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I'm looking at the coordinate differences between Google Maps and OSM. I'm a noob in the technicality behind GIS.

Having read:

  1. EPSG 3857 or 4326 for GoogleMaps, OpenStreetMap and Leaflet

  2. How to adjust the difference between the coordinate from OpenStreetMap and Google Maps

I'm not sure whether is it because of #1 or #2 that causes the difference for the same coordinate that appear few hundred meters on the projected map.

And the differences between the data stored and the projected map when using lat/long.

Thanks!

EDIT

This may be more specific to China and the infamous offset issue.

Looking at the following coordinates: 31.230548,121.470965 for the Shanghai Art Museum on Google Maps and ditu.

Having entered the same set of coordinate in OSM, it gives me the Shanghai Art Museum is about 400M away in the NW direction.

I don't understand why a physical point in space is different on the projected maps? Is this specific to China?

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2  
Please post a screenshot and/or code of what causes your confusion. Currently it is not clear what kind of answer/insight would help you. –  underdark May 5 '13 at 16:29
    
As Underdark has mentioned, we can't answer your question, unless we see what exact problem you are experiencing. Generally speaking there should not be a difference between the two service, but often there is, due to the origin of the datasets. –  Devdatta Tengshe May 5 '13 at 16:33
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Updated my original question to include a specific example. –  Nora Olsen May 5 '13 at 16:45
    
@DevdattaTengshe: here is code that shows the difference between the Google (WGS-84) coordinates, and the coordinate system that Baidu uses (GCJ-02? BD-09? I don't know): jsbin.com/panomu/2/watch?js,output –  Dan Dascalescu Apr 6 at 21:01
    
I've created a well-referenced Wikipedia article on the coordinate systems used in China (namely GCJ-02), which create the infamous China GPS shift problem. –  Dan Dascalescu Apr 7 at 12:27

1 Answer 1

If you look at the various data sources , you will see that OSM matches very well with Google's Satellite View.

You can check it with this excellent tool from GeoFabrik. Just mouse over various locations, and you will also see the mouse location on the other map.

http://tools.geofabrik.de/mc/#15/39.9076/116.3929&num=4&mt0=mapnik&mt1=google-map&mt2=bing-map&mt3=google-satellite

But when you check Google Maps, and the Google Satellite View, you will see that they do not match.

http://tools.geofabrik.de/mc/#15/39.9076/116.3929&num=4&mt0=google-hybrid&mt1=google-map&mt2=bing-map&mt3=google-satellite

The best way to observe this, is to open up Google Maps in Hybrid view, where you will see a clear shift between The satellite view and the Roads Layer.

So it is Google Maps, that on purpose are showing incorrect maps.

As to why this is happening, only Google can give you the authentic answer, but my experience says that this must be to meet the statutory requirements of Chinese laws.

A brief reading of The Google forums shows that this issue is well known. For example see these posts:

More reading indicates that this is done because of the laws of China. See these Blog Posts.


Update

I have seen that this offset has been attributed to the fact that China uses some other projection. This is utter nonsense.

There are many countries that have their data in other projections as well as datums other than WGS84. Web maps in these regions appear correct, and show no mismatch.

It is hard for me to believe that organisations such Google, Microsoft(Bing maps), Apple & Nokia (Here maps) are stopped in their tracks by a simple datum transformation or projection transformation, when the code for that is easily available in Repositories such as

Further more, when Chinese services such as Baidu can show a correct map, and Foreign services cannot, it indicates that the issues are not just techincal enter image description here

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Does this only happen within China? Because I did some checks on other coordinates around the world and it did match between OSM and Google. The coordinate should reflect what is in the physical space? –  Nora Olsen May 5 '13 at 17:15
    
It's very difficult to answer that. All I can say is that i've never seen this issue out of china. As your second point: there is often a difference between what should be and what is. –  Devdatta Tengshe May 5 '13 at 18:04
1  
Openstreetmap should aling with bing imagery, because they have allowed to digitze there imagery for Openstreetmap purpose, while Google did not. It may nethertheless be that both services have bought the same satellite pictures. –  AndreJ May 5 '13 at 18:16
    
@AndreJoost: The issue IMHO is that Google Maps does not match even Google Satellite View; That is why it does not match with anything else. All other sources (OSM, Google Satellite View, Bing) match with each other. –  Devdatta Tengshe May 6 '13 at 3:46
    
Here's a clean archive.org link for "All Maps in China are Transformed". This post goes in more technical detail. But I don't understand if GPS devices manufactured in China return different coordinates (vs. other GPS chips) to match Chinese maps, or if Chinese map APIs transform coordinates received from GPS chips into the proprietary system. Citations? –  Dan Dascalescu Apr 7 at 5:01

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