proj4js/lib/datum_transform.js

var PJD_3PARAM = 1;
var PJD_7PARAM = 2;
var PJD_GRIDSHIFT = 3;
var PJD_NODATUM = 5; // WGS84 or equivalent
var SRS_WGS84_SEMIMAJOR = 6378137; // only used in grid shift transforms
var SRS_WGS84_ESQUARED = 0.006694379990141316; //DGR: 2012-07-29
module.exports = function(source, dest, point) {
  var wp, i, l;

  function checkParams(fallback) {
    return (fallback === PJD_3PARAM || fallback === PJD_7PARAM);
  }
  // Short cut if the datums are identical.
  if (source.compare_datums(dest)) {
    return point; // in this case, zero is sucess,
    // whereas cs_compare_datums returns 1 to indicate TRUE
    // confusing, should fix this
  }

  // Explicitly skip datum transform by setting 'datum=none' as parameter for either source or dest
  if (source.datum_type === PJD_NODATUM || dest.datum_type === PJD_NODATUM) {
    return point;
  }

  //DGR: 2012-07-29 : add nadgrids support (begin)
  var src_a = source.a;
  var src_es = source.es;

  var dst_a = dest.a;
  var dst_es = dest.es;

  var fallback = source.datum_type;
  // If this datum requires grid shifts, then apply it to geodetic coordinates.
  if (fallback === PJD_GRIDSHIFT) {
    if (this.apply_gridshift(source, 0, point) === 0) {
      source.a = SRS_WGS84_SEMIMAJOR;
      source.es = SRS_WGS84_ESQUARED;
    }
    else {
      // try 3 or 7 params transformation or nothing ?
      if (!source.datum_params) {
        source.a = src_a;
        source.es = source.es;
        return point;
      }
      wp = 1;
      for (i = 0, l = source.datum_params.length; i < l; i++) {
        wp *= source.datum_params[i];
      }
      if (wp === 0) {
        source.a = src_a;
        source.es = source.es;
        return point;
      }
      if (source.datum_params.length > 3) {
        fallback = PJD_7PARAM;
      }
      else {
        fallback = PJD_3PARAM;
      }
    }
  }
  if (dest.datum_type === PJD_GRIDSHIFT) {
    dest.a = SRS_WGS84_SEMIMAJOR;
    dest.es = SRS_WGS84_ESQUARED;
  }
  // Do we need to go through geocentric coordinates?
  if (source.es !== dest.es || source.a !== dest.a || checkParams(fallback) || checkParams(dest.datum_type)) {
    //DGR: 2012-07-29 : add nadgrids support (end)
    // Convert to geocentric coordinates.
    source.geodetic_to_geocentric(point);
    // CHECK_RETURN;
    // Convert between datums
    if (checkParams(source.datum_type)) {
      source.geocentric_to_wgs84(point);
      // CHECK_RETURN;
    }
    if (checkParams(dest.datum_type)) {
      dest.geocentric_from_wgs84(point);
      // CHECK_RETURN;
    }
    // Convert back to geodetic coordinates
    dest.geocentric_to_geodetic(point);
    // CHECK_RETURN;
  }
  // Apply grid shift to destination if required
  if (dest.datum_type === PJD_GRIDSHIFT) {
    this.apply_gridshift(dest, 1, point);
    // CHECK_RETURN;
  }

  source.a = src_a;
  source.es = src_es;
  dest.a = dst_a;
  dest.es = dst_es;

  return point;
};