Does a Google Maps coordinate map to the same location in Baidu Maps?

Question

I'm implementing a map view in Baidu Maps (China's primary map service) using latitude & longitude originating from Google Maps. I am finding that my locations (primarily around Shanghai) are consistently 'off' by about a kilometer, though!

Doing a linear adjustment makes the Baidu locations appear in roughly (+- a few meters) the right spot:

//Declaring my Baidu map marker's properties
...
lat : gm_location.lat + 0.00575,
lng : gm_location.lng + 0.00668,
...

Although this will do for my particular needs, I dislike these magic numbers. Anyone have any insight where this offset is emerging from?

Answer 1

Which Google Maps do you use, google.com/map, or google.cn/map? Satellite or street view? In all cases, the answer is no - coordinates won't correspond to the Baidu Map POI, but for slightly different reasons.

First off, since Baidu doesn't offer any useful maps outside of China, let's focus on the question being,

Does a Google Maps coordinate within China, map to the same location in Baidu Maps?

The answer is no. Baidu Maps uses its own BD-09 coordinate system, while Google Maps uses either the Chinese GCJ-02 standard (for street maps on either google.com/maps or .cn), or WGS-84 for satellite imagery on google.com satellite view maps.

GPS coordinates (WGS-84) and Google Street Map coordinates (GCJ-02) do not map to the same location on Baidu Maps, as this snippet demonstrates (I didn't include the snippet in the answer because it doesn't work due to the way SO includes scripts - but you can see the code in this other answer I gave).

Using a linear correction around Shanghai might work, but the values will change for other regions in China.

How to compensate for the offset

The official method for converting Google coordinates to Baidu Map coordinates is to use the Baidu Map API for doing so.

There are two versions of the API:

1. An allegedly "server" version documented at http://developer.baidu.com/map/changeposition.htm that requires obtaining an application key

2. A version at http://api.map.baidu.com/ag/coord/convert that's used by the Baidu Map coordinates conversion demo via http://developer.baidu.com/map/jsdemo/demo/convertor.js and doesn't need an application key.

Let's take the Google Street Map (GCJ-02) coordinates of The People's Heroes Monument in Shanghai and convert them to Baidu Maps coordinates:

curl "http://api.map.baidu.com/ag/coord/convert?from=2&to=4&x=121.4914&y=31.2423"

will output

{"error":0,"x":"MTIxLjQ5Nzk5OTg3MzM4","y":"MzEuMjQ3OTc1ODQwMTk2"}

The coordinates are base64-encoded, and decode to

121.49799987338, 31.247975840196

If map.baidu.com allowed you to enter lat/lon coordinates, you'd land at the People's Heroes Monument.

Baidu's API only offers conversion to BD-09.

The conversion can also be run offline, and formulas can be found on Chinese blogs. Conversion between GCJ-02, WGS-84 and B-09 can be performed using libraries like geoChina or eviltransform.

Answer 2

Companies in China are required by law not to export map data without an offset to disguise foreign companies. See https://help.openstreetmap.org/questions/37560/map-offset-in-china for more details.

Coordinates you get from maps.google.com are probably wrong while those in baidu are probably correct.

Answer 3

Baidu map API does have a service to translate the Google Maps coordinates to Baidu map coordinates but I think it is only accurate for locations within China region. anywhere else is still off

Take a look at the official demo

http://developer.baidu.com/map/jsdemo.htm#a5_1

But beware there is a major delay before it returns your translated coordinate(s).

var x = 116.32715863448607;
var y = 39.990912172420714;
var ggPoint = new BMap.Point(x,y); // step 1 - create a Baidu map point from Google Maps coordinate


translateCallback = function (data){
  if(data.status === 0) {
    var marker = new BMap.Marker(data.points[0]);  // step 3 - u get the offset coordinate here
    bm.addOverlay(marker);
    bm.setCenter(data.points[0]);
  }
}

setTimeout(function(){
  var convertor = new BMap.Convertor();
  var pointArr = [];
  pointArr.push(ggPoint);
  convertor.translate(pointArr, 3, 5, translateCallback); // step 2 - pass the point as an array to the translate service
}, 1000);

Answer 4

My solution is

Replacing Lat and Long value in this link with current your coordinate (Google Lat and Long)

http://api.map.baidu.com/marker?location=39.916979519873,116.41004950566&output=html