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According to this answer on Travel.SE, the Chinese National Bureau of Surveying and Mapping requires that published maps of China must include a certain deviation between the map and the real world.

In the comments, the answerer says that although the statute doesn't provide a reason, it is commonly believed to be for national security.

Have any Chinese officials or official documents provided a reason why maps must include this deviation?

What the Law Says

After reading that statute, it turns out that it does not explicitly require that maps be obfuscated.

However, the law does require the usage of a certain datum. A datum is basically the mathematical model which describes the shape of the earth (see this question on GIS.SE for more detail). According to wikipedia, the agency responsible for implementing this model has created the GCJ-02 datum, which has a random disturbance - effectively slightly randomizing the location of objects on the map.

U.S. - Selective Availability

There are many unsourced claims online which say that this random disturbance is for security reasons. Although I couldn't locate an original source making this claim, there is precedent for it.

Prior to 2000, the United States Department of Defense required the usage of 'Selective Availability' (SA). SA was a technology built into the GPS system which introduced random errors. GPS.gov, the official U.S. source of information about GPS, describes it as:

Selective Availability (SA) is the deliberate introduction of error to the precise timekeeping of the GPS satellites, thereby reducing both positioning and timing accuracy for civilian users. It was designed to provide U.S. and Allied military forces with a navigational advantage in times of crisis or conflict.
(source: GPS.gov)

Just to clarify, this isn't at all like the SA case where bits were unavailable unless you knew the key. The Chinese don't have their own GPS satellites (duh) setting these coordinates. What this GCJ-02 business is is a non-disclosed, but not-so-hard-to-reverse engineer conversion algorithm from other coordinates. The big picture looks like this:

That's the reverse-engineered conversion from the "bog standard" WGS-84, displayed as a vector field. As the blog from which I too that image says, the offsets are basically constant locally, but they vary across China.

It's speculated that abides by this globally in order to please Beijing (and abide by their Chinese license that allows them to take street pictures there). Other maps providers (that don't operate locally in China) aren't so bothered.

On Google something funny happens even when you’re viewing in the US: the Chinese border crossing plaza (on the north side) doesn’t line up with the Macau crossing (on the south side). This is due to a law where companies are forbidden to map China accurately, OpenStreetMap’s ability to avoid that law, and the independence of Macau and also Hong Kong (which has the same discrepancy).

As for the reason, according to one Chinese developer:

Before the WGS-84 <--> GCJ-02 transform algorithm leaked and made available everywhere on the Internet (eg. this repo), some people tried to get a lot of samples of WGS-GCJ point pairs and sold the dataset to people in need. But this kind of dataset is no longer useful because we now have the algorithm itself.

Are all these in the Chinese law?
No. They are mostly in some vague administrative orders or industry regulations made by the government. There is a Code of Geological Survey (http://www.gov.cn/ziliao/flfg/2005-08/05/content_20947.htm, Chinese), though. But it only says data obtained through survey is classified by default (Article 29) and companies without a permission cannot do survey (Chapter 5). Nothing about GCJ-02 is made officially public.

Why does the Chinese government do this?
National security, they say. But people developing LBS apps tends to believe that it is because of money. The government charges Chinese companies for the "shift correction" feature. We need to make a reservation with the government and take the source code of our app to the bureau to get the feature compiled with our code.