Debugging an application that uses Google Maps

Thanks to Shuja and Guillaume that are kind enough to let me bug them all the time, I managed to finish the module to position the message on the map using latitude and longitude. It was hard because I had to play with three different reference systems: the canvas, the tile and finally the lat-long Mercator Projection.

To resources contributed to this little success: the first is this web site that allows you to debug a google map application, revealing the tile indexing system and the coordinates references. However, mind that the code offered as example is faulty. On a newsgroup I managed to find the good code that I am posting below. The code allow to convert a Google Map tile index, to latitude and longitude. Additionally, I converted the javascript into python ūüôā

Gmaps Debug

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def _inv_project(self, x, y, zoom):

¬† ¬† ¬† ¬† “””returns a Rectangle2D with x = lon degrees , y = lat degrees,

          width=lonSpan, height=latSpan for an x,y,zoom as used by google.

¬† ¬† ¬† ¬† ¬† “””

        lon = -180.0 # x

        lonWidth = 360.0 # width 360

       

        # double lat = -90;  // y

        # double latHeight = 180; // height 180

        lat      = -1.0

        latHeight = 2.0

       

        tilesAtThisZoom = 1 << (17 Рzoom)

        lonWidth  = 360.0 / tilesAtThisZoom

        lon      = -180.0 + (x * lonWidth)

        latHeight = -2.0 / tilesAtThisZoom

        lat      = 1.0 + (y * latHeight)

       

        # convert lat and latHeight to degrees in a mercator projection

        # note that in fact the coordinates go from

        # about -85 to +85 not -90 to 90!

       

        latHeight += lat

        latHeight = (2.0 * math.atan(math.exp(math.pi * latHeight))) Р(math.pi / 2.0)

        latHeight *= (180.0 / math.pi);

       

        lat = (2.0 * math.atan(math.exp(math.pi * lat))) Р(math.pi / 2.0)

        lat *= (180.0 / math.pi)

       

        latHeight -= lat

       

        if (lonWidth < 0.0):

          lon      = lon + lonWidth

          lonWidth = -lonWidth 

       

        if (latHeight < 0.0):

          lat      = lat + latHeight

          latHeight = -latHeight

        return (lon, lat, lonWidth, latHeight)

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