
Why Digital Maps Are Inaccurate in China - cpeterso
http://www.travelandleisure.com/articles/digital-maps-skewed-china
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contingencies
I was dealing with multi-source map authoring with coordinate system
conversions here in China years ago (~2004)... that's before Google Maps came
out.

Later I found that in China a given city's satellite data for a will be off by
some standard drift, usually pretty much effectively constant for a whole city
area, and by co-relating a few known points throughout your area of interest
across multiple sources' coordinate systems you can scale things well, even if
you are remaining 100% projection agnostic (black box treatment).

I have never studied computational cartography, GIS and only have a basic
self-taught understanding of matrices but didn't find this all that difficult
to hack up at the time.

We were using it to generate little point-of-interest area maps to help people
find locations throughout the city on a sort of city-focused whats on style
magazine operation. Maps were generated in multiple languages.

We were shut down by the publishing bureau but subsequently kept running
anyway, apparently with no problems. The business continues today, though I
have nothing to do with it.

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tristanj
Topic discussed 1 month ago (205 points) here:
[https://news.ycombinator.com/item?id=10964450](https://news.ycombinator.com/item?id=10964450)

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zcbenz
There is a Chinese idiom describing things like this: Plugging One's Ears
While Stealing a Bell.

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jzwinck
South Korea also has problems with Google Maps. And also for political
reasons. Apparently they have some disagreement with Google which prohibits
Maps from supporting routing aka navigation. It also prohibits full resolution
display, although the map data is basically present.

It seems to be a way to retain control over what is undoubtedly economically
valuable information. But at what cost?

~~~
coldtea
> _It seems to be a way to retain control over what is undoubtedly
> economically valuable information. But at what cost?_

At the cost of not giving up some of your national assets such as geographic
info just to get better GPS gadgets from foreign companies? They can still
create and enjoy their own maps and tools.

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ovis
The article implies that the source of trouble is the GCJ-02 datum.

Is this really such a major problem though? In North America, both the NAD27
and NAD83 datums are still common. NAD83 nearly matches WGS84, but NAD27 can
be quite off. Datum transformation is a well understood topic, so what's so
special about China?

~~~
HillRat
Yes, this. I've had to deal with inter-datum conversions in the past, and as
far as I can tell, though after a _very_ brief review, GCJ-02 is just another
datum, albeit one not well documented. The "random offset" described in
various documents may be easily explained by the datum being optimized for
accuracy in a relatively small area, and thus errors increase as you expand
your mapped area.

(For those who don't speak GIS, datums are essentially a way of mapping an
orange peel to a table; because any attempt to dedimensionalize points on a
sphere must by necessity introduce errors, datums are designed to either
minimize that error in all locations -- which is what WGS84 does -- or to gain
greater accuracy by focusing on a smaller part of the globe, such as the
"state plane" datums. When you see misaligned points on maps that derive data
from various sources, it's usually a good idea to make sure that multiple
unconverted datums aren't the source of the problem.)

~~~
maxerickson
The state plane coordinate systems use NAD27 or NAD83 (plus improvements).

[https://en.wikipedia.org/wiki/State_Plane_Coordinate_System](https://en.wikipedia.org/wiki/State_Plane_Coordinate_System)

The flattening of an orange peel you mention is more about projection, a datum
is a mathematical description of the earth that maps out how geographic
coordinates will vary. Given geographic coordinates, a projection system is
needed to display them on a flat surface (or really any surface that doesn't
match the shape of the geographic coordinate system).

Differences between datums do arise from different modeling choices and should
be accounted for when translating coordinates, but as the fine article
discusses, the offsets in GCJ-02 are algorithmic and are not a refinement of
WGS84, they obscure true location.

[https://en.wikipedia.org/wiki/Restrictions_on_geographic_dat...](https://en.wikipedia.org/wiki/Restrictions_on_geographic_data_in_China#GCJ-02)

------
est
in case anyone missed, the source code of evil transform between WGS-84 and
GCJ-02 in C/C#/Go/Javascript/Python/PHP/Swift

[https://github.com/googollee/eviltransform](https://github.com/googollee/eviltransform)

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digi_owl
Its funny how some of the most paranoid seeming nations on the planet has
within the last 100 years or so experienced foreign military meddling in
internal affairs.

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_greim_
Interesting. Why can't maps be produced using satellite photos?

~~~
jackvalentine
The problem isn't of producing a map, it's of publishing it. If you go ahead
and do that be prepared to find yourself locked out of anything to do with
China.

~~~
_greim_
But if the bottleneck is publication, not data gathering, then why does the
Chinese government bother cracking down on map making activities?

> Three British geology students discovered this the hard way while
> “collecting data” on a 2009 field trip...

~~~
est
> then why does the Chinese government bother cracking down on map making
> activities

Most China polices can be explained by the "fuck you" treatment. What is "fuck
you" treatment? Well fuck you go figure it out yourself.

~~~
b6
See also: "no why".

Paranoid psychopaths are so obsessed with keeping a stranglehold on every
possible thing that they design algorithms to screw up maps, causing untold
problems, inconvenience, extra costs, incompatibility, etc. It's reverse-
engineered and compensated for by everyone else, but they just keep doing it.
Why? No why. #china. My cynicism is, I think, my way of dealing with
heartbrokenness.

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ck2
We like to think of North Korea as the crazy country but we forget it has a
much older brother who is far more dangerous because it appears to be mature
and acceptable yet is almost as nuts.

~~~
coldtea
Said the country that evaporated 250.000 Japanese (men, women and children),
sprayed Vietnam farmers with chemicals, had segregation until the sixties, has
the biggest number of incarcerations in the world, has been tens of times at
war with places thousands of miles away from its borders, has supported all
kinds of dictatorships abroad as long as they have been favorable for its
interests, had 2 open occupations of foreign countries just until a few years
ago, is full of religious nuts, has a huge surveillance system that spans the
whole world, its police kills more people (mostly blacks) in a year that the
rest of the Western world combined does in 10, and has elected Bush for two
terms, while now half of it celebrates Trump...

~~~
leesalminen
Oh no you didn't!

