Discussion:
China, GCJ-02 & cartography
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Eric Scace
2018-05-15 13:38:21 UTC
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The following was published on an email list to which I subscribe. Can others on this list can shed more light on CGJ-02 vs WGS-84, and some of the representations made in this article?

— Eric

The Problem with Chinese GPS
If you’re in a foreign country and try to read a map, you may find it difficult -- unless your host nation’s language is the same as your home nation’s, the words are going to be different and, assuming you’re not bilingual, will require some translation. But the locations of the roads, rivers, buildings, and the like should be the same, regardless of whether the map is in English, Spanish, or Chinese, right? Language aside, Google Maps should work the same everywhere, right?

Well
 no.

Pictured above is a map of the China/Hong Kong border via Google Maps; you can see it yourself by clicking here <https://nowiknow.us1.list-manage.com/track/click?u=2889002ad89d45ca21f50ba46&id=913c66c8ec&e=730199ccc5>. The map is your standard road map overlaid upon a satellite image. As you can see, the roads -- the light grey lines -- don’t match up with reality. There are roundabouts which purport to be in public parks, bridges which don’t exist, and multi-lane highways which seem to be underwater. The whole thing is a big navigational mess. Go far enough into Hong Kong, though, and the problem abates.

What’s going on? The map data, basically, is being lost in translation.

The world -- China aside -- uses something called the World Geodetic System 1984 <https://nowiknow.us1.list-manage.com/track/click?u=2889002ad89d45ca21f50ba46&id=ed4deb1753&e=730199ccc5> (“WGS-84”) as the basis for the digital maps. Virtually all the navigation tools we use online today -- the maps apps on our phones, the GPS systems in our cars, the missile guidance systems in use by the military, and yes, Google Maps -- all use WGS-84. China, though, goes its own way.

The Chinese use something called GCJ-02, an alternative system which the cartography world colloquially refers to it as the “Mars Coordinates” as it may as well be made for another planet. The Google Maps screenshot and link, above, shows the problem: the road map data comes from the Chinese government, which uses GCJ-02, but the satellite data is from a non-Chinese source and uses WGS-84. (As China exerts control over, and takes responsibility for mapping out the border between itself and Hong Kong, the problem bleeds into the neighboring pseudo-sovereign state.) The two data sets, effectively, are speaking different languages.

China isn’t just trying to be different, though; they’re trying to be difficult. The government has long seen map data as a matter of national security. There’s a “Surveying and Mapping Law of the People's Republic of China <https://nowiknow.us1.list-manage.com/track/click?u=2889002ad89d45ca21f50ba46&id=1b29948166&e=730199ccc5>” which greatly restricts who can make maps. One needs a cartography license, one which comes with many strings; if you’re creating digital map data, for example, it needs to use GCJ-02 and has to be hosted on servers within China. And this isn’t one of those anachronistic laws which go ignored and unenforced. In 2015, for example, the country announced that those who violate the law could face fines of 200,000 yuan (about $30,000 at the time) and, according to CityLab <https://nowiknow.us1.list-manage.com/track/click?u=2889002ad89d45ca21f50ba46&id=cd460fdbd8&e=730199ccc5>, “if the violation is deemed serious enough, [those who run afoul of the law] can even find themselves booked on criminal charges.”

So why not just make a tool which translates GCJ-02 to WGS-84? Well, there are a few, but they’re typically hard to come by and not all that reliable. Multinational corporations like Google don’t want to deploy them as it could hurt their standing with the Chinese government. And even if they did, the results wouldn’t be great. GCJ-02 isn’t just an alternative coordinate system; it’s an often unpredictable one. As Wikipedia explains <https://nowiknow.us1.list-manage.com/track/click?u=2889002ad89d45ca21f50ba46&id=6e3841c617&e=730199ccc5>, “it uses an obfuscation algorithm which adds apparently random offsets to both the latitude and longitude.” And even if you can get around those issues, it won’t matter much if you’re in China itself. If you use a mobile device there, per Travel and Leisure <https://nowiknow.us1.list-manage.com/track/click?u=2889002ad89d45ca21f50ba46&id=ba7f66addd&e=730199ccc5>, “Chinese geographic regulations demand that GPS functions must either be disabled on handheld devices or they must be made to display a similar offset.”

So if you're traveling to China, knowing Chinese may be a lot more helpful than you'd think.
Bob kb8tq
2018-05-15 16:13:19 UTC
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Hi

Running with a very normal WGS-84 GPS “by the sea shore” can easily show you underwater. That
is very much a normal result of the model. It does not tell you what high (or low) tide level is going to
be at your location. That stuff is simply to complex.

The rest of it …… a lot of countries ( and states in the US ) run their own coordinate systems. It is
not very unusual. Was *is* unusual is that the data providers for this or that have not re-normalized
the data. They have had to do that pretty much everywhere else ….

Bob
Post by Eric Scace
The following was published on an email list to which I subscribe. Can others on this list can shed more light on CGJ-02 vs WGS-84, and some of the representations made in this article?
— Eric
The Problem with Chinese GPS
If you’re in a foreign country and try to read a map, you may find it difficult -- unless your host nation’s language is the same as your home nation’s, the words are going to be different and, assuming you’re not bilingual, will require some translation. But the locations of the roads, rivers, buildings, and the like should be the same, regardless of whether the map is in English, Spanish, or Chinese, right? Language aside, Google Maps should work the same everywhere, right?
Well… no.
Pictured above is a map of the China/Hong Kong border via Google Maps; you can see it yourself by clicking here <https://nowiknow.us1.list-manage.com/track/click?u=2889002ad89d45ca21f50ba46&id=913c66c8ec&e=730199ccc5>. The map is your standard road map overlaid upon a satellite image. As you can see, the roads -- the light grey lines -- don’t match up with reality. There are roundabouts which purport to be in public parks, bridges which don’t exist, and multi-lane highways which seem to be underwater. The whole thing is a big navigational mess. Go far enough into Hong Kong, though, and the problem abates.
What’s going on? The map data, basically, is being lost in translation.
The world -- China aside -- uses something called the World Geodetic System 1984 <https://nowiknow.us1.list-manage.com/track/click?u=2889002ad89d45ca21f50ba46&id=ed4deb1753&e=730199ccc5> (“WGS-84”) as the basis for the digital maps. Virtually all the navigation tools we use online today -- the maps apps on our phones, the GPS systems in our cars, the missile guidance systems in use by the military, and yes, Google Maps -- all use WGS-84. China, though, goes its own way.
The Chinese use something called GCJ-02, an alternative system which the cartography world colloquially refers to it as the “Mars Coordinates” as it may as well be made for another planet. The Google Maps screenshot and link, above, shows the problem: the road map data comes from the Chinese government, which uses GCJ-02, but the satellite data is from a non-Chinese source and uses WGS-84. (As China exerts control over, and takes responsibility for mapping out the border between itself and Hong Kong, the problem bleeds into the neighboring pseudo-sovereign state.) The two data sets, effectively, are speaking different languages.
China isn’t just trying to be different, though; they’re trying to be difficult. The government has long seen map data as a matter of national security. There’s a “Surveying and Mapping Law of the People's Republic of China <https://nowiknow.us1.list-manage.com/track/click?u=2889002ad89d45ca21f50ba46&id=1b29948166&e=730199ccc5>” which greatly restricts who can make maps. One needs a cartography license, one which comes with many strings; if you’re creating digital map data, for example, it needs to use GCJ-02 and has to be hosted on servers within China. And this isn’t one of those anachronistic laws which go ignored and unenforced. In 2015, for example, the country announced that those who violate the law could face fines of 200,000 yuan (about $30,000 at the time) and, according to CityLab <https://nowiknow.us1.list-manage.com/track/click?u=2889002ad89d45ca21f50ba46&id=cd460fdbd8&e=730199ccc5>, “if the violation is deemed serious enough, [those who run afoul of th
e law] can even find themselves booked on criminal charges.”
Post by Eric Scace
So why not just make a tool which translates GCJ-02 to WGS-84? Well, there are a few, but they’re typically hard to come by and not all that reliable. Multinational corporations like Google don’t want to deploy them as it could hurt their standing with the Chinese government. And even if they did, the results wouldn’t be great. GCJ-02 isn’t just an alternative coordinate system; it’s an often unpredictable one. As Wikipedia explains <https://nowiknow.us1.list-manage.com/track/click?u=2889002ad89d45ca21f50ba46&id=6e3841c617&e=730199ccc5>, “it uses an obfuscation algorithm which adds apparently random offsets to both the latitude and longitude.” And even if you can get around those issues, it won’t matter much if you’re in China itself. If you use a mobile device there, per Travel and Leisure <https://nowiknow.us1.list-manage.com/track/click?u=2889002ad89d45ca21f50ba46&id=ba7f66addd&e=730199ccc5>, “Chinese geographic regulations demand that GPS functions must eith
er be disabled on handheld devices or they must be made to display a similar offset.”
Post by Eric Scace
So if you're traveling to China, knowing Chinese may be a lot more helpful than you'd think.
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Eric Scace
2018-05-15 18:11:37 UTC
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I live at the edge of Boston Harbor, in the Charlestown Navy Yard.

Tide stations, such as this one at Boston Harbor’s Fort Point Channel <https://tidesandcurrents.noaa.gov/stationhome.html?id=8443970>, report their tides against a locally-defined datum <https://tidesandcurrents.noaa.gov/datums.html?id=8443970> of mean lower low water (MLLW, the average of the lower of the two low tides reported each day, for all days observed in the current national tidal datum epoch of 1983-2001).

For this station, MLLW = +5.51 ft elevation, referenced to NAVD88.

There is a National Geodetic Survey benchmark <https://www.ngs.noaa.gov/cgi-bin/get_image.prl?PROCESSING=get_figure&IID=63035> close to the tide gage. <https://www.ngs.noaa.gov/cgi-bin/get_image.prl?PROCESSING=get_figure&IID=63034>

The benchmark record <https://www.ngs.noaa.gov/cgi-bin/ds2.prl?retrieval_type=by_pid&PID=MY0555> states:
MY0555 *CURRENT SURVEY CONTROL
MY0555 ______________________________________________________________________
MY0555* NAD 83(1986) POSITION- 42 21 18.1 (N) 071 03 03.4 (W) HD_HELD2
MY0555* NAVD 88 <https://www.ngs.noaa.gov/datums/vertical/VerticalDatums.shtml#NAVD88> ORTHO HEIGHT - 4.105 (meters) 13.47 (feet) ADJUSTED
MY0555 ______________________________________________________________________
MY0555 GEOID HEIGHT - -27.738 (meters) GEOID12B
MY0555 DYNAMIC HEIGHT - 4.104 (meters) 13.46 (feet) COMP
MY0555 MODELED GRAVITY - 980,381.5 (mgal) NAVD 88
MY0555
MY0555 VERT ORDER - FIRST CLASS II

As I understand it, here comes the messy part of getting from NAVD88 geopotential geoid to WGS84 ellipsoid. I used NOAA's on-line vertical data transformation tool <https://vdatum.noaa.gov/vdatumweb/?dump_app_trace=false&db_debug=false>, using the 2009 May 16 benchmark recovery date specified in its record.
long = -71.0509444
lat = +42.355028
NAVD88 height = +4.105 m

The WGS84 result was:
long = -71.0509458
lat = 42.3550371
height = -24.850 m ±0.076m

If I did this correctly, to convert from NAVD88 height to WGS84 elevation at this location requires subtracting 28.954m. The MLLW datum of +5.51 ft (+1.679m) becomes -27.275m in WGS84.

The benchmark record above states that the 4.105m NAVD88 height is -27.738m height in GEOID12B at this location. One could double check the NAVD88 → WGS84 conversion by starting with the GEOID12B height. Sadly, GEOID12B does not appear to be an option for the vertical data transformation tool that I used.

But I’m a complete novice at these transformations.

— Eric

p.s.: To confuse matter further, at the moment my iPhone’s Compass tool reports my current height as "50 feet”. I don’t know what is the basis for that determination, but it doesn’t seem correct for a NAVD88 or WGS84 height.
Running with a very normal WGS-84 GPS “by the sea shore” can easily show you underwater. That
is very much a normal result of the model. It does not tell you what high (or low) tide level is going to
be at your location. That stuff is simply to complex.
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