WGS 84 coordinates

28.10.2004

Submission:

Previous writers have concentrated on the conversion of coordinates from local to WGS84 as opposed to WGS84 survey. I, too, recommend the use of WGS84 survey rather than the conversion of local coordinates, which may be questionable and less accurate than that required. However this may not be possible if there are other constraints (financial, political, etc.). Regardless of how the coordinates are obtained the real test is in flight. As stated previously by other authors, GPS will accurately fly you to a wrong point, if that is in your system. We have had the situation where "licensed" surveyors produced "certified" WGS84 coordinates which, well, weren't. The proof is on the day when the flight check places you accurately (and repeatedly) off centreline all the way from straight initial or IWP to MAPt. Even from 15nm out it is often obvious that something is amiss. With more than 400 published approaches, we have had approximately 20 (5%) where the data did not reference the datum the provider guaranteed. The procedures have all been flown by day in VMC in "test" mode (manually entered coordinates which are then corroborated against flight plan tracks and distances, CDI scale set to 0.3 for the whole approach, and hand flown using pre-release procedure plates).

I suggest to all designers, that you include a cost to flight validate ALL procedures, as a QA check. It should include validation of RWY threshold coordinates, the flight path, and the segment critical obstacles/altitudes (design altitude - MOC). In an IFR capable light twin, a $500 investment may prove or disprove your design assumptions and safeguard the travelling public.


Ray Romano
Procedure Design Specialist - Australia