'Initializes a positioning error object.
@param pdop: The position dilution of precision. L{None} if unknown.
@type pdop: C{float} or L{None}
@param hdop: The horizontal dilution of precision. L{None} if unknown.
@type hdop: C{float} or L{None}
@param vdop: The vertical dilution of precision. L{None} if unknown.
@type vdop: C{float} or L{None}
@param testInvariant: Flag to test if the DOP invariant is valid or
not. If C{True}, the invariant (PDOP = (HDOP**2 + VDOP**2)*.5) is
checked at every mutation. By default, this is false, because the
vast majority of DOP-providing devices ignore this invariant.
@type testInvariant: c{bool}'

def __init__(self, pdop=None, hdop=None, vdop=None, testInvariant=False):

    self._pdop = pdop
    self._hdop = hdop
    self._vdop = vdop
    self._testInvariant = testInvariant
    self._testDilutionOfPositionInvariant()
