GeoUK/GeoUK/OSTN02Transform.cs

using GeoUK.Coordinates;
using GeoUK.Ellipsoids;
using GeoUK.Projections;
using System;
using System.Collections.Generic;
using System.IO;
using System.Reflection;

namespace GeoUK
{
    public static class OSTN02Transform
    {
        /// <summary>
        /// Performs an ETRS89 to OSGB36/ODN datum transformation. Accuracy is approximately 10 centimeters.
        /// Whilst very accurate this method is much slower than the Helmert transformation.
        /// </summary>
        public static Osgb36 Etrs89ToOsgb(LatitudeLongitude coordinates)
        {
            var enCoordinates = Convert.ToEastingNorthing(new Grs80(), new BritishNationalGrid(), coordinates);
            return Etrs89ToOsgb(enCoordinates, coordinates.ElipsoidalHeight);
        }

        private static Osgb36 Etrs89ToOsgb(EastingNorthing coordinates, double ellipsoidHeight)
        {
            var dataStream = GetEmbeddedOSTN02();

            TextReader tr = new StreamReader(dataStream);
            List<int> recordNumbers = new List<int>();
            string[] records = new string[4];

            //determine record numbers
            int eastIndex = (int)(coordinates.Easting / 1000.0);
            int northIndex = (int)(coordinates.Northing / 1000.0);

            double x0 = eastIndex * 1000;
            double y0 = northIndex * 1000;

            //work out the four records
            recordNumbers.Add(CalculateRecordNumber(eastIndex, northIndex));
            recordNumbers.Add(CalculateRecordNumber(eastIndex + 1, northIndex));
            recordNumbers.Add(CalculateRecordNumber(eastIndex + 1, northIndex + 1));
            recordNumbers.Add(CalculateRecordNumber(eastIndex, northIndex + 1));

            //get records from the data file
            int recordsFound = 0;
            while (recordsFound < 4)
            {
                string csvRecord = tr.ReadLine();
                for (int index = 0; index < 4; index++)
                {
                    if (csvRecord.StartsWith(recordNumbers[index].ToString().Trim() + ",", StringComparison.Ordinal))
                    {
                        //dont use add as we need to keep these in same order as record numbers
                        records[index] = csvRecord;
                        recordsFound++;
                    }
                }
            }

            //populate the properties
            string[] fields0 = records[0].Split(",".ToCharArray());
            string[] fields1 = records[1].Split(",".ToCharArray());
            string[] fields2 = records[2].Split(",".ToCharArray());
            string[] fields3 = records[3].Split(",".ToCharArray());

            var se0 = System.Convert.ToDouble(fields0[3]);
            var se1 = System.Convert.ToDouble(fields1[3]);
            var se2 = System.Convert.ToDouble(fields2[3]);
            var se3 = System.Convert.ToDouble(fields3[3]);

            var sn0 = System.Convert.ToDouble(fields0[4]);
            var sn1 = System.Convert.ToDouble(fields1[4]);
            var sn2 = System.Convert.ToDouble(fields2[4]);
            var sn3 = System.Convert.ToDouble(fields3[4]);

            var sg0 = System.Convert.ToDouble(fields0[5]);
            var sg1 = System.Convert.ToDouble(fields1[5]);
            var sg2 = System.Convert.ToDouble(fields2[5]);
            var sg3 = System.Convert.ToDouble(fields3[5]);

            var dx = coordinates.Easting - x0;
            var dy = coordinates.Northing - y0;

            var t = dx / 1000.0;
            var u = dy / 1000.0;

            var se = (1 - t) * (1 - u) * se0 + t * (1 - u) * se1 + t * u * se2 + (1 - t) * u * se3;
            var sn = (1 - t) * (1 - u) * sn0 + t * (1 - u) * sn1 + t * u * sn2 + (1 - t) * u * sn3;
            var sg = (1 - t) * (1 - u) * sg0 + t * (1 - u) * sg1 + t * u * sg2 + (1 - t) * u * sg3;

            var geoidDatum = (Osgb36GeoidDatum)System.Convert.ToInt32(fields0[6]);

            var easting = coordinates.Easting + se;
            var northing = coordinates.Northing + sn;
            var height = ellipsoidHeight - sg;

            return new Osgb36(easting, northing, height, geoidDatum);
        }

        /// <summary>
        /// Calculates a data file record number.
        /// </summary>
        /// <param name="eastIndex"></param>
        /// <param name="northIndex"></param>
        /// <returns></returns>
        private static int CalculateRecordNumber(int eastIndex, int northIndex)
        {
            return eastIndex + (northIndex * 701) + 1;
        }

        private static Stream GetEmbeddedOSTN02()
        {
            return ResourceManager.GetEmbeddedResourceStream(typeof(OSTN02Transform).GetTypeInfo().Assembly, "OSTN02_OSGM02_GB.txt");
        }
    }
}