code: def getAltitudeFromLatLon(self, lat, lon):
          
          # print "-----\nFromLatLon", lon, lat
          lat -= self.lat
          lon -= self.lon
          # print "lon, lat", lon, lat
          if lat < 0.0 or lat >= 1.0 or lon < 0.0 or lon >= 1.0:
              raise WrongTileError(self.lat, self.lon, self.lat+lat, self.lon+lon)
          x = lon * (self.size - 1)
          y = lat * (self.size - 1)
          # print "x,y", x, y
          x_int = int(x)
          x_frac = x - int(x)
          y_int = int(y)
          y_frac = y - int(y)
          # print "frac", x_int, x_frac, y_int, y_frac
          value00 = self.getPixelValue(x_int, y_int)
          value10 = self.getPixelValue(x_int+1, y_int)
          value01 = self.getPixelValue(x_int, y_int+1)
          value11 = self.getPixelValue(x_int+1, y_int+1)
          value1 = self._avg(value00, value10, x_frac)
          value2 = self._avg(value01, value11, x_frac)
          value  = self._avg(value1,  value2, y_frac)
          # print "%4d %4d | %4d\n%4d %4d | %4d\n-------------\n%4d" % (
          #        value00, value10, value1, value01, value11, value2, value)
          return value
docs: Get the altitude of a lat lon pair, using the four neighbouring
              pixels for interpolation.