code: function(x,z){
  				var idx = z * this.width + x;
  				if(idx < 0 || idx > this.__elevationLength){
  					throw new Error("the given terrain cell is invalid: "+x+ ";"+z);
  				}
  				return idx;
  			}
docs: Computes the array index for the given cell coords & checks if they're in
  bounds. If not an {@link IndexOutOfBoundsException} is thrown.
  @param {Number} x
  @param {Number} z
  @return {Number} array index