rough estimate of the time offset from GMT from latitude longitude

Question

Is there a way to estimate the offset from GMT (or time zone) from a latitude/longitude? I've seen geonames, but this would need to work long term and we don't really want to rely on a web service. It'd just be used for determining whether to display "today" or "tonight" when giving information to various users so it wouldn't need to be too accurate (an hour or two off wouldn't be bad).

Answer 1

Basing the time zone on the longitude alone is wildly inaccurate outside of international waters. See the map on this page:

http://askgeo.com/database/TimeZone

The vertical colored stripes in the deep ocean are the so-called natural time zones derived from longitude alone, and the colors of the land are the actual time zones per the governing laws. You can see that they don't line up very well at all.

I actually ran into this problem while working on a different project and did substantial research and development on it. First my research:

• First, time zones are not typically encoded by just an offset from GMT (aka UTC). That fails to take into account Daylight Savings Time, and changes in the time zones over the years. Instead, time zone IDs are used to designate a geographic area in which the official clock time has been the same throughout the area for a given period of time (e.g., since 1970). The most important system of such IDs is the "Olson time zone ID" (together these IDs and their offset rules are known as the "tz database"), which is used by Linux and other Unix operating systems. Most programming languages and operating systems have native or third party support for Olson time zone IDs.

In terms of existing solutions to convert latitude and longitude to time zone:

• GeoNames.org has a vast database of point locations (centers of cities, airports, public buildings, etc.), each of which is annotated with a bunch of useful metadata, including the Olson time zone ID. And they have a nice API to let you access these via the web. The trouble is that if unless the point you are querying is right on top of a record in their database, you might get a result that is on the other side of a time zone border, or you might get no response at all if your query is far from their nearest point. The web service is also painfully slow, and they limit the number of queries you can make in a day to a relatively small number.

• Earth Tools (http://www.earthtools.org/webservices.htm) also has a service for this, and it is much faster than GeoNames, but it returns just an offset from GMT, not a time zone ID, and it doesn't handle Daylight Savings Time correctly for most of the world. Also, it seems to not be maintained, so I'm not sure if the data is accurate anymore (time zones change over time).

After reviewing those options and searching for other possibilities without success, I decided to build my own solution, and have released it at:

http://askgeo.com

AskGeo is based on a time zone map of the world, so it returns a valid time zone for every valid latitude and longitude. It returns the standard Olson time zone ID (e.g., "America/Los_Angeles") used on Linux and most other operating systems and programming frameworks. It also returns the current offset, taking full account of daylight savings time.

It is extremely easy to use and usage is documented on the main page of the site. The API supports batch queries, so if you need to do a lot of look-ups, please use the batch interface rather than bog down our servers with serial requests. The bulk queries are also much faster, so everybody wins.

When we first launched this, we built it on Google App Engine (GAE) and made it free to all users. This was possible because GAE's prices were so low at that time. Since then, our server load has increased substantially and GAE's prices went way up. Both factors combined led us to switch to Amazon Web Services for hosting and to start charging for commercial use, while keeping the service free for non-profit, non-commercial open source projects, and researchers. For commercial users, we provide 1000 free queries to let potential customers evaluate the API to make sure it meets their needs. See the web site for pricing and terms.

The underlying library was written in Java and due to popular demand, we also released the library under a commercial license. Full documentation of the library and pricing details are on the web site.

I hope this is useful. It certainly was useful for the project I was working on.