Get the value of a pixel from the data, handling voids in the
            SRTM data.

def getPixelValue(self, x, y):
        """Get the value of a pixel from the data, handling voids in the
            SRTM data."""
        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