Map shifts can occur for a number or reasons such as incorrect ppos initialization of the IRS, spurious ground Navavid data, NavDB error, or excessive drift of the IRS (especially where navaid update not available). On modern aircraft, this problem is mostly illiminated if the aircraft is equipped with GPS and the FMS uses GPS as the primary position source (cross-checked against DME/VOR/IRS for reasonableness). The actual position blending can differ between aircraft types. The most important thing in all of this is that the EGPWS should ideally receive a position based on GPS which is by far the most accurate (since selective availablilty was turned off - thanks to Bill Clinton). In the event that the FMS position may become shifted from the actual position, it is essential that EGPWS has a GPS based position to ensure that the last line of defence against CFIT can provide adequate protection against FMS blended position errors.