code: function percentVariationExplainedByFittedPlane(vertices, centroid, normal, range) {
      var numVertices = vertices.length,
          diff = 0;
      for (var i = 0; i < numVertices; i++) {
          var fittedZ = Math.sqrt(
                  (vertices[i].x - centroid.x) * (vertices[i].x - centroid.x) +
                  (vertices[i].y - centroid.y) * (vertices[i].y - centroid.y)
              ) * Math.tan(normal.z * Math.PI) + centroid.z;
          diff += (vertices[i].z - fittedZ) * (vertices[i].z - fittedZ);
      }
      return 1 - Math.sqrt(diff / numVertices) * 2 / range;
  }
docs: A measure of correlation between a terrain and its fitted plane.
  
  This uses a different approach than the common one (R^2, aka Pearson's
  correlation coefficient) because the range is constricted and the data is
  often non-normal. The approach taken here compares the differences between
  the terrain elevations and the fitted plane at each vertex, and divides by
  half the range to arrive at a dimensionless value.
  
  @param {THREE.Vector3[]} vertices
  The terrain vertices.
  @param {THREE.Vector3} centroid
  The fitted plane centroid.
  @param {THREE.Vector3} normal
  The fitted plane normal.
  @param {Number} range
  The allowed range in elevations.
  
  @return {Number}
  Returns a number between 0 and 1 indicating how well the fitted plane
  explains the variation in terrain elevation. 1 means entirely explained; 0
  means not explained at all.