Question

I am building a Tile Overlay server for Google maps in C#, and have found a few different code examples for calculating Y from Latitude. After getting them to work in general, I started to notice certain cases where the overlays were not lining up properly. To test this, I made a test harness to compare Google Map’s Mercator LatToY conversion against the formulas I found online. As you can see below, they do not match in certain cases.

Case #1

Zoomed Out: The problem is most
evident when zoomed out. Up close,
the problem is barely visible.

Case #2

Point Proximity to Top & Bottom of
viewing bounds: The problem is worse
in the middle of the viewing bounds,
and gets better towards the edges.
This behavior can negate the behavior
of Case #1

The Test:

I created a google maps page to
display red lines using the Google Map
API’s built in Mercator conversion,
and overlay this with an image using
the reference code for doing Mercator
conversion. These conversions are
represented as black lines. Compare
the difference.

The Results:
Equator http://www.kayak411.com/Mercator/MercatorComparison%20-%20Equator.png
North Zoomed Out http://www.kayak411.com/Mercator/MercatorComparison%20-%20North%20Zoomed%20Out.png

Check out the top-most and bottom-most lines:
North Top & Bottom Example http://www.kayak411.com/Mercator/MercatorComparison%20-%20North%20Zoomed%20Out%20-%20TopAndBottom.png

The problem gets visually larger but numerically smaller as you zoom in:
alt text http://www.kayak411.com/Mercator/MercatorComparison%20-%20North%20Zoomed%20Midway.png

And it all but disappears at closer zoom levels, regardless of screen orientation.
alt text http://www.kayak411.com/Mercator/MercatorComparison%20-%20North%20Zoomed%20In.png

The Code:

Google Maps Client Side Code:

            var lat = 0;
        for (lat = -80; lat <= 80; lat += 5) {
            map.addOverlay(new GPolyline([new GLatLng(lat, -180), new GLatLng(lat, 0)], "#FF0033", 2));
            map.addOverlay(new GPolyline([new GLatLng(lat, 0), new GLatLng(lat, 180)], "#FF0033", 2));
        }

Server Side Code:

Tile Cutter :
http://mapki.com/wiki/Tile_Cutter

OpenStreetMap Wiki :
http://wiki.openstreetmap.org/wiki/Mercator

 protected override void ImageOverlay_ComposeImage(ref Bitmap ZipCodeBitMap)
        {
            Graphics LinesGraphic = Graphics.FromImage(ZipCodeBitMap);

            Int32 MapWidth = Convert.ToInt32(Math.Pow(2, zoom) * 255);

            Point Offset =
                Cartographer.Mercator2.toZoomedPixelCoords(North, West, zoom);

            TrimPoint(ref Offset, MapWidth);

            for (Double lat = -80; lat <= 80; lat += 5)
            {
                Point StartPoint = Cartographer.Mercator2.toZoomedPixelCoords(lat, -179, zoom);
                Point EndPoint = Cartographer.Mercator2.toZoomedPixelCoords(lat, -1, zoom);

                TrimPoint(ref StartPoint, MapWidth);
                TrimPoint(ref EndPoint, MapWidth);

                StartPoint.X = StartPoint.X - Offset.X;
                EndPoint.X = EndPoint.X - Offset.X;

                StartPoint.Y = StartPoint.Y - Offset.Y;
                EndPoint.Y = EndPoint.Y - Offset.Y;


                LinesGraphic.DrawLine(new Pen(Color.Black, 2),
                    StartPoint.X,
                    StartPoint.Y,
                    EndPoint.X,
                    EndPoint.Y);

                LinesGraphic.DrawString(
                    lat.ToString(),
                    new Font("Verdana", 10),
                    new SolidBrush(Color.Black),
                    new Point(
                        Convert.ToInt32((width / 3.0) * 2.0),
                        StartPoint.Y));
            }
        }

        protected void TrimPoint(ref Point point, Int32 MapWidth)
        {
            point.X = Math.Max(point.X, 0);
            point.X = Math.Min(point.X, MapWidth - 1);

            point.Y = Math.Max(point.Y, 0);
            point.Y = Math.Min(point.Y, MapWidth - 1);
        }

So, Anyone ever experienced this? Dare I ask, resolved this? Or simply have a better C# implementation of Mercator Project coordinate conversion?

Thanks!

Answer 1

Thank you all for your suggestions & assistance.

What I eventually found out is that it’s not a formula or technical problem, I believe it’s a methodology problem.

You can’t define the viewing area in Lat/Lng format, and expect to populate it with the appropriate Mercator projections. That’s where the distortion happens. Instead, you have to define the correct viewing box in Mercator, and project Mercator.

Doing that I was able to correctly match up with Google maps.