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An orthophoto is an undistorted aerial photograph with a completely uniform scale that allows it to function as a map. A significant amount of geometric correction, known as orthorectification, is required to bring about this high level of uniformity. An orthophoto is effective because, like a picture, it is highly detailed but it is also completely uniform, like a map. In many cases, map features, such as lines for roads and various markers for specific notable buildings, are superimposed over such a photo, increasing its effectiveness as a map. Many different methods of correction are used to produce orthophotos; some involve manual manipulation of the film, while other more common methods use computer systems for geometric correction.
Optical methods of manipulating orthophotos have almost completely been replaced by digital methods as sufficiently powerful computers have become much more affordable, convenient, and available. While a significant amount of human work is still required to produce a particularly good orthophoto, powerful computer programs and algorithms have automated a great deal of the process. This level of automation greatly reduces the cost and increases the efficiency of orthophoto production.
An orthophoto is particularly superior to a normal aerial photo because it can be directly used for measurement. All distortion from the lens is corrected for, as is the tilt of the camera as a whole. Topographical relief is also corrected—the distances across valleys, mountains, and other distortions in the landscape are adjusted so the elevation is considered in a calculation of distance. If, for example, one were to attempt to measure the distance between two points on an aerial photo, he would measure an incorrect distance if significant changes in elevation were present between the two points. Such elevation distances are corrected in orthophotos, so such measurements can be taken without need for an actual map as long as the photo's scale is known.
Errors can enter the process of orthophoto production at almost any stage. While most are completely harmless and correctable, others can actually affect the usability of the orthophoto. Sometimes, features of the natural environment being photographed, such as a reflection from the surface of a body of water, can cause distortions to the photo. In some cases, these are merely aesthetically displeasing, while in others they can lead to difficulties in the geometric correction of an aerial photo. Human errors can also cause problems; these can range from scratching the lens or film to improperly carrying out the digital manipulation of the photo.
@pleonasm - The fact that they use computer programs to develop their maps makes me wonder how accurate they are. I know computers might be less likely to make a mistake than a human, but they don't have the flexibility of humans either.
You hear about random people who have discovered strange things using Google Maps all the time, which to me implies that those random people are the first ones to take a really good look at those particular images.
Back when people had to look at every single photo in order to process it, that would never happen. But with a digital orthophoto there's no need for it, so many of them go without attention.
I just hope the computers know what they are doing considering how many people use and depend on Google Maps.
@pastanaga - As it says in the article, there are powerful computer algorithms that can correct errors in the photos that are used in Google Maps and with any other similar service.
I'm sure a lot of work goes into it as well, but it's not entirely human effort that goes into every single photo.
And they aren't just doing it from the goodness of their hearts. Admittedly, they could charge a small fee if they wanted, but where Google really makes its money is through advertising and information. Every time you look for directions or examine a particular map, that information goes off to Google who can use it to target ads for you.
They aren't a bad company, but they wouldn't be able to pay for developing all that sophisticated orthophoto technology if they were completely altruistic.
When you read about how much work needs to be put into these sorts of photos before they can be used as a map, it really makes you realize how much work Google puts into the Google Maps function, which they provide to everyone for free.
They must have to go through and connect up all the different satellite photos in order to make comprehensive aerial maps out of them, as well as adding all the different roads and buildings and so forth. And they do this for almost every country in the world. Isn't that incredible?
Yet it's something that gets taken completely for granted.
When you think about how rare and precious accurate maps were even a couple of hundred years ago, it makes me really happy I live in a world where I can type a command and have instant directions to anywhere within a few minutes.