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March 16, 2009
About Google Earth Imagery
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 new Google Earth 5, Google added a new historical imagery feature, you're not limited to just the imagery shown by default in Google Earth. Google has archives of imagery much larger than a single view of any one place. 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.]
Posted by FrankTaylor at March 16, 2009 8:55 AM
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Comments
Posted by: Joe L. at March 16, 2009 9:38 AM
The one thing that frustrates me the most about GE is the poor resolution of rural areas. I like to post photos to Panoramio and it is hard to geotag photos that I have taken in some rural areas because the poor resolution can make it impossible to accurately locate something. I hope that the new satellite will improve that situation over time.
Posted by: Acitta at March 16, 2009 11:39 AM
After reading this post, I guess I'm surprised that imagery of my city, which is the 2nd largest metropolitan area in Idaho (I know...that's not saying much) is 5-6 years old and still in low res. I watch closely for an update, but clearly Southeast Idaho is not high on anybody's list of important locations. Strangely, completely uninhabited, desolate areas just over the border in Wyoming are much more recent and higher resolution. Hmmm...
Posted by: jeff at March 16, 2009 1:37 PM
GE's imagery is at its best when it is accurately "placed" on the globe so that lat/long is correct. There are lots of examples of this mis-alignment, where a tower on top of a mt. is shown half-way down the side, or a river is shown half-way up the canyon wall.
Getting this cleaned up can be daunting. I suggest the Google come up with a way to crowd-source this. Like their Street View, where I can move an address icon to the right house, I should be able to skew a piece of imagery and get GE to make that change permanent.
How about devoting a conference to skewing imagery? That would keep me busy for weeks!
Posted by: Ernie at March 16, 2009 1:57 PM
Ernie, I'd take a guess and say the errors you describe, although they may be partly caused by poor registration, are most like caused by several other factors.
Firstly, the elevation data which is used to create the terrain models is generally quite course and as a result it is far from perfect. When you drape higher resolution aerial imagery over the top it may not match perfectly due to this mismatch in resolutions.
Secondly, imagery obtained from satellites and aerial surveys is again not perfect. Differences in elevation (eg bottom of a canyon vs the plateau) combined with the angle of the camera/sensor can cause distortions/displacements in the image. See here: http://www.geog.ucsb.edu/~jeff/115a/lectures/geometry_of_aerial_photographs_notes.html
Cleaning this up is far from straightforward and certainly not something the average user could accurately do. It is not a case of simply skewing an image to make it fit, it requires a far more complex process called orthorectification which itself requires various other data sources to correct the imagery.
Posted by: Brendan at March 16, 2009 3:03 PM
In my town, Braga, Portugal the last imaginary do not fit with the old one. We see that very clearly in GE 5, with the history images.
So all the 3D buildings, Panoramio photos, wikipedia articles are in the current imaginary out of place. I like to know if this is common in other places, and what can we do to correct the error.
Posted by: Joseolgon at March 16, 2009 4:08 PM
I'm really a fan of the "travell time" bar! I couldn't find some cool spots from http://www.caughtfromabove.com/ without it!!
Posted by: jorge at March 16, 2009 5:41 PM
Credit/Blame Neal Stephenson's book, Snow Crash, his "Google Earth" was real-time.
Posted by: Mark at March 17, 2009 2:59 PM
Trust me Frank I'm not surprised at all, we get this question regularly too, plus the other usual questions of can I read a license plate and see a persons face. No matter how many times you explain the limitations of public imagery, and physical limitations which make some things impossible for even Government satellites people will believe what they see on TV.
Posted by: Bull at March 17, 2009 5:05 PM
I like the way the imagery updates live on screen..very clever. I am waiting for my dog to come back home :(
Posted by: Billybob at March 23, 2009 2:54 PM
Why dont the Google people make a clear statement about the frequency of their satellites scanning specific zones? Are the Military/ Espionage purpose images very different from Google images in terms of being more recent or having better resolution? Lastly can the Google people control the image taking infrastructure to take more frequent pics when there are more interesting events taking place in a certain zone?
Posted by: Amar Madnani at August 21, 2009 2:44 PM
While in google earth on an Imac google suggested downloading an update. I did and now when I try to start the app again it says "not supported on this architecture" can't go any further. Can you help? Thanks, Bill claudio
Posted by: Bill Claudio at November 23, 2009 2:22 PM
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NOTE: Please use English. Comments are moderated.
"You would be surprised how many people initially think Google Earth will show imagery in real-time."
Yeah, and ironically, to non-technophiles who understand why real-time is not possible, this can make Google Maps/Earth less impressive.
I was showing my mother-in-law, a decided luddite, Google Maps a couple years ago, and of course we zoomed right into their house in Satellite view. She said "ooh, can we see your car outside?" I was genuinely puzzled and told her that no, these images were months or years old. She was visibly disappointed, and after that, the allure of looking at Google Maps was gone for her.