One possible culprit is that people have asked to have their homes blurred for privacy reasons. Google explains on its Maps privacy page how to request this, and it's a relatively simple process.
Famous people often file such requests. There's a gap in the Street View imagery outside Mark Zuckerberg's house in Palo Alto, for instance, although his home was visible in the past. But a lot of regular folk have asked to have their homes removed too, judging by the prevalence of blurred images in residential areas.
In some cases, those blurring requests may have caused some of gaps in Street View. If you approach the edge of gap on a residential street, you can often zoom in and see a house in the missing section that's been blurred out. It's hard to be sure about the causes, though, since Google isn't talking.
In extreme cases, whole countries are affected. Privacy has been a particular issue in Germany, where many people objected to the roll-out of Street View. Google now has Street View images only for big cities in Germany, like Berlin and Frankfurt, and appears to have given up on the rest of the country completely. Zoom out over Europe in Street View mode and Germany is mostly a blank island in a sea of blue.
Some users have noted the gaps in a Google Maps forum, and they point to some strange examples. On the tiny island of San Andres in the Caribbean Sea, a Street View car appears to have driven a third of the way around the island's perimeter road and then stopped for no obvious reason. There are no apparent obstacles or construction -- though there does happen to be a bar by the side of the road where the car stopped capturing images.
There's another curiosity in San Francisco's Presidio Terrace, a gated community with a circular road and about two dozen homes. Most of it doesn't appear on Street View, but there are a few short stretches deep inside the complex that do. A telltale shadow shows that one of Google's cars was in there, but for some reason Google displays images for only a fraction of it.
Other reasons for gaps in Street View could include temporary lane closures, the construction of new roads since Google photographed an area, and safety issues that prevented drivers visiting certain streets, said Jon Froehlich, an assistant professor of computer science at the University of Maryland, College Park, who has used Street View imagery to study accessibility problems.
Plenty of exotic locales are impressively covered, however, and Google is adding more all the time. If you want to use Street View to check out the Great Barrier Reef -- where the fish apparently have no concerns about privacy -- you can really go to town.
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