/// <summary>
/// Performs an OSGB36/ODN to ETRS89 datum transformation. Accuracy is approximately 10 centimeters.
/// Whilst very accurate this method is much slower than the Helmert transformation.
/// </summary>
public static LatitudeLongitude OsgbToEtrs89(Osgb36 coordinates, OstnVersionEnum ostnVersion = OstnVersionEnum.OSTN15)
{
//calculate shifts from OSGB36 point
double errorN = double.MaxValue;
double errorE = double.MaxValue;
EastingNorthing enCoordinates = null;
Shifts shiftsA = GetShifts(coordinates, ostnVersion);
//0.0001 error meters
int iter = 0;
while ((errorN > 0.0001 || errorE > 0.0001) && iter < 10)
{
enCoordinates = new EastingNorthing(coordinates.Easting - shiftsA.Se, coordinates.Northing - shiftsA.Sn);
Shifts shiftsB = GetShifts(enCoordinates, ostnVersion);
errorE = Math.Abs(shiftsA.Se - shiftsB.Se);
errorN = Math.Abs(shiftsA.Sn - shiftsB.Sn);
shiftsA = shiftsB;
iter++;
}
return(Convert.ToLatitudeLongitude(new Wgs84(), new BritishNationalGrid(), enCoordinates));
}
/// <summary>
/// Retrieves the parsed OSTN data from the embedded resource file.
/// </summary>
/// <param name="ostnVersion"></param>
/// <returns></returns>
private static Dictionary <int, OstnDataRecord> GetOstnData(OstnVersionEnum ostnVersion)
{
switch (ostnVersion)
{
case OstnVersionEnum.OSTN02:
return(ResourceManager.Ostn02Data);
case OstnVersionEnum.OSTN15:
return(ResourceManager.Ostn15Data);
default:
throw new NotImplementedException();
}
}
/// <summary>
/// Gets the embedded OSTN data
/// </summary>
/// <param name="ostnVersion"></param>
/// <returns></returns>
private static Dictionary <int, OstnDataRecord> RetrieveEmbeddedOSTN(OstnVersionEnum ostnVersion)
{
Stream stream;
switch (ostnVersion)
{
case OstnVersionEnum.OSTN02:
stream = GetEmbeddedResourceStream(typeof(Transform).GetTypeInfo().Assembly, "OSTN02_OSGM02_GB.txt");
break;
case OstnVersionEnum.OSTN15:
stream = GetEmbeddedResourceStream(typeof(Transform).GetTypeInfo().Assembly, "OSTN15_OSGM15_DataFile.txt");
break;
default:
throw new NotImplementedException();
}
Dictionary <int, OstnDataRecord> data = new Dictionary <int, OstnDataRecord>();
using (StreamReader reader = new StreamReader(stream))
{
// Skipping the header row
reader.ReadLine();
string line;
while ((line = reader.ReadLine()) != null)
{
if (string.IsNullOrWhiteSpace(line))
{
continue;
}
string[] values = line.Split(',');
OstnDataRecord record = new OstnDataRecord
{
Point_ID = int.Parse(values[0]),
ETRS89_Easting = double.Parse(values[1]),
ETRS89_Northing = double.Parse(values[2]),
ETRS89_OSGB36_EShift = double.Parse(values[3]),
ETRS89_OSGB36_NShift = double.Parse(values[4]),
ETRS89_ODN_HeightShift = double.Parse(values[5]),
Height_Datum_Flag = double.Parse(values[6]),
};
data[record.Point_ID] = record;
}
}
return(data);
}
private static Shifts GetShifts(EastingNorthing coordinates, OstnVersionEnum ostnVersion)
{
//See OS Document: Transformations and OSGM02/OSGM15 user guide chapter 3
Dictionary <int, OstnDataRecord> ostnData = GetOstnData(ostnVersion);
List <int> recordNumbers = new List <int>();
OstnDataRecord[] records = new OstnDataRecord[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 the corresponding records from the data dictionary
for (int index = 0; index < 4; index++)
{
records[index] = ostnData[recordNumbers[index]];
}
//populate the properties
double se0 = System.Convert.ToDouble(records[0].ETRS89_OSGB36_EShift);
double se1 = System.Convert.ToDouble(records[1].ETRS89_OSGB36_EShift);
double se2 = System.Convert.ToDouble(records[2].ETRS89_OSGB36_EShift);
double se3 = System.Convert.ToDouble(records[3].ETRS89_OSGB36_EShift);
double sn0 = System.Convert.ToDouble(records[0].ETRS89_OSGB36_NShift);
double sn1 = System.Convert.ToDouble(records[1].ETRS89_OSGB36_NShift);
double sn2 = System.Convert.ToDouble(records[2].ETRS89_OSGB36_NShift);
double sn3 = System.Convert.ToDouble(records[3].ETRS89_OSGB36_NShift);
double sg0 = System.Convert.ToDouble(records[0].ETRS89_ODN_HeightShift);
double sg1 = System.Convert.ToDouble(records[1].ETRS89_ODN_HeightShift);
double sg2 = System.Convert.ToDouble(records[2].ETRS89_ODN_HeightShift);
double sg3 = System.Convert.ToDouble(records[3].ETRS89_ODN_HeightShift);
double dx = coordinates.Easting - x0;
double dy = coordinates.Northing - y0;
double t = dx / 1000.0;
double u = dy / 1000.0;
Shifts shifts = new Shifts
{
Se = (1 - t) * (1 - u) * se0 + t * (1 - u) * se1 + t * u * se2 + (1 - t) * u * se3,
Sn = (1 - t) * (1 - u) * sn0 + t * (1 - u) * sn1 + t * u * sn2 + (1 - t) * u * sn3,
Sg = (1 - t) * (1 - u) * sg0 + t * (1 - u) * sg1 + t * u * sg2 + (1 - t) * u * sg3,
GeoidDatum = (Osgb36GeoidDatum)System.Convert.ToInt32(records[0].Height_Datum_Flag)
};
return(shifts);
}
private static Osgb36 Etrs89ToOsgb(EastingNorthing coordinates, double ellipsoidHeight, OstnVersionEnum ostnVersion = OstnVersionEnum.OSTN15)
{
Shifts shifts = GetShifts(coordinates, ostnVersion);
double easting = coordinates.Easting + shifts.Se;
double northing = coordinates.Northing + shifts.Sn;
double height = ellipsoidHeight - shifts.Sg;
return(new Osgb36(easting, northing, height, shifts.GeoidDatum));
}
