digitalglobe

You would be surprised how many people initially think Google Earth will show imagery in real-time. Or, that surely it will only be a day old. I guess part of this thinking comes from watching the weather satellite photos which are only a few hours old, or live weather radar. But, the problems of getting high resolution imagery are very challenging. Weather satellites are at geosynchronous orbits (36,000 km). High resolution satellites (e.g. those operated by commercial satellite companies like GeoEye or DigitalGlobe) operate just a few hundred kilometers above the Earth. This means they only see a small part of the Earth with their camera as they orbit over. They typically go around the Earth every 90 minutes, but only cover about 1% of the Earth on each pass (you can see strips of imagery if you look at the imagery in Google Earth) – but, most of the area covered in a pass is water. Not only that, but imagery for Google Earth is only going to be good if the sun is at a high angle when the satellite goes over (fewer shadows), when there are no clouds, and as little haze/pollution as possible. Believe it or not, the times when these factors all come together are pretty rare.

Once the imagery is taken, it takes time to process the data by a commercial provider like GeoEye before it is available to customers. Google is one of these customers (a really big one). Google has to evaluate the new imagery against the current imagery to determine whether the new is better than the current. I assume they are trying to automate as much of this as possible. But, for important areas with large populations the process most likely involves people. This process takes time – especially when you think about the quantities of land mass of the Earth. Once an image is selected, it has to be processed into the format and coordinate system of Google Earth’s databases. Then it has to go through a quality control process and fed into a processing system before it gets distributed to the live Google Earth database servers. This is one reason why you usually do not find any imagery younger than about 6 months in Google Earth. And why updates only happen about once every 60 days.

Not all the imagery in Google Earth comes from satellites. A lot of the imagery comes from aerial photographers mostly in airplanes with special high resolution cameras. Some of the imagery even comes from kites and balloons. Google acquires imagery from a variety of providers. Some of the imagery is given to Google by city or state governments. The age of the imagery varies greatly, but most of the high resolution imagery is between 6 months and 5 years of age. Again, because the imagery comes from a variety of sources, the process to get this imagery into Google Earth is complex and involves a great deal of time and effort.

Another reason why you don’t find imagery that is newer is that it can cost a great deal of money to acquire quality aerial imagery. The companies who spend this money need a way to recover their costs. More recent imagery is more valuable than older imagery. As a result, these companies are reluctant to have their newest imagery available for free for anyone to view in Google Earth. Read the agreements for Google Earth before you try to use its imagery for business applications (more information). You can’t sell or use the imagery from Google Earth for business purposes without permission.

Google has been known to release much more recent imagery in GE for unique events. For example, for the 2008 Beijing Olympics Google released 2-week old imagery for the Beijing area.
However, near real-time imagery of Earth is available in Google Earth! “What?! After all that you are saying it is available?” you ask. Sure, first there’s the new Clouds layer. Found under the Weather layer folder. The clouds are actually taken from weather satellites and are a global picture of the clouds as recent as 3 hours old. Ok, so that’s not the kind of imagery you meant.

NASA has a layer they call DailyPlanet which shows the entire Earth at a medium resolution (about 250 meter resolution per pixel). You can view DailyPlanet in Google Earth . The imagery is taken by the MODIS Terra satellite and is processed as quickly as possible and shows the entire Earth between 6 – 12 hours old. The imagery is continuously updating. When you download the network link above you see the Earth with clouds. Clouds may not line up properly because the satellite has to make multiple passes to get a full picture and the images have to be spliced together. As you zoom in, you will see higher resolution imagery load – up to a point (since this is medium resolution imagery). Remember the fires in California last October? You could have watched the smoke visible from space with this layer (except the layer just became available in December). You can see dust storms, large fires, volcanoes, haze conditions, droughts, floods, and – of course – clouds. Right now, this layer from NASA is the most recent, highest resolution imagery of the Earth continuously updating available to the general public.

And, with the release of Google Earth 5, Google added a new historical imagery feature, so you’re not limited to just the imagery shown by default in Google Earth. Google has archives of imagery from many sources and dates. Now for many places, Google has 2, 3, or even 30 different images over time for any one location. In some cases, you can even find newer imagery than the one shown by default. Usually in a case where older imagery looks better than the newer. The historical imagery feature is a an amazing resource, which I encourage everyone to check out.
Anyway, I hope this article helps provide a better understanding of the imagery in Google Earth and how it all works. This is a high-level overview and is based on my own observations and opinions. Feel free to comment below. (Originally posted Feb 2008)

[NOTE: This article also available in Spanish, and in French.]