Why must Chinese maps be obfuscated? Unicorn Meta Zoo #1: Why another podcast? Announcing the arrival of Valued Associate #679: Cesar ManaraIs the ruling Chinese party seen more favorably than it was 25 years ago?Is the Chinese Communist Party really an autocracy?Chinese contingency plans due to N. Korea collapsingWhy don't Western countries penalize Chinese companies in the same way that China restricts Western companies working on its territory?Why won't the Western countries block Chinese websites in retaliation for China blocking American and European online services?Would a Social Credit System like the one contemplated in China be Constitutional In The U.SWhat is the fundamental difference between post WW2 German and post Cold War Chinese model of industrialization?Why doesn't the Chinese foreign minister speak English?Chinese government internet censorship - statistics on evasion?Why doesn't the Chinese Communist Party induct everyone into the party?
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Why must Chinese maps be obfuscated?
Unicorn Meta Zoo #1: Why another podcast?
Announcing the arrival of Valued Associate #679: Cesar ManaraIs the ruling Chinese party seen more favorably than it was 25 years ago?Is the Chinese Communist Party really an autocracy?Chinese contingency plans due to N. Korea collapsingWhy don't Western countries penalize Chinese companies in the same way that China restricts Western companies working on its territory?Why won't the Western countries block Chinese websites in retaliation for China blocking American and European online services?Would a Social Credit System like the one contemplated in China be Constitutional In The U.SWhat is the fundamental difference between post WW2 German and post Cold War Chinese model of industrialization?Why doesn't the Chinese foreign minister speak English?Chinese government internet censorship - statistics on evasion?Why doesn't the Chinese Communist Party induct everyone into the party?
According to this answer on Travel.SE, the Chinese National Bureau of Surveying and Mapping requires that published maps of China must include a certain deviation between the map and the real world.
In the comments, the answerer says that although the statute doesn't provide a reason, it is commonly believed to be for national security.
Have any Chinese officials or official documents provided a reason why maps must include this deviation?
china
add a comment |
According to this answer on Travel.SE, the Chinese National Bureau of Surveying and Mapping requires that published maps of China must include a certain deviation between the map and the real world.
In the comments, the answerer says that although the statute doesn't provide a reason, it is commonly believed to be for national security.
Have any Chinese officials or official documents provided a reason why maps must include this deviation?
china
Well, anyone familiar with China government ambiguity over many things, you can simply assume this is typical red tape to safeguard those in the bureaucrat, to avoid possible political correctness implication.
– mootmoot
39 mins ago
add a comment |
According to this answer on Travel.SE, the Chinese National Bureau of Surveying and Mapping requires that published maps of China must include a certain deviation between the map and the real world.
In the comments, the answerer says that although the statute doesn't provide a reason, it is commonly believed to be for national security.
Have any Chinese officials or official documents provided a reason why maps must include this deviation?
china
According to this answer on Travel.SE, the Chinese National Bureau of Surveying and Mapping requires that published maps of China must include a certain deviation between the map and the real world.
In the comments, the answerer says that although the statute doesn't provide a reason, it is commonly believed to be for national security.
Have any Chinese officials or official documents provided a reason why maps must include this deviation?
china
china
edited 59 mins ago
JJJ
7,75932965
7,75932965
asked 16 hours ago
indigochildindigochild
19.5k258142
19.5k258142
Well, anyone familiar with China government ambiguity over many things, you can simply assume this is typical red tape to safeguard those in the bureaucrat, to avoid possible political correctness implication.
– mootmoot
39 mins ago
add a comment |
Well, anyone familiar with China government ambiguity over many things, you can simply assume this is typical red tape to safeguard those in the bureaucrat, to avoid possible political correctness implication.
– mootmoot
39 mins ago
Well, anyone familiar with China government ambiguity over many things, you can simply assume this is typical red tape to safeguard those in the bureaucrat, to avoid possible political correctness implication.
– mootmoot
39 mins ago
Well, anyone familiar with China government ambiguity over many things, you can simply assume this is typical red tape to safeguard those in the bureaucrat, to avoid possible political correctness implication.
– mootmoot
39 mins ago
add a comment |
2 Answers
2
active
oldest
votes
What the Law Says
After reading that statute, it turns out that it does not explicitly require that maps be obfuscated.
However, the law does require the usage of a certain datum. A datum is basically the mathematical model which describes the shape of the earth (see this question on GIS.SE for more detail). According to wikipedia, the agency responsible for implementing this model has created the GCJ-02 datum, which has a random disturbance - effectively slightly randomizing the location of objects on the map.
U.S. - Selective Availability
There are many unsourced claims online which say that this random disturbance is for security reasons. Although I couldn't locate an original source making this claim, there is precedent for it.
Prior to 2000, the United States Department of Defense required the usage of 'Selective Availability' (SA). SA was a technology built into the GPS system which introduced random errors. GPS.gov, the official U.S. source of information about GPS, describes it as:
Selective Availability (SA) is the deliberate introduction of error to the precise timekeeping of the GPS satellites, thereby reducing both positioning and timing accuracy for civilian users. It was designed to provide U.S. and Allied military forces with a navigational advantage in times of crisis or conflict.
(source: GPS.gov)
1
Not done yet, but this is what I have so far.
– indigochild
10 hours ago
1
Upvote, but the "error" in GCJ-02 datum does not seem to be random, i.e. it's security by obscurity (of the [conversion] algorithm). Quite a different thing.
– Fizz
2 hours ago
3
@Fizz "Random" has many meanings. An algorithm generating a predictable stream of numbers is still called a "random number generator". The output of an encryption algorithm is supposed to look random to the observer, etc.
– pipe
2 hours ago
add a comment |
Just to clarify, this isn't at all like the SA case where bits were unavailable unless you knew the key. The Chinese don't have their own GPS satellites (duh) setting these coordinates. What this GCJ-02 business is is a non-disclosed, but not-so-hard-to-reverse engineer conversion algorithm from other coordinates. The big picture looks like this:
That's the reverse-engineered conversion from the "bog standard" WGS-84, displayed as a vector field. As the blog from which I too that image says, the offsets are basically constant locally, but they vary across China.
It's speculated that Google abides by this globally in order to please Beijing (and abide by their Chinese license that allows them to take street pictures there). Other maps providers (that don't operate locally in China) aren't so bothered.
On Google something funny happens even when you’re viewing in the US: the Chinese border crossing plaza (on the north side) doesn’t line up with the Macau crossing (on the south side). This is due to a law where companies are forbidden to map China accurately, OpenStreetMap’s ability to avoid that law, and the independence of Macau and also Hong Kong (which has the same discrepancy).
As for the reason, according to one Chinese developer:
Before the WGS-84 <--> GCJ-02 transform algorithm leaked and made available everywhere on the Internet (eg. this repo), some people tried to get a lot of samples of WGS-GCJ point pairs and sold the dataset to people in need. But this kind of dataset is no longer useful because we now have the algorithm itself.
Are all these in the Chinese law?
No. They are mostly in some vague administrative orders or industry regulations made by the government. There is a Code of Geological Survey (http://www.gov.cn/ziliao/flfg/2005-08/05/content_20947.htm, Chinese), though. But it only says data obtained through survey is classified by default (Article 29) and companies without a permission cannot do survey (Chapter 5). Nothing about GCJ-02 is made officially public.
Why does the Chinese government do this?
National security, they say. But people developing LBS apps tends to believe that it is because of money. The government charges Chinese companies for the "shift correction" feature. We need to make a reservation with the government and take the source code of our app to the bureau to get the feature compiled with our code.
Unless the Chinese decide to change that (leaked code) and ban all prior maps, this doesn't compare to the SAASM, which supposedly changed keys (on the satellites and receivers) once in a while. SAASM even allows rekeying over insecure channels, while the older PPS-SM required a secure channel (e.g. taking the GPS device to an authorized/secure location for rekeying.)
I suppose I should mention that the Chinese are building their own GPS alternative, the BDS (BeiDou Navigation Satellite System), which apparently became operational in December last year, but that's a different story.
What does this add over the existing answer?
– indigochild
12 mins ago
The Chinese don't have their own GPS satellites
they do have a global navigation satellite system; it's called BeiDou.
– Nosrac
10 mins ago
@indigochild: corrects your various misleading claims like "GCJ-02 datum [...] has a random disturbance" and adds a quote from a Chinese dev on his 2 cents how and why this is the way it is. The latter also contradicts your claim that "the law does require the usage of a certain datum".
– Fizz
6 mins ago
@Nosrac: that's covered in the last para if you take the time to read the whole anwer. And that's not same thing as them having GPS satellites (with Chinese codes). You need a different receiver for BDS to work. (True there are some Chinese mixed receivers, but then so are mixed GPS/GLONASS receivers--I have one).
– Fizz
6 mins ago
Thanks for the clarification. I will use that to improve my answer also. Would you consider making the answer to the question more explicit? To me, it was not obvious among all the technical details.
– indigochild
49 secs ago
add a comment |
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2 Answers
2
active
oldest
votes
2 Answers
2
active
oldest
votes
active
oldest
votes
active
oldest
votes
What the Law Says
After reading that statute, it turns out that it does not explicitly require that maps be obfuscated.
However, the law does require the usage of a certain datum. A datum is basically the mathematical model which describes the shape of the earth (see this question on GIS.SE for more detail). According to wikipedia, the agency responsible for implementing this model has created the GCJ-02 datum, which has a random disturbance - effectively slightly randomizing the location of objects on the map.
U.S. - Selective Availability
There are many unsourced claims online which say that this random disturbance is for security reasons. Although I couldn't locate an original source making this claim, there is precedent for it.
Prior to 2000, the United States Department of Defense required the usage of 'Selective Availability' (SA). SA was a technology built into the GPS system which introduced random errors. GPS.gov, the official U.S. source of information about GPS, describes it as:
Selective Availability (SA) is the deliberate introduction of error to the precise timekeeping of the GPS satellites, thereby reducing both positioning and timing accuracy for civilian users. It was designed to provide U.S. and Allied military forces with a navigational advantage in times of crisis or conflict.
(source: GPS.gov)
1
Not done yet, but this is what I have so far.
– indigochild
10 hours ago
1
Upvote, but the "error" in GCJ-02 datum does not seem to be random, i.e. it's security by obscurity (of the [conversion] algorithm). Quite a different thing.
– Fizz
2 hours ago
3
@Fizz "Random" has many meanings. An algorithm generating a predictable stream of numbers is still called a "random number generator". The output of an encryption algorithm is supposed to look random to the observer, etc.
– pipe
2 hours ago
add a comment |
What the Law Says
After reading that statute, it turns out that it does not explicitly require that maps be obfuscated.
However, the law does require the usage of a certain datum. A datum is basically the mathematical model which describes the shape of the earth (see this question on GIS.SE for more detail). According to wikipedia, the agency responsible for implementing this model has created the GCJ-02 datum, which has a random disturbance - effectively slightly randomizing the location of objects on the map.
U.S. - Selective Availability
There are many unsourced claims online which say that this random disturbance is for security reasons. Although I couldn't locate an original source making this claim, there is precedent for it.
Prior to 2000, the United States Department of Defense required the usage of 'Selective Availability' (SA). SA was a technology built into the GPS system which introduced random errors. GPS.gov, the official U.S. source of information about GPS, describes it as:
Selective Availability (SA) is the deliberate introduction of error to the precise timekeeping of the GPS satellites, thereby reducing both positioning and timing accuracy for civilian users. It was designed to provide U.S. and Allied military forces with a navigational advantage in times of crisis or conflict.
(source: GPS.gov)
1
Not done yet, but this is what I have so far.
– indigochild
10 hours ago
1
Upvote, but the "error" in GCJ-02 datum does not seem to be random, i.e. it's security by obscurity (of the [conversion] algorithm). Quite a different thing.
– Fizz
2 hours ago
3
@Fizz "Random" has many meanings. An algorithm generating a predictable stream of numbers is still called a "random number generator". The output of an encryption algorithm is supposed to look random to the observer, etc.
– pipe
2 hours ago
add a comment |
What the Law Says
After reading that statute, it turns out that it does not explicitly require that maps be obfuscated.
However, the law does require the usage of a certain datum. A datum is basically the mathematical model which describes the shape of the earth (see this question on GIS.SE for more detail). According to wikipedia, the agency responsible for implementing this model has created the GCJ-02 datum, which has a random disturbance - effectively slightly randomizing the location of objects on the map.
U.S. - Selective Availability
There are many unsourced claims online which say that this random disturbance is for security reasons. Although I couldn't locate an original source making this claim, there is precedent for it.
Prior to 2000, the United States Department of Defense required the usage of 'Selective Availability' (SA). SA was a technology built into the GPS system which introduced random errors. GPS.gov, the official U.S. source of information about GPS, describes it as:
Selective Availability (SA) is the deliberate introduction of error to the precise timekeeping of the GPS satellites, thereby reducing both positioning and timing accuracy for civilian users. It was designed to provide U.S. and Allied military forces with a navigational advantage in times of crisis or conflict.
(source: GPS.gov)
What the Law Says
After reading that statute, it turns out that it does not explicitly require that maps be obfuscated.
However, the law does require the usage of a certain datum. A datum is basically the mathematical model which describes the shape of the earth (see this question on GIS.SE for more detail). According to wikipedia, the agency responsible for implementing this model has created the GCJ-02 datum, which has a random disturbance - effectively slightly randomizing the location of objects on the map.
U.S. - Selective Availability
There are many unsourced claims online which say that this random disturbance is for security reasons. Although I couldn't locate an original source making this claim, there is precedent for it.
Prior to 2000, the United States Department of Defense required the usage of 'Selective Availability' (SA). SA was a technology built into the GPS system which introduced random errors. GPS.gov, the official U.S. source of information about GPS, describes it as:
Selective Availability (SA) is the deliberate introduction of error to the precise timekeeping of the GPS satellites, thereby reducing both positioning and timing accuracy for civilian users. It was designed to provide U.S. and Allied military forces with a navigational advantage in times of crisis or conflict.
(source: GPS.gov)
edited 9 hours ago
answered 10 hours ago
indigochildindigochild
19.5k258142
19.5k258142
1
Not done yet, but this is what I have so far.
– indigochild
10 hours ago
1
Upvote, but the "error" in GCJ-02 datum does not seem to be random, i.e. it's security by obscurity (of the [conversion] algorithm). Quite a different thing.
– Fizz
2 hours ago
3
@Fizz "Random" has many meanings. An algorithm generating a predictable stream of numbers is still called a "random number generator". The output of an encryption algorithm is supposed to look random to the observer, etc.
– pipe
2 hours ago
add a comment |
1
Not done yet, but this is what I have so far.
– indigochild
10 hours ago
1
Upvote, but the "error" in GCJ-02 datum does not seem to be random, i.e. it's security by obscurity (of the [conversion] algorithm). Quite a different thing.
– Fizz
2 hours ago
3
@Fizz "Random" has many meanings. An algorithm generating a predictable stream of numbers is still called a "random number generator". The output of an encryption algorithm is supposed to look random to the observer, etc.
– pipe
2 hours ago
1
1
Not done yet, but this is what I have so far.
– indigochild
10 hours ago
Not done yet, but this is what I have so far.
– indigochild
10 hours ago
1
1
Upvote, but the "error" in GCJ-02 datum does not seem to be random, i.e. it's security by obscurity (of the [conversion] algorithm). Quite a different thing.
– Fizz
2 hours ago
Upvote, but the "error" in GCJ-02 datum does not seem to be random, i.e. it's security by obscurity (of the [conversion] algorithm). Quite a different thing.
– Fizz
2 hours ago
3
3
@Fizz "Random" has many meanings. An algorithm generating a predictable stream of numbers is still called a "random number generator". The output of an encryption algorithm is supposed to look random to the observer, etc.
– pipe
2 hours ago
@Fizz "Random" has many meanings. An algorithm generating a predictable stream of numbers is still called a "random number generator". The output of an encryption algorithm is supposed to look random to the observer, etc.
– pipe
2 hours ago
add a comment |
Just to clarify, this isn't at all like the SA case where bits were unavailable unless you knew the key. The Chinese don't have their own GPS satellites (duh) setting these coordinates. What this GCJ-02 business is is a non-disclosed, but not-so-hard-to-reverse engineer conversion algorithm from other coordinates. The big picture looks like this:
That's the reverse-engineered conversion from the "bog standard" WGS-84, displayed as a vector field. As the blog from which I too that image says, the offsets are basically constant locally, but they vary across China.
It's speculated that Google abides by this globally in order to please Beijing (and abide by their Chinese license that allows them to take street pictures there). Other maps providers (that don't operate locally in China) aren't so bothered.
On Google something funny happens even when you’re viewing in the US: the Chinese border crossing plaza (on the north side) doesn’t line up with the Macau crossing (on the south side). This is due to a law where companies are forbidden to map China accurately, OpenStreetMap’s ability to avoid that law, and the independence of Macau and also Hong Kong (which has the same discrepancy).
As for the reason, according to one Chinese developer:
Before the WGS-84 <--> GCJ-02 transform algorithm leaked and made available everywhere on the Internet (eg. this repo), some people tried to get a lot of samples of WGS-GCJ point pairs and sold the dataset to people in need. But this kind of dataset is no longer useful because we now have the algorithm itself.
Are all these in the Chinese law?
No. They are mostly in some vague administrative orders or industry regulations made by the government. There is a Code of Geological Survey (http://www.gov.cn/ziliao/flfg/2005-08/05/content_20947.htm, Chinese), though. But it only says data obtained through survey is classified by default (Article 29) and companies without a permission cannot do survey (Chapter 5). Nothing about GCJ-02 is made officially public.
Why does the Chinese government do this?
National security, they say. But people developing LBS apps tends to believe that it is because of money. The government charges Chinese companies for the "shift correction" feature. We need to make a reservation with the government and take the source code of our app to the bureau to get the feature compiled with our code.
Unless the Chinese decide to change that (leaked code) and ban all prior maps, this doesn't compare to the SAASM, which supposedly changed keys (on the satellites and receivers) once in a while. SAASM even allows rekeying over insecure channels, while the older PPS-SM required a secure channel (e.g. taking the GPS device to an authorized/secure location for rekeying.)
I suppose I should mention that the Chinese are building their own GPS alternative, the BDS (BeiDou Navigation Satellite System), which apparently became operational in December last year, but that's a different story.
What does this add over the existing answer?
– indigochild
12 mins ago
The Chinese don't have their own GPS satellites
they do have a global navigation satellite system; it's called BeiDou.
– Nosrac
10 mins ago
@indigochild: corrects your various misleading claims like "GCJ-02 datum [...] has a random disturbance" and adds a quote from a Chinese dev on his 2 cents how and why this is the way it is. The latter also contradicts your claim that "the law does require the usage of a certain datum".
– Fizz
6 mins ago
@Nosrac: that's covered in the last para if you take the time to read the whole anwer. And that's not same thing as them having GPS satellites (with Chinese codes). You need a different receiver for BDS to work. (True there are some Chinese mixed receivers, but then so are mixed GPS/GLONASS receivers--I have one).
– Fizz
6 mins ago
Thanks for the clarification. I will use that to improve my answer also. Would you consider making the answer to the question more explicit? To me, it was not obvious among all the technical details.
– indigochild
49 secs ago
add a comment |
Just to clarify, this isn't at all like the SA case where bits were unavailable unless you knew the key. The Chinese don't have their own GPS satellites (duh) setting these coordinates. What this GCJ-02 business is is a non-disclosed, but not-so-hard-to-reverse engineer conversion algorithm from other coordinates. The big picture looks like this:
That's the reverse-engineered conversion from the "bog standard" WGS-84, displayed as a vector field. As the blog from which I too that image says, the offsets are basically constant locally, but they vary across China.
It's speculated that Google abides by this globally in order to please Beijing (and abide by their Chinese license that allows them to take street pictures there). Other maps providers (that don't operate locally in China) aren't so bothered.
On Google something funny happens even when you’re viewing in the US: the Chinese border crossing plaza (on the north side) doesn’t line up with the Macau crossing (on the south side). This is due to a law where companies are forbidden to map China accurately, OpenStreetMap’s ability to avoid that law, and the independence of Macau and also Hong Kong (which has the same discrepancy).
As for the reason, according to one Chinese developer:
Before the WGS-84 <--> GCJ-02 transform algorithm leaked and made available everywhere on the Internet (eg. this repo), some people tried to get a lot of samples of WGS-GCJ point pairs and sold the dataset to people in need. But this kind of dataset is no longer useful because we now have the algorithm itself.
Are all these in the Chinese law?
No. They are mostly in some vague administrative orders or industry regulations made by the government. There is a Code of Geological Survey (http://www.gov.cn/ziliao/flfg/2005-08/05/content_20947.htm, Chinese), though. But it only says data obtained through survey is classified by default (Article 29) and companies without a permission cannot do survey (Chapter 5). Nothing about GCJ-02 is made officially public.
Why does the Chinese government do this?
National security, they say. But people developing LBS apps tends to believe that it is because of money. The government charges Chinese companies for the "shift correction" feature. We need to make a reservation with the government and take the source code of our app to the bureau to get the feature compiled with our code.
Unless the Chinese decide to change that (leaked code) and ban all prior maps, this doesn't compare to the SAASM, which supposedly changed keys (on the satellites and receivers) once in a while. SAASM even allows rekeying over insecure channels, while the older PPS-SM required a secure channel (e.g. taking the GPS device to an authorized/secure location for rekeying.)
I suppose I should mention that the Chinese are building their own GPS alternative, the BDS (BeiDou Navigation Satellite System), which apparently became operational in December last year, but that's a different story.
What does this add over the existing answer?
– indigochild
12 mins ago
The Chinese don't have their own GPS satellites
they do have a global navigation satellite system; it's called BeiDou.
– Nosrac
10 mins ago
@indigochild: corrects your various misleading claims like "GCJ-02 datum [...] has a random disturbance" and adds a quote from a Chinese dev on his 2 cents how and why this is the way it is. The latter also contradicts your claim that "the law does require the usage of a certain datum".
– Fizz
6 mins ago
@Nosrac: that's covered in the last para if you take the time to read the whole anwer. And that's not same thing as them having GPS satellites (with Chinese codes). You need a different receiver for BDS to work. (True there are some Chinese mixed receivers, but then so are mixed GPS/GLONASS receivers--I have one).
– Fizz
6 mins ago
Thanks for the clarification. I will use that to improve my answer also. Would you consider making the answer to the question more explicit? To me, it was not obvious among all the technical details.
– indigochild
49 secs ago
add a comment |
Just to clarify, this isn't at all like the SA case where bits were unavailable unless you knew the key. The Chinese don't have their own GPS satellites (duh) setting these coordinates. What this GCJ-02 business is is a non-disclosed, but not-so-hard-to-reverse engineer conversion algorithm from other coordinates. The big picture looks like this:
That's the reverse-engineered conversion from the "bog standard" WGS-84, displayed as a vector field. As the blog from which I too that image says, the offsets are basically constant locally, but they vary across China.
It's speculated that Google abides by this globally in order to please Beijing (and abide by their Chinese license that allows them to take street pictures there). Other maps providers (that don't operate locally in China) aren't so bothered.
On Google something funny happens even when you’re viewing in the US: the Chinese border crossing plaza (on the north side) doesn’t line up with the Macau crossing (on the south side). This is due to a law where companies are forbidden to map China accurately, OpenStreetMap’s ability to avoid that law, and the independence of Macau and also Hong Kong (which has the same discrepancy).
As for the reason, according to one Chinese developer:
Before the WGS-84 <--> GCJ-02 transform algorithm leaked and made available everywhere on the Internet (eg. this repo), some people tried to get a lot of samples of WGS-GCJ point pairs and sold the dataset to people in need. But this kind of dataset is no longer useful because we now have the algorithm itself.
Are all these in the Chinese law?
No. They are mostly in some vague administrative orders or industry regulations made by the government. There is a Code of Geological Survey (http://www.gov.cn/ziliao/flfg/2005-08/05/content_20947.htm, Chinese), though. But it only says data obtained through survey is classified by default (Article 29) and companies without a permission cannot do survey (Chapter 5). Nothing about GCJ-02 is made officially public.
Why does the Chinese government do this?
National security, they say. But people developing LBS apps tends to believe that it is because of money. The government charges Chinese companies for the "shift correction" feature. We need to make a reservation with the government and take the source code of our app to the bureau to get the feature compiled with our code.
Unless the Chinese decide to change that (leaked code) and ban all prior maps, this doesn't compare to the SAASM, which supposedly changed keys (on the satellites and receivers) once in a while. SAASM even allows rekeying over insecure channels, while the older PPS-SM required a secure channel (e.g. taking the GPS device to an authorized/secure location for rekeying.)
I suppose I should mention that the Chinese are building their own GPS alternative, the BDS (BeiDou Navigation Satellite System), which apparently became operational in December last year, but that's a different story.
Just to clarify, this isn't at all like the SA case where bits were unavailable unless you knew the key. The Chinese don't have their own GPS satellites (duh) setting these coordinates. What this GCJ-02 business is is a non-disclosed, but not-so-hard-to-reverse engineer conversion algorithm from other coordinates. The big picture looks like this:
That's the reverse-engineered conversion from the "bog standard" WGS-84, displayed as a vector field. As the blog from which I too that image says, the offsets are basically constant locally, but they vary across China.
It's speculated that Google abides by this globally in order to please Beijing (and abide by their Chinese license that allows them to take street pictures there). Other maps providers (that don't operate locally in China) aren't so bothered.
On Google something funny happens even when you’re viewing in the US: the Chinese border crossing plaza (on the north side) doesn’t line up with the Macau crossing (on the south side). This is due to a law where companies are forbidden to map China accurately, OpenStreetMap’s ability to avoid that law, and the independence of Macau and also Hong Kong (which has the same discrepancy).
As for the reason, according to one Chinese developer:
Before the WGS-84 <--> GCJ-02 transform algorithm leaked and made available everywhere on the Internet (eg. this repo), some people tried to get a lot of samples of WGS-GCJ point pairs and sold the dataset to people in need. But this kind of dataset is no longer useful because we now have the algorithm itself.
Are all these in the Chinese law?
No. They are mostly in some vague administrative orders or industry regulations made by the government. There is a Code of Geological Survey (http://www.gov.cn/ziliao/flfg/2005-08/05/content_20947.htm, Chinese), though. But it only says data obtained through survey is classified by default (Article 29) and companies without a permission cannot do survey (Chapter 5). Nothing about GCJ-02 is made officially public.
Why does the Chinese government do this?
National security, they say. But people developing LBS apps tends to believe that it is because of money. The government charges Chinese companies for the "shift correction" feature. We need to make a reservation with the government and take the source code of our app to the bureau to get the feature compiled with our code.
Unless the Chinese decide to change that (leaked code) and ban all prior maps, this doesn't compare to the SAASM, which supposedly changed keys (on the satellites and receivers) once in a while. SAASM even allows rekeying over insecure channels, while the older PPS-SM required a secure channel (e.g. taking the GPS device to an authorized/secure location for rekeying.)
I suppose I should mention that the Chinese are building their own GPS alternative, the BDS (BeiDou Navigation Satellite System), which apparently became operational in December last year, but that's a different story.
edited 56 mins ago
answered 2 hours ago
FizzFizz
16.9k242108
16.9k242108
What does this add over the existing answer?
– indigochild
12 mins ago
The Chinese don't have their own GPS satellites
they do have a global navigation satellite system; it's called BeiDou.
– Nosrac
10 mins ago
@indigochild: corrects your various misleading claims like "GCJ-02 datum [...] has a random disturbance" and adds a quote from a Chinese dev on his 2 cents how and why this is the way it is. The latter also contradicts your claim that "the law does require the usage of a certain datum".
– Fizz
6 mins ago
@Nosrac: that's covered in the last para if you take the time to read the whole anwer. And that's not same thing as them having GPS satellites (with Chinese codes). You need a different receiver for BDS to work. (True there are some Chinese mixed receivers, but then so are mixed GPS/GLONASS receivers--I have one).
– Fizz
6 mins ago
Thanks for the clarification. I will use that to improve my answer also. Would you consider making the answer to the question more explicit? To me, it was not obvious among all the technical details.
– indigochild
49 secs ago
add a comment |
What does this add over the existing answer?
– indigochild
12 mins ago
The Chinese don't have their own GPS satellites
they do have a global navigation satellite system; it's called BeiDou.
– Nosrac
10 mins ago
@indigochild: corrects your various misleading claims like "GCJ-02 datum [...] has a random disturbance" and adds a quote from a Chinese dev on his 2 cents how and why this is the way it is. The latter also contradicts your claim that "the law does require the usage of a certain datum".
– Fizz
6 mins ago
@Nosrac: that's covered in the last para if you take the time to read the whole anwer. And that's not same thing as them having GPS satellites (with Chinese codes). You need a different receiver for BDS to work. (True there are some Chinese mixed receivers, but then so are mixed GPS/GLONASS receivers--I have one).
– Fizz
6 mins ago
Thanks for the clarification. I will use that to improve my answer also. Would you consider making the answer to the question more explicit? To me, it was not obvious among all the technical details.
– indigochild
49 secs ago
What does this add over the existing answer?
– indigochild
12 mins ago
What does this add over the existing answer?
– indigochild
12 mins ago
The Chinese don't have their own GPS satellites
they do have a global navigation satellite system; it's called BeiDou.– Nosrac
10 mins ago
The Chinese don't have their own GPS satellites
they do have a global navigation satellite system; it's called BeiDou.– Nosrac
10 mins ago
@indigochild: corrects your various misleading claims like "GCJ-02 datum [...] has a random disturbance" and adds a quote from a Chinese dev on his 2 cents how and why this is the way it is. The latter also contradicts your claim that "the law does require the usage of a certain datum".
– Fizz
6 mins ago
@indigochild: corrects your various misleading claims like "GCJ-02 datum [...] has a random disturbance" and adds a quote from a Chinese dev on his 2 cents how and why this is the way it is. The latter also contradicts your claim that "the law does require the usage of a certain datum".
– Fizz
6 mins ago
@Nosrac: that's covered in the last para if you take the time to read the whole anwer. And that's not same thing as them having GPS satellites (with Chinese codes). You need a different receiver for BDS to work. (True there are some Chinese mixed receivers, but then so are mixed GPS/GLONASS receivers--I have one).
– Fizz
6 mins ago
@Nosrac: that's covered in the last para if you take the time to read the whole anwer. And that's not same thing as them having GPS satellites (with Chinese codes). You need a different receiver for BDS to work. (True there are some Chinese mixed receivers, but then so are mixed GPS/GLONASS receivers--I have one).
– Fizz
6 mins ago
Thanks for the clarification. I will use that to improve my answer also. Would you consider making the answer to the question more explicit? To me, it was not obvious among all the technical details.
– indigochild
49 secs ago
Thanks for the clarification. I will use that to improve my answer also. Would you consider making the answer to the question more explicit? To me, it was not obvious among all the technical details.
– indigochild
49 secs ago
add a comment |
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Well, anyone familiar with China government ambiguity over many things, you can simply assume this is typical red tape to safeguard those in the bureaucrat, to avoid possible political correctness implication.
– mootmoot
39 mins ago