GPS offset in Chinese Maps

GPS offset in Chinese Maps

Cartographic companies introduce Trap Streets to trap potential copyright violators of the map. A trap street is a fictious entry in the form of a misrepresented street on a map. If that street later shows up on any other company’s maps, then they have the proof required to sue for copyright infringement. What if, instead of just one random street or locality, an entire map is misleading?!

This can be seen in the case of digital mapping products in China. There, every street, location, and water body are just slightly off its mark, skewed for reasons of national and economic security. Due to national security concerns, the use of geographic information in the People’s Republic of China is restricted to entities that obtain a special authorization from the administrative department for surveying and mapping under the State Council.” Users of services like Google/Bing Maps often see streets tearing through buildings or freeways disappearing into water bodies when they toggle to satellite view on Google/Bing Maps.


Screenshots taken from Google Maps for the same coordinates window. Pickup: any location label and compare them in the above screenshots

The reason for this GPS offset in China is a combination of technology (Different datums) and sovereign interests – an aspect that the Chinese government seems to have done an excellent work of. China uses an indigenous coordinate system unlike the rest of the world. The GCJ-02 standard is used instead of the WGS-84 standard used by GPS satellites and the vast majority of maps. China prefers service providers to obtain a sign-off from state council for all maps and it also mandates that the maps use GCJ-02. This causes WGS-84 locations, such as GPS tracks from a GPS receiver, to appear shifted when plotted on Chinese street maps. A different coordinate system isn’t normally a problem, but China chose to encrypt GCJ-02 to enhance its security, so there was no straightforward transformation.

Datums are used to translate map positions to their real positions on Earth. Datums can be thought of as ellipsoid projections or models of the Earth’s curved surface, like a stretched globe, on which positional measurements can be made to determine a location’s precise position on Earth. There are about 200 datums in use As all datums are not same, there would be a difference between datums and is known as datum displacement or shift which can result in inaccuracies.  Depending on the region, the shift between coordinates of differing datums can be just a couple of meters or several kilometers. Luckily, GPS uses just one datum, WGS-84 (World Geodetic System 1984). It is considered as the best global geodetic reference system for applications of mapping and navigation.

China uses datum standard GCJ-02. This was created by the Chinese State Bureau of Surveying and Mapping and it is based on WGS-84, but it uses an obfuscation algorithm to randomly offset coordinates.GCJ-02 in Chinese is referred to as “地形图非线性保密处理算法”, which translates to “Topographic map non-linear confidentiality algorithm”. It means that the Chinese GPS shift problem that outsiders encounter is a design feature. Hence, unlike typical datum shift caused by differing projections, the offset in Chinese maps is a feature and due to a confidentiality algorithm.  And on top of it, mapping companies like Baidu uses a different datum called BD-09 which is a derivative of GCJ-02 with further obfuscation.

Anyone working on any products or solutions using maps, should understand these nuances and apply the required transformations to accurately position and track. It involves the check to find if the coordinates are falling in Chinese geography within a bounding coordinates window, which covers china and some portions of neighbouring countries. Once it is determined that the coordinates are in China, a multi-step reverse obfuscation algorithm needs to be applied to convert the coordinates from WGS-84 to GCJ-02. The resulting coordinates can be plotted on maps like Google to accurately position them. If the same needs to be plotted on Baidu maps, one another transformation to be applied.

At Safetrax, we specialize in location-based services and provide solutions for Employee Transportation or School Bus Tracking. Our solutions aim at high accuracy in real-time tracking, distance travelled and route optimization and other safety workflows like SoS, Safereach etc.

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