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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?

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    Historically all governments have learned from history. During WW2 the allied forces planned the D-Day invasion using maps form the National Geographic magazine. They were so accurate that you could depend on them to plan large-scale military manoeuvres. Since then all governments have to some degree or other restricted super accurate land survey mapping. A lot of democratic countries have issues with balancing needing to keep your maps secret and public use so some countries only "censor" specific locations - this in itself is problematic since the censorship itself reveals intelligence – slebetman Apr 26 at 4:29
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    @slebetman Eh. Any country that can launch precision guided weapons from outside your borders has probably made its own maps of your country by photographing it from space. Conversely, anything that's piloted (a bomber plane or even drone) isn't going to be so confused by you messing with the map a bit. – David Richerby Apr 26 at 17:48
  • On a similar topic, I recently became interested in the exact layout of underground railway lines in the between Gants Hill and Leytonstone in East London. There are several map sources that pretend to show the route. They differ quite considerably. They don't align with a couple of easily visible ventilation shafts, or with local observation (I used to live in a house directly above the track, I KNEW where it went!). Obfuscation for military purposes? In WW2 the tunnel, as yet uncomissioned as a railway, was used as a 'secret' factory by Plessey, a manufacturer of war equipment. – Laurence Payne Apr 27 at 12:20
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Just to clarify, this isn't at all like the GPS Selective Availability 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:

enter image description here

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.

Google bought the China map data from an officially approved Chinese source company. Other maps providers (that didn't/don't operate locally in China) aren't skittish to transform/align the Chinese maps to WGS84.

enter image description here

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 in a post from April 2015:

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.

Since then Xinhua reported new regulations on maps being passed at the end of 2015, and taking effect in 2016. Amazingly these are available in English, but they say nothing about using some specific coordinates; so the ambiguity noted in that Chinese dev comment basically continues. The new regulations do say that maps must be submitted for government review, except in some very limited circumstances.

For on-line maps there's this verbiage in the new regs (which doesn't clarify much):

Article 33. To provide the public with services such as geospatial positioning, geoinformation uploading and labeling, and map database development, a provider of Internet map services shall obtain the necessary qualification certificate for surveying and mapping in accordance with law.

To engage in Internet map publishing, a provider of Internet map services shall be subject to review by and approval of the competent publication administration department of the State Council in accordance with law.

Article 34. A provider of Internet map services shall place its map data servers within the territory of the People’s Republic of China, and shall establish the management system and introduce safeguard measures for the security of Internet map data.

The competent administration departments for surveying, mapping and geoinformation of the people’s governments at or above the county level shall, jointly with the relevant departments, strengthen supervision and administration of the security of Internet map data.

[...]

Article 38 A provider of Internet map services shall use maps that have been reviewed and approved in accordance with law, and strictly verify newly added content of the Internet maps, and it shall, in accordance with the relevant provisions of the State, file such content with the competent administration department of the State Council for surveying, mapping and geoinformation or the competent administration department for surveying, mapping and geoinformation of the province, autonomous region, or municipality directly under the Central Government.

Basically it seems to be a system of: give us your maps and will give them back to you as we think they should be published... which at some point probably involves the coordinate system, but it's not mentioned explicitly in these regs. There are some up to 100,000 yuan fines for not following these regulations.

Also according to a Chinese page, GCJ-02 (although developed by the State Bureau of Surveying and Mapping) is not the same as the official Chinese CGCS2000 system, for which a lot public info is available.

So it's hard to answer a "why" for something that's hard to even find an official record that it needs to happen...


Unless the Chinese decide to change that (leaked code) and ban all prior maps, this doesn't compare to the SAASM, which supposedly changed keys (on the satellites and receivers) once in a while. SAASM even allows rekeying over insecure channels, while the older PPS-SM required a secure channel (e.g. taking the GPS device to an authorized/secure location for rekeying.)

I suppose I should mention that the Chinese are building their own GPS alternative, the BDS (BeiDou Navigation Satellite System), which apparently became operational in December last year, but that's a different story. And BDS broadcasts its location info in the aforementioned CGC[S]2000.

Finally, Baidu "Total View" has a Street View equivalent with altered photos ... like [badly] disappeared buildings etc. You'd probably be hard pressed to find an official explanation for this either. That article contains some speculation about the reasons, but that's about it.

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    The Chinese don't have their own GPS satellites they do have a global navigation satellite system; it's called BeiDou. – Nosrac Apr 25 at 14:06
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    @indigochild: corrects your various misleading claims like "GCJ-02 datum [...] has a random disturbance" and adds a quote from a Chinese dev on his 2 cents how and why this is the way it is. The latter also contradicts your claim that "the law does require the usage of a certain datum". – Fizz Apr 25 at 14:10
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    @Nosrac: that's covered in the last para if you take the time to read the whole anwer. And that's not same thing as them having GPS satellites (with Chinese codes). You need a different receiver for BDS to work. (True there are some Chinese mixed receivers, but then so are mixed GPS/GLONASS receivers--I have one). – Fizz Apr 25 at 14:10
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    @indigochild it seems to me that clarifying the very nature of the obfuscation is a fairly important part of explaining why it was done. – DaveInCaz Apr 25 at 15:42
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    @indigochild Your answer says nothing at all about the "why" and is very inaccurate about the "what". – David Richerby Apr 25 at 18:03
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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 an official 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. The U.S. previously required another technology which also altered location data for defense purposes.

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)

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    Not done yet, but this is what I have so far. – indigochild Apr 25 at 4:07
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    Upvote, but the "error" in GCJ-02 datum does not seem to be random, i.e. it's security by obscurity (of the [conversion] algorithm). Quite a different thing. – Fizz Apr 25 at 11:38
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    @Fizz "Random" has many meanings. An algorithm generating a predictable stream of numbers is still called a "random number generator". The output of an encryption algorithm is supposed to look random to the observer, etc. – pipe Apr 25 at 11:54
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    @pipe Such an algorithm is properly described as a "pseudorandom number generator". In the case of this answer, it's particularly misleading to use the same word, "random", to describe two completely different phenomena. GPS selective availability is the addition of pseudorandom noise to the GPS signal so people with non-classified receivers receive less accurate information. In particular, A GPS unit will give different answers about the same location, when SA is in use. But the Chinese system applies a fixed distortion to the map that doesn't change over time. That distortion is "random"... – David Richerby Apr 25 at 18:00
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    ... only in the very weakest sense of the word, meaning that it displays no particularly obvious pattern. – David Richerby Apr 25 at 18:01

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