How to convert NAD 27 to WebMercator in using the ESRI Silverlight API? (C#)

Question

I am working with the ESRI Silverlight API. I found THIS post in the ESRI forums and THIS OTHER which show how to convert between WGS84 geographic coordinates and WebMercator coordinates. I want to do the same thing but from NAD27 Geographic coordinates to WebMercator. I found the spatial reference for NAD27 at the spatialreference website with the WKID 4267 and changed the code accordingly with the radius of 6378206.4 and degree 0.01745329251994328 respectively. I ended up with this function (in c#):

private MapPoint GeographicToWebMercator(MapPoint point)
{
        double a = point.Y * 0.01745329251994328;
        double y = (6378206.4/2) * Math.Log((1.0 + Math.Sin(a)) / (1.0 - Math.Sin(a)));

        double x = 6378206.4 * (point.X * 0.01745329251994328);
        return new MapPoint(x, y);
}

Unfortunately, my dots are displaced from where they should be. So, this is not working as intended. I was wondering if someone can take a look at it and tell me what is wrong... or if I can do this projection this way to begin with. The function, as is, is not checking for null "point" or any other data just yet. But, please, don't focus on that. I am just interested in the calculation per se. Thanks!

Answer 1

If you don't mind making a round trip to the server, and you have a 10.1 geometry service available to you, you can use the Esri Rest API Project Geometries operation.

If Transformation is not specified, a search is made through a set of default GeoTransformations. Currently, the following default transformations are used when applicable:

esriSRGeoTransformation_NAD_1927_TO_NAD_1983_NADCON, forward and reverse, WKID=1241

esriSRGeoTransformation_NAD1983_To_WGS1984_1, forward and reverse, WKID=1188

esriSRGeoTransformation_NAD1927_To_WGS1984_4, forward and reverse, WKID=1173

Answer 2

I'd recommend you try the Proj.NET library (a Proj4 implementation for .Net) in your application and let Proj.NET do the coordinate transformations for you. It should be compatible with Silverlight by way of C#.

Here's a blog post with a C# example showing implementation.

I took a crack at it in a simple Windows Form app. Here's a screenshot of it..

And this is the C# code I used, of course you will need to have referenced the ProjNet.dll in your project..

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;

//using ProjNet.Converters; // didn't need this one
using ProjNet.Converters.WellKnownText;
using ProjNet.CoordinateSystems;
using ProjNet.CoordinateSystems.Transformations;

namespace ProjNetTest
{
    public partial class projNetTestForm : Form
    {
        public projNetTestForm()
        {
            InitializeComponent();

            // BassPro, Springfield MO
            // long: -93.29592962, lat: 37.18069720
            xTB.Text = "-93.29592962";
            yTB.Text = "37.18069720";
        }

        private void convertButton_Click(object sender, EventArgs e)
        {
            // Get the NAD27 Point Coords as X(long) Y(Lat)..
            double fromX = Convert.ToDouble(xTB.Text);
            double fromY = Convert.ToDouble(yTB.Text);
            double[] nad27Point = new double[] { fromX, fromY };

            // To begin with, I tested against WGS84 coords.
            // Init the WGS84 rules..
            // ICoordinateSystem gcs_WGS84 = GeographicCoordinateSystem.WGS84;

            // Init the NAD27 rules..
            string epsg4267 = "GEOGCS[\"NAD27\",DATUM[\"North_American_Datum_1927\",SPHEROID[\"Clarke 1866\",6378206.4,294.9786982138982,AUTHORITY[\"EPSG\",\"7008\"]],AUTHORITY[\"EPSG\",\"6267\"]],PRIMEM[\"Greenwich\",0,AUTHORITY[\"EPSG\",\"8901\"]],UNIT[\"degree\",0.01745329251994328,AUTHORITY[\"EPSG\",\"9122\"]],AUTHORITY[\"EPSG\",\"4267\"]]";
            ICoordinateSystem gcs_NAD27 = CoordinateSystemWktReader.Parse(epsg4267) as ICoordinateSystem;

            // Init the WebMerc rules, and note the "gotchas!" at that link (this was eating me alive at first)..
            // READ THIS:
            // http://alastaira.wordpress.com/2011/01/23/the-google-maps-bing-maps-spherical-mercator-projection/
            string epsg3857_HACK = "PROJCS[\"Popular Visualisation CRS / Mercator\",GEOGCS[\"Popular Visualisation CRS\",DATUM[\"WGS84\",SPHEROID[\"WGS84\", 6378137.0, 298.257223563, AUTHORITY[\"EPSG\",\"7059\"]],AUTHORITY[\"EPSG\",\"6055\"]],PRIMEM[\"Greenwich\", 0, AUTHORITY[\"EPSG\", \"8901\"]],UNIT[\"degree\", 0.0174532925199433, AUTHORITY[\"EPSG\", \"9102\"]],AXIS[\"E\", EAST], AXIS[\"N\", NORTH], AUTHORITY[\"EPSG\",\"4055\"]],PROJECTION[\"Mercator\"],PARAMETER[\"semi_minor\",6378137],PARAMETER[\"False_Easting\", 0],PARAMETER[\"False_Northing\", 0],PARAMETER[\"Central_Meridian\", 0],PARAMETER[\"Latitude_of_origin\", 0],UNIT[\"metre\", 1, AUTHORITY[\"EPSG\", \"9001\"]],AXIS[\"East\", EAST], AXIS[\"North\", NORTH],AUTHORITY[\"EPSG\",\"3785\"]]";
            IProjectedCoordinateSystem gsc_WebMerc = CoordinateSystemWktReader.Parse(epsg3857_HACK) as IProjectedCoordinateSystem;

            // Perform the coordinate transformation between these systems.
            CoordinateTransformationFactory transformer = new CoordinateTransformationFactory();
            ICoordinateTransformation coordTransform = transformer.CreateFromCoordinateSystems(gcs_NAD27, gsc_WebMerc as ICoordinateSystem);

            double[] webMercPoint = coordTransform.MathTransform.Transform(nad27Point);

            double webMercX = (double)webMercPoint[0];
            double webMercY = (double)webMercPoint[1];

            resultTB.Text = "X: " + webMercX.ToString() + "\r\nY: " + webMercY.ToString();
        }
    }
}

And finally, here is where QGIS, with the Bing aerial (OpenLayers plugin) shows my point computation. I hovered my mouse over the position in question so it appears in the status area, below:

If you want to compare it with where Google places that Lat/Long, this is the link: http://maps.google.com/?ll=37.18069720,-93.29592962