'Tests if this positioning error object satisfies the dilution of
position invariant (PDOP = (HDOP**2 + VDOP**2)*.5), unless the
C{self._testInvariant} instance variable is C{False}.
@return: L{None} if the invariant was not satisfied or not tested.
@raises ValueError: Raised if the invariant was tested but not
satisfied.'

def _testDilutionOfPositionInvariant(self):

    if (not self._testInvariant):
        return
    for x in (self.pdop, self.hdop, self.vdop):
        if (x is None):
            return
    delta = abs((self.pdop - (((self.hdop ** 2) + (self.vdop ** 2)) ** 0.5)))
    if (delta > self._ALLOWABLE_TRESHOLD):
        raise ValueError(('invalid    combination    of    dilutions    of    precision:    position:    %s,    horizontal:    %s,    vertical:    %s' % (self.pdop, self.hdop, self.vdop)))
