Surveying

According to articles 7, 26, 40 and 42 of the Surveying and Mapping Law of the People's Republic of China, private surveying and mapping activities have been illegal in mainland China since 2002. The law prohibits^[4]

publishing, without authorization, significant geographic information and data concerning the territorial air, land and waters, as well as other sea areas under the jurisdiction of the People's Republic of China.

— The National Administration of Surveying, Mapping and Geoinformation of China, Surveying and Mapping Law of the People's Republic of China

Article 1 says:

This Law is enacted to strengthen the administration of the surveying and mapping undertaking, promote its development and ensure that it renders service to development of the national economy, the building up of national defence, and progress of the society.^[5]

Fines range from 10,000 to 500,000 CNY ($1550 – $77519 USD). Foreign individuals or organizations that wish to conduct surveying must form a Chinese-foreign joint venture.^[1]

Between 2006 and 2011, the authorities pursued around 40 illegal cases of mapping and surveying.^[6] The media has reported on other cases of unlawful surveys:

• March 7, 2007 - Japanese and Korean scholars fined; joint-venture Weihai hired foreign surveyors without approval from the government^[7]
• March 25, 2008 - China's State Bureau of Surveying and Mapping cracks down on some of the 10,000 websites that publish maps in China, most without authorization.^[8]
• January 6, 2009 - Chinese authorities fine UK students for “illegal map-making activities”.^[9]
• 2010 - Chinese authorities to crack down on the unregistered or illegal among 42,000 online map providers, targeting incorrect information and leaks of sensitive information involving state secrets. New standards require all Internet map providers to keep servers storing map data inside China.^[10]
• March 14, 2014 - Coca-Cola is accused of illegal mapping.^[11]

As a consequence, major digital camera manufacturers including Panasonic, Leica, FujiFilm, Nikon and Samsung restrict location information within China.^[12]

OpenStreetMap, the crowdsourced project to assemble a map of the world, advises that "private surveying and mapping activities are illegal in China".^[13]

Map content

Chinese Wikisource has original text related to this article:

Category:PRC maps

Chinese law and regulations also rule on the contents of any published map:

• The 公开地图内容表示若干规定 ("some rules on the content presentation of public maps") of 2003 by the Bureau of Surveying and Mapping (BSM) prohibits the representation of sensitive information such as airports (except those specially listed), military bases, and waterway depths. It also describes the handling and naming of disputed territories and former Qing territories ceded to Russia.^[14]
• The 公开地图内容表示补充规定（试行）("supplementary rules on the content presentation of public maps, trial version") of 2009 from the BSM additionally prohibits, among other things:^[15]
  • Locations having a precision of finer than 50 meters and the use of grids finer than 100 meters for elevation data.
  • Showing the locations of key infrastructure (power, dams, weather stations) and public safety installations (prisons, mandatory drug rehab facilities).
  • Showing the internal structures of airports and ferry ports.
  • Showing the weight and height limits of bridges, roads, and tunnels; showing the material of roads.
• The 地图管理条例 ("map management regulation") of 2015 from the State Council mandates that all Internet maps must be stored in mainland China, among other rules about national security. The law also details punishments, some of which criminal, for violations.^[16]

In 2016, a large-scale search by Chinese law enforcement found 253 types of problematic paper maps and 1000 problematic online map websites, most pertaining to the depiction of Taiwan and 9-dash line.^[17]

GCJ-02

GCJ-02 (officially Chinese: 地形图非线性保密处理算法; lit. 'Topographic map non-linear confidentiality algorithm')^[21] is a geodetic datum used by the Chinese State Bureau of Surveying and Mapping (Chinese: 国测局; pinyin: guó-cè-jú), and based on WGS-84.^[22] It uses an obfuscation algorithm^[23] which adds apparently random offsets to both the latitude and longitude, with the alleged goal of improving national security.^[20][24] There is a licence fee associated with using this mandatory algorithm in China.^[21]

A marker with GCJ-02 coordinates will be displayed at the correct location on a GCJ-02 map. However, the offsets can result in a 100–700 meter error from the actual location if a WGS-84 marker (such as a GPS location) is placed on a GCJ-02 map, or vice versa. The Google Maps street map is offset by 50–500 meters from its satellite imagery.^[11][25] Yahoo! Maps also displayed the street map without major errors when compared to the satellite imagery.^[26] MapQuest overlays OpenStreetMap data perfectly as well.^[27]

Despite the secrecy surrounding the GCJ-02 obfuscation, several open-source projects exist that provide conversions between GCJ-02 and WGS-84, for languages including C#,^[28] C, Go, Java, JavaScript, PHP,^[29] Python,^[30] R,^[20] and Ruby.^[31][32] They appear to be based on leaked code for the WGS to GCJ part.^[33] Other solutions to the conversion involve interpolating coordinates based on regression from a data set of Google China and satellite imagery coordinates.^[34] An attempt by Wu Yongzheng using fast Fourier transform analysis gave a result much like the leaked code.^[35]

From the leaked code,^[28][36] GCJ-02 uses parameters from the SK-42 reference system. The parameters were used to calculate lengths of one degree of latitude and longitude, so that offsets in meters previously calculated can be converted to degrees for the WGS-84 input coordinates.

BD-09

BD-09 is a geographic coordinate system used by Baidu Maps, adding further obfuscation to GCJ-02 "to better protect users' privacy".^[37][19] Baidu provides an API call to convert from Google or GPS (WGS-84), GCJ-02, BD-09, MapBar or 51ditu coordinates into Baidu or GCJ-02 coordinates.^[38][18] As required by local law,^[38] there is no API to convert into WGS-84, but open source implementations in R^[20] and various other languages^[29] exist.

Reverse transformation

As the actual algorithm is now available in open source form (see above), the text below is obsolete.

GCJ-02 appears to use multiple high-frequency noises of the form ${\displaystyle 20n\sin {}(180k\times lat_{rad})}$, effectively generating a transcendental equation and thus eliminating analytical solutions. However, the open-source "reverse" transformations make use of the properties of GCJ-02 that the transformed coordinates are not too far from WGS-84 and are mostly monotonic related to corresponding WGS-84 coordinates:^[39][20]

from typing import Callable

# Represent coordinates with complex numbers for simplicity
coords = complex
# Coords-to-coords function
C2C = Callable[[coords], coords]

def rev_transform_rough(bad: coords, worsen: C2C) -> coords:
    """Roughly reverse the ``worsen`` transformation.

    Since ``bad = worsen(good)`` is close to ``good``,
    ``worsen(bad) - bad`` can be used to approximate ``bad - good``.

    First seen in eviltransform.
    """
    return bad - (worsen(bad) - bad)

def rev_transform(bad: coords, worsen: C2C) -> coords:
    """More precisely reverse the ``worsen`` transformation.

    Similar to ``rev_transform_rough``,
    ``worsen(a) - worsen(b)`` can be used to approximate ``a - b``.

    First seen in geoChina/R/cst.R (caijun 2014).
    Iteration-only version (without rough initialization) has been known
    since fengzee-me/ChinaMapShift (November 2013).
    """
    eps = 1e-6
    wgs = bad
    improvement = 99 + 99j  # dummy value

    while abs(improvement) > eps:
        improvement = worsen(wgs) - bad
        wgs = wgs - improvement
    return wgs

The rough method is reported to give some 1~2 meter accuracy for wgs2gcj,^[29] while the exact (fixed point iteration) method is able to get "centimeter accuracy" in two calls to the forward function.^{[note 1][40][39]} Since the two properties ensure some basic functionality of the coordinate system, it is unlikely that the methods will change with new coordinate systems. The BD-to-GCJ code works in a manner much like the rough method, except that it removes the explicitly-applied constant shift of ~20 seconds of arc on both coordinates first and works in polar coordinates like the forward function does.^[20]

The establishment of working conversion methods both ways largely renders obsolete datasets for deviations mentioned below.^[41]

Hong Kong and Macau

Under One Country Two Systems, legislation in mainland China does not apply in Hong Kong and Macau SARs and there are no similar restrictions in the SARs. Therefore, the GPS shift problem does not apply. However, at the border between the SARs and mainland China, the data shown by online maps such as Google Maps^[51] are broken where the shifted data and correct data overlap. This poses problems to users travelling across the border, especially visitors not aware of the issue.