Beijing is concerned that field data collected by foreigners could be used by hostile powers to help guided missiles hit key military and civilian installations, mainland experts say.
The development of intercontinental ballistic missiles has increased the demand for precise location data because a tiny error could send a long-range missile hundreds of kilometres off its target. In recent years, spy satellites have enabled countries to pinpoint a location from space.
But Xu Shijie, a guided missile expert at Beihang University, said satellite images alone were not enough for a precise hit. He said satellite photos were enough to guide a missile during the initial and middle phases of a flight, but more detailed input would be needed in the final leg to determine which building to hit.
'A satellite does not hover directly above a target to take a snapshot. When it looks at something on Earth, it is at an angle. Mapping onsite with a GPS (Global Positioning System) device could help calculate the angle very precisely, and the data could help a missile calibrate its approach to the target,' he said.
That concern was the main reason behind Beijing's recent crackdown on surveying and mapping in sensitive areas without permission. But the move was also helped by prevailing xenophobic sentiments on the mainland.
A popular article widely circulating in mainland internet chat rooms titled 'How to Catch a Foreign Spy Mapping Chinese Terrain' expresses such sentiments. The author, whose identity is unknown, urged mainlanders to watch out for foreigners using GPS and doing mapping-related activities.
'A foreigner walks down a highway and records the road on his GPS device. Whenever he comes across a key intersection, he will mark it down. When the war breaks out, packs of JDAM (Joint Direct Attack Munition) bombs will land on these targets precisely,' the article said.
'Therefore, when you see a foreigner walking down the street who looks attentively at his PDA for a long time, don't hesitate. Call the police immediately.'
Onsite mapping used to be a critical military intelligence activity, so much so that the various participants in the second world war sent thousands of spies to pinpoint the locations of important military and civilian targets for air raids.
Li Shengyu, a topography expert at the Henan Polytechnic University's School of Surveying and Land Information, said technological advances had made precision mapping using GPS devices easier, inexpensive and difficult to detect.
'[With the most sophisticated equipment,] you could turn on the device and walk around an airport and immediately know how large the area is, as well as other valuable information, to within 1cm,' Dr Li said. 'They used to be quite big, but in recent years the equipment has shrunk to the size of a cellphone.'
Wang Junzhuo, sales manager of Huachen Bingpeng Technology and Trade, a company that sells GPS devices in Beijing, said he would not use GPS equipment openly on the mainland if he were a foreigner, because a professional mapping device was not much different from an ordinary GPS-enabled handheld one.
'Even close up, you cannot tell if someone is using a cellphone or a spying device,' he said. 'They look quite similar, with the same buttons and screens and sizes, thanks to revolutionary technological advancements in recent years.
'If the police were to hold you as a suspect, they would have to detain you while they send your device for professional tests, which could take a long time. You might turn out to be innocent, but the drama would ruin the whole trip.'
How GPS works
The Earth is constantly circled by 24 Global Positioning System satellites and at least three are always in range of any one point. GPS receivers pick up signals that give the satellites?? locations and the exact time of sending. The receiver then triangulates the user??s latitude and longitude to within 10 metres