Why You Can't Trust GPS in China

Why You Can't Trust GPS in China

Beijing China Map
39°54'11.8"N 116°23'29.0"E in Beijing Google Maps
Beijing China Map
39°54'11.8"N 116°23'29.0"E in Beijing
Google Maps

One of the most interesting, if unanticipated, side effects of modern copyright law is the practice by which cartographic companies will introduce a fake street—a road, lane, or throughway that does not, in fact, exist on the ground—into their maps. If that street later shows up on a rival company’s products, then they have all the proof they need for a case of copyright infringement. Known as trap streets, these imaginary roads exist purely as figments of an overactive legal imagination.

Trap streets are also compelling evidence that maps don’t always equal the territory. What if not just one random building or street, however, but an entire map is deliberately wrong? This is the strange fate of digital mapping products in China: there, every street, building, and freeway is just slightly off its mark, skewed for reasons of national and economic security.

The result is an almost ghostly slippage between digital maps and the landscapes they document. Lines of traffic snake through the centers of buildings; monuments migrate into the midst of rivers; one’s own position standing in a park or shopping mall appears to be nearly half a kilometer away, as if there is more than one version of you on the loose. Stranger yet, your morning running route didn’t quite go where you thought it did.

It is, in fact, illegal for foreign individuals or organizations to make maps in China without official permission. As stated in the “Surveying and Mapping Law of the People’s Republic of China,” for example, mapping—even casually documenting “the shapes, sizes, space positions, attributes, etc. of man-made surface installations”—is considered a protected activity for reasons of national defense and “progress of the society.” Those who do receive permission must introduce a geographic offset into their products, a kind of preordained cartographic drift. An entire world of spatial glitches is thus deliberately introduced into the resulting map.

The central problem is that most digital maps today rely upon a set of coordinates known as the World Geodetic System 1984, or WGS-84; the U.S. National Geospatial-Intelligence Agency describes it as “the reference frame upon which all geospatial-intelligence is based.” However, as software engineer Dan Dascalescu writes in a Stack Exchange post, digital mapping products in China instead use something called “the GCJ-02 datum.” As he points out, an apparently random algorithmic offset “causes WGS-84 coordinates, such as those coming from a regular GPS chip, to be plotted incorrectly on GCJ-02 maps.” GCJ-02 data are also somewhat oddly known as “Mars Coordinates,” as if describing the geography of another planet. Translations back and forth between these coordinate systems—to bring China back to Earth, so to speak—are easy enough to find online, but they are also rather intimidating to non-specialists.

While algorithmic offsets introduced into digital maps might sound like nothing more than a matter of speculative concern—something more like a dinner conversation for fans of William Gibson novels—it is actually a very concrete issue for digital product designers. Releasing an app, for example, whose location functions do not work in China has immediate and painfully evident user-experience, not to mention financial, implications.

Shanghai China Map
Google Maps

One such app designer posted on the website Stack Overflow to ask about Apple’s “embeddable map viewer.” To make a long story short, when used in China, Apple’s maps are subject to “a varying offset [of] 100-600m which makes annotations display incorrectly on the map.” In other words, everything there—roads, nightclubs, clothing stores—appears to be 100-600 meters away from its actual, terrestrial position. The effect of this is that, if you check the GPS coordinates of your friends, as blogger Jon Pasden writes, “you’ll likely see they’re standing in a river or some place 500 meters away even if they’re standing right next to you.”

The same thread on Stack Overflow goes on to explain that Google also has its own algorithmically derived offset, known as “_applyChinaLocationShift” (or more humorously as “eviltransform”). The key, of course, to offering an accurate app is to account for this Chinese location shift before it ever happens—to distort the distortions before they occur.

In addition to all this, Chinese geographic regulations demand that GPS functions must either be disabled on handheld devices or they must be made to display a similar offset. If a given device—such as a smartphone or camera—detects that it is in China, then its ability to geo-tag photos is either temporarily unavailable or strangely compromised. Once again, you would find that your hotel is not quite where your camera wants it to be, or that the restaurant you and your friends want to visit is not, in fact, where your smartphone thinks it has guided you. Your physical footsteps and your digital tracks no longer align.

It is worth pointing out that this raises interesting geopolitical questions. If a traveler finds herself in, say, Tibet or on a short trip to the artificial islands of the South China Sea—or perhaps simply in Taiwan—are she and her devices really “in China”? This seemingly abstract question might already be answered, without the traveler even knowing that it’s been asked, by circuits inside her phone or camera. Depending on the insistence of China’s territorial claims and the willingness of certain manufacturers to acknowledge those assertions, a device might no longer offer accurate GPS readings.

Put another way, you might not think you’ve crossed an international border—but your devices have. This is just one, relatively small example of how complex geopolitical questions can be embedded in the functionality of our handheld devices: cameras and smartphones are suddenly thrust to the front line of much larger conversations about national sovereignty.

These sorts of examples might sound like inconsequential travelers’ trivia, but for China, at least, cartographers are seen as a security threat: China’s Ministry of Land and Resources recently warned that “the number of foreigners conducting surveys in China is on the rise,” and, indeed, the government is increasingly cracking down on those who flout the mapping laws. Three British geology students discovered this the hard way while “collecting data” on a 2009 field trip through the desert state of Xinjiang, a politically sensitive area in northwest China. The students’ data sets were considered “illegal map-making activities,” and they were fined nearly $3,000.

What remains so oddly compelling here is the uncanny gulf between the world and its representations. In a well-known literary parable called “On Exactitude in Science," from Collected Fictions, Argentine fabulist Jorge Luis Borges describes a kingdom whose cartographic ambitions ultimately get the best of it. The imperial mapmakers, Borges writes, devised “a Map of the Empire whose size was that of the Empire, and which coincided point for point with it.” This 1:1 map, however, while no doubt artistically and conceptually wondrous, was seen as utterly useless by future generations. Rather than enlighten or educate, this sprawling and inescapable super-map merely smothered the very territory whose connections it sought to clarify.

Mars Coordinates, eviltransform, _applyChinaLocationShift, the “China GPS Offset Problem”—whatever name you want to describe this contemporary digital phenomenon of full-scale digital maps sliding precariously away from their referents, the gap between map and territory is suitably Borgesian.

Indeed, Borges ends his tiniest of parables with an image of animals and beggars living wild amidst the “tattered ruins” of an abandoned map, unaware of what its original purpose might have been—perhaps foreshadowing the possibility that travelers several decades from now will wander amidst remote Chinese landscapes with outdated GPS devices in hand, marveling at their apparent discovery of some parallel, dislocated version of the world that had been hiding in plain view.

Geoff wishes to thank Twitter user @0xdeadbabe for first pointing out “Mars Coordinates” to him. Follow Geoff on Twitter at @bldgblog.

Promoted Stories
Explore More
More from T+L