Waypoints, Fixpoints and Turning Points
When entering Waypoints into a piece of accurate GPS equipment there are two kinds of Waypoints entered.
First are turning points. These are nearly always taken from a LFC, measured with a protractor and entered with an accuracy of about 0.5nm.
The slight errors entered are not critical, because they are simply tools to help find your way around a route in VMC. You often have no intention of overflying these points just acquiring and then turning.
Second are precision entries of points that it is critical for you to make.
These can include DZ's, LZ's, Targets, Runway Threshholds, Final approach fixes anything that is critical that you have to approach precisely.
You can get the accurate positions from a variety of sources including TAP,s, DZ and LZ handbooks. For a one off you might send a man with a GPS, ahead of your mission.
As a last resort you will again plot out from a chart, but this time you will use an Ordnance Survey and be looking for errors of a few metres.
Waypoint A N5518.50 W00548.00 looks like a turning point, the numbers are too regular. It is close enough to the lighthouse to allow that to be the planned TP.
Even if those co- ordinates are the precise location for an LZ, there is another problem with theory. In most conditions, especially poor weather, the crew would have pre planned their approach and you would see another Waypoint between Belfast and the Mull