terminology - How do I achieve the technique which creates the illusion of busy locations being empty?

I remember seeing a series of photos where popular tourist locations are shown to be completely empty. I'm assuming it was done by taking a series of photos and then combining them in Photoshop. But what is the name of that technique so that I can research it further? Bonus points if you actually post the required steps within your answer.

Answer

One straightforward way to achieve this effect is to take multiple shots of the same scene and combine them using median blending. This technique is commonly used for noise reduction, and does it very effectively — so effectively, in fact, that it can even hide "noise" such as random people walking through the scene.

What median blending effectively does is combine multiple shots of the same scene, replacing the color of each pixel (or, rather, each RGB color channel of each pixel) with the median of the colors of that pixel in each shot — that is, a color value chosen to be lighter than in 50% of the shots and darker than in the other 50%.

Compared to simple averaging (either by digital stacking, or simply by taking a longer exposure), median blending is a lot more effective at rejecting "outliers", i.e. pixel values that happen to lie far from the "consensus" value, whether due to noise or due to an obstacle in the frame. In practical terms, what this means is that properly done median blending doesn't suffer from the faint blurry "ghosts" that moving people or other obstacles tend to produce in averaged images or in long exposures.

The main drawback of median blending is that, to guarantee successful obstacle removal, each pixel in the image must be unobscured in over 50% of the shots. Otherwise, it's possible that the "true" unobscured color of some pixels will actually end up getting rejected as an outlier, leading to more or less random fragments of the foreground obstacles showing up in the results. As such, median blending is poorly suited for very crowded scenes where the requirement of 50% unobscured coverage cannot be reliably met. (In such cases manual stitching, as suggested by Jahaziel, may be more effective at the cost of extra labor.)

Also, median blending can give poor results if the actual scene you wish to capture contains some moving elements such as waving flags or clouds moving across the sky. Whereas a long exposure simply causes moving clouds to blur into streaks, median blending will try to color each pixel either "cloud gray" or "sky blue", depending on which color happens to appear in a majority of the shots, potentially producing weird and ugly results. Again, it may be possible to fix this by manual masking (e.g. by using a single shot for the sky and only applying median blending to the non-sky parts of the picture).

Also, as with any image stacking technique, you obviously need a static scene and the ability to take multiple shots of it from the exact same viewpoint. To some extent you can compensate for minor camera shake with digital image alignment, but a good tripod is definitely recommended.

(Unfortunately, I don't personally have any good pictures available to illustrate this technique with. However, the PetaPixel post I linked to in my first paragraph above does have some, as does this tutorial that I found via Google.)