code: def getPixelValue(self, x, y):
          
          assert x < self.size, "x: %d<%d" % (x, self.size)
          assert y < self.size, "y: %d<%d" % (y, self.size)
          # Same as calcOffset, inlined for performance reasons
          offset = x + self.size * (self.size - y - 1)
          #print offset
          value = self.data[offset]
          if value == -32768:
              return -1 # -32768 is a special value for areas with no data
          return value
docs: Get the value of a pixel from the data, handling voids in the
              SRTM data.