion, we will focus on ARIN (American Registry for Internet Numbers) which handles IP addresses in North America. ARIN would be similar to a telephone company that distributes telephone numbers when you move to a new house. Except in this case, they hand out IP addresses to ISPs and Corporations.

It’s Public Information
ARIN (as do all “registries” as they are called) require that “owners” of IP addresses have a physical address on record. This information is available to the public at various locations.

Lets take a real world example: My IP address for my computer at home is: 24.137.199.17. If I type that address in the ARIN website, I get the following result:

Aurora Cable Internet ACI2 (NET-24-137-192-0-1)
                                  24.137.192.0 - 24.137.223.255
Aurora Cable Internet HE-24-137-192-0-19 (NET-24-137-192-0-2)
                                  24.137.192.0 - 24.137.223.255

Great! I know that a company called “Aurora Cable Internet” is responsible for my IP address and any other IP address that is inside the ranges listed above.

The string after the company name (“ACI2″ and “HE-24-137-192-0-19″) will be able to provide us with more information about that company. Plugging in “ACI2″ into the same search box on the ARIN site reveals (among other information):

OrgName:    Aurora Cable Internet
OrgID:      ACI-38
Address:    350 Industrial Parkway South
City:       Aurora
StateProv:  ON
PostalCode: L4G-3L6
Country:    CA

Now we all know that I live in Aurora, ON! More importantly though, we have just associated a physical location with a IP address.

What’s this 82% business?
The database that we use is only 82% accurate within 40kms of true location.

So what happens if you are in the 18%? The weather defaults to Toronto (further proof that we’re the centre of the universe). From there you can click on the “change city” link to save a new location.

I live in Canmore, but the weather for Calgary shows up. Why?
We’ve only discussed one part of how this system works. We can now identify which city you are probably located in. However, we have weather data for hundreds of cities across Canada and we need to determine which city with a weather station is closest to you.

Currently the weather widget only knows about 21 cities in Canada (we will be expanding this shortly). To find the closest city with weather station to Canmore we use a calculation called “great circle distance“. Put simply, its the distance between two points on a sphere (the sphere in this case is the Earth).

Holy Math Batman! The Great Circle Formula

Using the latitude and longitude of Canmore we calculate the great circle distance to the 21 supported cities and return the one that is closest and display that data in the widget. This calculation only happens once and the information is saved as a cookie in your browser.

Tip of the Iceberg
The weather widget is just a sample of some of the possibilities with the location based website customization. Some of the possibilities can include:

• Biasing your searches on the website to rank items that are physically closer to you more important than others. For example if you search for “Conrad Black” and you are located in Vancouver, show news stories of Conrad Black in Vancouver at the top of the results list.
• Customizing the news page. Similar to the above, display your local news before or above the national news.
• Display the proper radio and television schedules for your area automatically.
• Show you more “relevant” advertisements based on your location.

I’m sure you will see more of this type of customization as cbc.ca evolves over the next little while.

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