geojson-utils.js

(function () {
  var gju = this.gju = {};

  // Export the geojson object for **CommonJS**
  if (typeof module !== 'undefined' && module.exports) {
    module.exports = gju;
  }

  // adapted from http://www.kevlindev.com/gui/math/intersection/Intersection.js
  gju.lineStringsIntersect = function (l1, l2) {
    var intersects = [];
    for (var i = 0; i <= l1.coordinates.length - 2; ++i) {
      for (var j = 0; j <= l2.coordinates.length - 2; ++j) {
        var a1 = {
          x: l1.coordinates[i][1],
          y: l1.coordinates[i][0]
        },
          a2 = {
            x: l1.coordinates[i + 1][1],
            y: l1.coordinates[i + 1][0]
          },
          b1 = {
            x: l2.coordinates[j][1],
            y: l2.coordinates[j][0]
          },
          b2 = {
            x: l2.coordinates[j + 1][1],
            y: l2.coordinates[j + 1][0]
          },
          ua_t = (b2.x - b1.x) * (a1.y - b1.y) - (b2.y - b1.y) * (a1.x - b1.x),
          ub_t = (a2.x - a1.x) * (a1.y - b1.y) - (a2.y - a1.y) * (a1.x - b1.x),
          u_b = (b2.y - b1.y) * (a2.x - a1.x) - (b2.x - b1.x) * (a2.y - a1.y);
        if (u_b != 0) {
          var ua = ua_t / u_b,
            ub = ub_t / u_b;
          if (0 <= ua && ua <= 1 && 0 <= ub && ub <= 1) {
            intersects.push({
              'type': 'Point',
              'coordinates': [a1.x + ua * (a2.x - a1.x), a1.y + ua * (a2.y - a1.y)]
            });
          }
        }
      }
    }
    if (intersects.length == 0) intersects = false;
    return intersects;
  }

  // Bounding Box

  function boundingBoxAroundPolyCoords (coords) {
    var xAll = [], yAll = []

    for (var i = 0; i < coords[0].length; i++) {
      xAll.push(coords[0][i][1])
      yAll.push(coords[0][i][0])
    }

    xAll = xAll.sort(function (a,b) { return a - b })
    yAll = yAll.sort(function (a,b) { return a - b })

    return [ [xAll[0], yAll[0]], [xAll[xAll.length - 1], yAll[yAll.length - 1]] ]
  }

  gju.pointInBoundingBox = function (point, bounds) {
    return !(point.coordinates[1] < bounds[0][0] || point.coordinates[1] > bounds[1][0] || point.coordinates[0] < bounds[0][1] || point.coordinates[0] > bounds[1][1]) 
  }

  // Point in Polygon
  // http://www.ecse.rpi.edu/Homepages/wrf/Research/Short_Notes/pnpoly.html#Listing the Vertices

  function pnpoly (x,y,coords) {
    var vert = [ [0,0] ]

    for (var i = 0; i < coords.length; i++) {
      for (var j = 0; j < coords[i].length; j++) {
        vert.push(coords[i][j])
      }
      vert.push([0,0])
    }

    var inside = false
    for (var i = 0, j = vert.length - 1; i < vert.length; j = i++) {
      if (((vert[i][0] > y) != (vert[j][0] > y)) && (x < (vert[j][1] - vert[i][1]) * (y - vert[i][0]) / (vert[j][0] - vert[i][0]) + vert[i][1])) inside = !inside
    }

    return inside
  }

  gju.pointInPolygon = function (p, poly) {
    var coords = (poly.type == "Polygon") ? [ poly.coordinates ] : poly.coordinates

    var insideBox = false
    for (var i = 0; i < coords.length; i++) {
      if (gju.pointInBoundingBox(p, boundingBoxAroundPolyCoords(coords[i]))) insideBox = true
    }
    if (!insideBox) return false

    var insidePoly = false
    for (var i = 0; i < coords.length; i++) {
      if (pnpoly(p.coordinates[1], p.coordinates[0], coords[i])) insidePoly = true
    }

    return insidePoly
  }

  gju.numberToRadius = function (number) {
    return number * Math.PI / 180;
  }

  gju.numberToDegree = function (number) {
    return number * 180 / Math.PI;
  }

  // written with help from @tautologe
  gju.drawCircle = function (radiusInMeters, centerPoint, steps) {
    var center = [centerPoint.coordinates[1], centerPoint.coordinates[0]],
      dist = (radiusInMeters / 1000) / 6371,
      // convert meters to radiant
      radCenter = [gju.numberToRadius(center[0]), gju.numberToRadius(center[1])],
      steps = steps || 15,
      // 15 sided circle
      poly = [[center[0], center[1]]];
    for (var i = 0; i < steps; i++) {
      var brng = 2 * Math.PI * i / steps;
      var lat = Math.asin(Math.sin(radCenter[0]) * Math.cos(dist)
              + Math.cos(radCenter[0]) * Math.sin(dist) * Math.cos(brng));
      var lng = radCenter[1] + Math.atan2(Math.sin(brng) * Math.sin(dist) * Math.cos(radCenter[0]),
                                          Math.cos(dist) - Math.sin(radCenter[0]) * Math.sin(lat));
      poly[i] = [];
      poly[i][1] = gju.numberToDegree(lat);
      poly[i][0] = gju.numberToDegree(lng);
    }
    return {
      "type": "Polygon",
      "coordinates": [poly]
    };
  }

  // assumes rectangle starts at lower left point
  gju.rectangleCentroid = function (rectangle) {
    var bbox = rectangle.coordinates[0];
    var xmin = bbox[0][0],
      ymin = bbox[0][1],
      xmax = bbox[2][0],
      ymax = bbox[2][1];
    var xwidth = xmax - xmin;
    var ywidth = ymax - ymin;
    return {
      'type': 'Point',
      'coordinates': [xmin + xwidth / 2, ymin + ywidth / 2]
    };
  }

  // from http://www.movable-type.co.uk/scripts/latlong.html
  gju.pointDistance = function (pt1, pt2) {
    var lon1 = pt1.coordinates[0],
      lat1 = pt1.coordinates[1],
      lon2 = pt2.coordinates[0],
      lat2 = pt2.coordinates[1],
      dLat = gju.numberToRadius(lat2 - lat1),
      dLon = gju.numberToRadius(lon2 - lon1),
      a = Math.pow(Math.sin(dLat / 2), 2) + Math.cos(gju.numberToRadius(lat1))
        * Math.cos(gju.numberToRadius(lat2)) * Math.pow(Math.sin(dLon / 2), 2),
      c = 2 * Math.atan2(Math.sqrt(a), Math.sqrt(1 - a));
    return (6371 * c) * 1000; // returns meters
  },

  // checks if geometry lies entirely within a circle
  // works with Point, LineString, Polygon
  gju.geometryWithinRadius = function (geometry, center, radius) {
    if (geometry.type == 'Point') {
      return gju.pointDistance(geometry, center) <= radius;
    } else if (geometry.type == 'LineString' || geometry.type == 'Polygon') {
      var point = {};
      var coordinates;
      if (geometry.type == 'Polygon') {
        // it's enough to check the exterior ring of the Polygon
        coordinates = geometry.coordinates[0];
      } else {
        coordinates = geometry.coordinates;
      }
      for (var i in coordinates) {
        point.coordinates = coordinates[i];
        if (gju.pointDistance(point, center) > radius) {
          return false;
        }
      }
    }
    return true;
  }

  // adapted from http://paulbourke.net/geometry/polyarea/javascript.txt
  gju.area = function (polygon) {
    var area = 0;
    // TODO: polygon holes at coordinates[1]
    var points = polygon.coordinates[0];
    var j = points.length - 1;
    var p1, p2;

    for (var i = 0; i < points.length; j = i++) {
      var p1 = {
        x: points[i][1],
        y: points[i][0]
      };
      var p2 = {
        x: points[j][1],
        y: points[j][0]
      };
      area += p1.x * p2.y;
      area -= p1.y * p2.x;
    }

    area /= 2;
    return area;
  },

  // adapted from http://paulbourke.net/geometry/polyarea/javascript.txt
  gju.centroid = function (polygon) {
    var f, x = 0,
      y = 0;
    // TODO: polygon holes at coordinates[1]
    var points = polygon.coordinates[0];
    var j = points.length - 1;
    var p1, p2;

    for (var i = 0; i < points.length; j = i++) {
      var p1 = {
        x: points[i][1],
        y: points[i][0]
      };
      var p2 = {
        x: points[j][1],
        y: points[j][0]
      };
      f = p1.x * p2.y - p2.x * p1.y;
      x += (p1.x + p2.x) * f;
      y += (p1.y + p2.y) * f;
    }

    f = gju.area(polygon) * 6;
    return {
      'type': 'Point',
      'coordinates': [y / f, x / f]
    };
  },

  gju.simplify = function (source, kink) { /* source[] array of geojson points */
    /* kink	in metres, kinks above this depth kept  */
    /* kink depth is the height of the triangle abc where a-b and b-c are two consecutive line segments */
    kink = kink || 20;
    source = source.map(function (o) {
      return {
        lng: o.coordinates[0],
        lat: o.coordinates[1]
      }
    });

    var n_source, n_stack, n_dest, start, end, i, sig;
    var dev_sqr, max_dev_sqr, band_sqr;
    var x12, y12, d12, x13, y13, d13, x23, y23, d23;
    var F = (Math.PI / 180.0) * 0.5;
    var index = new Array(); /* aray of indexes of source points to include in the reduced line */
    var sig_start = new Array(); /* indices of start & end of working section */
    var sig_end = new Array();

    /* check for simple cases */

    if (source.length < 3) return (source); /* one or two points */

    /* more complex case. initialize stack */

    n_source = source.length;
    band_sqr = kink * 360.0 / (2.0 * Math.PI * 6378137.0); /* Now in degrees */
    band_sqr *= band_sqr;
    n_dest = 0;
    sig_start[0] = 0;
    sig_end[0] = n_source - 1;
    n_stack = 1;

    /* while the stack is not empty  ... */
    while (n_stack > 0) {

      /* ... pop the top-most entries off the stacks */

      start = sig_start[n_stack - 1];
      end = sig_end[n_stack - 1];
      n_stack--;

      if ((end - start) > 1) { /* any intermediate points ? */

        /* ... yes, so find most deviant intermediate point to
        either side of line joining start & end points */

        x12 = (source[end].lng() - source[start].lng());
        y12 = (source[end].lat() - source[start].lat());
        if (Math.abs(x12) > 180.0) x12 = 360.0 - Math.abs(x12);
        x12 *= Math.cos(F * (source[end].lat() + source[start].lat())); /* use avg lat to reduce lng */
        d12 = (x12 * x12) + (y12 * y12);

        for (i = start + 1, sig = start, max_dev_sqr = -1.0; i < end; i++) {

          x13 = source[i].lng() - source[start].lng();
          y13 = source[i].lat() - source[start].lat();
          if (Math.abs(x13) > 180.0) x13 = 360.0 - Math.abs(x13);
          x13 *= Math.cos(F * (source[i].lat() + source[start].lat()));
          d13 = (x13 * x13) + (y13 * y13);

          x23 = source[i].lng() - source[end].lng();
          y23 = source[i].lat() - source[end].lat();
          if (Math.abs(x23) > 180.0) x23 = 360.0 - Math.abs(x23);
          x23 *= Math.cos(F * (source[i].lat() + source[end].lat()));
          d23 = (x23 * x23) + (y23 * y23);

          if (d13 >= (d12 + d23)) dev_sqr = d23;
          else if (d23 >= (d12 + d13)) dev_sqr = d13;
          else dev_sqr = (x13 * y12 - y13 * x12) * (x13 * y12 - y13 * x12) / d12; // solve triangle
          if (dev_sqr > max_dev_sqr) {
            sig = i;
            max_dev_sqr = dev_sqr;
          }
        }

        if (max_dev_sqr < band_sqr) { /* is there a sig. intermediate point ? */
          /* ... no, so transfer current start point */
          index[n_dest] = start;
          n_dest++;
        } else { /* ... yes, so push two sub-sections on stack for further processing */
          n_stack++;
          sig_start[n_stack - 1] = sig;
          sig_end[n_stack - 1] = end;
          n_stack++;
          sig_start[n_stack - 1] = start;
          sig_end[n_stack - 1] = sig;
        }
      } else { /* ... no intermediate points, so transfer current start point */
        index[n_dest] = start;
        n_dest++;
      }
    }

    /* transfer last point */
    index[n_dest] = n_source - 1;
    n_dest++;

    /* make return array */
    var r = new Array();
    for (var i = 0; i < n_dest; i++)
      r.push(source[index[i]]);

    return r.map(function (o) {
      return {
        type: "Point",
        coordinates: [o.lng, o.lat]
      }
    });
  }

  // http://www.movable-type.co.uk/scripts/latlong.html#destPoint
  gju.destinationPoint = function (pt, brng, dist) {
    dist = dist/6371;  // convert dist to angular distance in radians
    brng = gju.numberToRadius(brng);

    var lat1 = gju.numberToRadius(pt.coordinates[0]);
    var lon1 = gju.numberToRadius(pt.coordinates[1]);

    var lat2 = Math.asin( Math.sin(lat1)*Math.cos(dist) +
                          Math.cos(lat1)*Math.sin(dist)*Math.cos(brng) );
    var lon2 = lon1 + Math.atan2(Math.sin(brng)*Math.sin(dist)*Math.cos(lat1),
                                 Math.cos(dist)-Math.sin(lat1)*Math.sin(lat2));
    lon2 = (lon2+3*Math.PI) % (2*Math.PI) - Math.PI;  // normalise to -180..+180º

    return {
      'type': 'Point',
      'coordinates': [gju.numberToDegree(lat2), gju.numberToDegree(lon2)]
    };
  };

})();