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Comment by olalonde | Hacker News Reader

To be fair, Google Maps in China isn't great either. See "Map View" vs "Satellite View" here: https://maps.google.com/?ll=22.637323,114.030954&spn=0.0.... It's off by a few hundred meters (the corresponding satellite view for the previous map view is actually here: https://maps.google.com/?ll=22.640363,114.025936&spn=0.0...). I assume Google is aware of this issue since they don't overlay map data over their satellite view like they do almost everywhere else around the world. The fact that they can't drive their Street View cars in China doesn't help either.

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My Chinese friends tell me that there is a government requirement that maps databases have an offset. I'm guessing the Google car isn't permitted to drive around in China, because they lease maps from Mapabc [1], which presumably has the offset. If you go to ditu.google.cn you get accurate maps without the offset (but it's only in Chinese).

[1] http://en.wikipedia.org/wiki/Google_Maps (see the "Copyright" section)

Ha, that explains it. This kind of censorship wouldn't be unheard of in China and it seems much more plausible than Google doing such a poor job.

A problem I had while driving on ChongMing Island, just outside of Shanghai, less than a year ago was that although maps in Google Maps was great, unfortunately many of the roads where not built in the south part of island by then. So ended up at lots of dead ends and huge construction sites.

Not saying AutoNavi is better in that regard, but will give more testing. From my limited testing though it seemed to handle searches in Chinese a tad better (hopefully).

> there is a government requirement that maps databases have an offset.

What do you mean by offset? Mapping companies are forced to make their data be inaccurate?

Yes. The US did this with GPS - co-ordinates would be slightly scrambled, unless you had a military decoder.

Some people worked around it by getting the GPS location at a fixed point (i.e. a lighthouse) then figuring out the offset for the area.

The reason being, maps and location data has military uses.

You'll also find it's illegal to make maps of certain infrastructure (generators, power lines, dams) in many countries.

How far off is this with GPS? Is there a hack for it on Android?

Edit: just saw this post[0], looks like it's no longer the case.

0: http://hackerne.ws/item?id=4549289

there are patches for Android that correct the Google maps offset in China. tried it there, it works ;)

Not quite the same thing, although they do have some similarities.

The Selective Availability "feature" of GPS worked by fuzzing the time readings output by the GPS satellites. The fuzz was time-varying but only changed every few seconds. Because it affected nearby receivers with similar amounts of error, if you had access to two receivers in the same area you could quite accurately measure their positions relative to each other.

The Chinese map obfuscation scheme is basically a secret map projection (it's not a constant offset, it varies over locations). GPS coordinates go into a black box and out come obfuscated coordinates, which you then use to plot on the map. Because there wouldn't be much point to GPS receivers that constantly show your location on the wrong point on the map, receivers sold in China actually incorporate the algorithm, and it probably wouldn't be too difficult to reverse engineer.

I'm trying to figure out if you are serious about "Chinese Map Obfuscation Schemes". Your description of SA and DGPS is spot on, but I can't think of any reason that China would want to obfuscate their maps now that satellites have mapped their country.

Paranoia, maybe? But yeah, it's rather pointless. Even more so since GPS vendors have access to the algorithm.

It's required by the chinese government http://home.wangjianshuo.com/archives/20081109_all_maps_in_c...

All GPS used to have an offset. However the US Government (who controls GPS) turned it off a in 2000 (or set it to 0), and it's been boon to the public ever since.

The offset is still there on ditu.google.cn, it's just that they've also munged the satellite imagery to have the same offset.

Nothing in this space can be great in China; accurate maps are illegal.

Japan under iOS 6, on the other hand, is pretty inexcusable. Japan has led the world in map accuracy and detail since the dawning of the GPS era.

I don't have a source handy but China has been known to censor highly accurate map data because they considered it a threat to national security. I've seen satellite photos overlayed with map data where almost all the streets were off by many meters or were slightly skewed.

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