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Greetings Mission Planners,

For those of you fascinated by GeoInt (and who isn't?) it's time to start thinking about GeoInt2004 in New Orleans.  For those of you  fascinated by New Orleans it's time to start pretending your fascinated by GeoInt.  GeoInt2004 runs from October 12th thru 14th, and  includes keynote speakers like LtGen James R. Clapper, Jr., USAF (Ret.), Director, National Geospatial-Intelligence Agency, Representative Peter Hoekstra and former Senator (and current member of The National Commission on Terrorist Attacks Upon the United States) Robert Kerrey.  For more details see the Press Release

An error has been found in the July 2004 AMC Valid DZ Database affecting Phillips DZ in Arizona  .  Files downloaded after 2 July 2004 have the correct information.  You can download the Valid DZ file from AMC's ZAR website at https://www.amc.af.mil/do/dok/zar.htm

Mission Planning Tip: Geographic Line Styles Part 2

Last week I discussed Great Circle lines, Rhumb lines and their differences.  Today we'll get into specific examples of FalconView's depiction of Airspace Boundaries and Airways, and TaskView's depiction of the ACO on the FalconView map.

Airspace Boundaries: DAFIF defines boundaries using Great Circle lines, Rhumb lines and Left/Right hand arcs.  Unfortunately the publications that define the boundaries don't always make it clear what line style the Host Nation wants used.  In the case of North/South lines it doesn't matter because both line styles follow an identical track.  In the case of East/West lines it's clear that the boundary's intended to follow an even degree of Latitude - so a Rhumb line is required.  This still leaves a lot of cases where it isn't clear what line style should be used and it's where things get interesting...

Today's example is the border between the Edmonton FIR and the Sondrestrom FIR at the NADMA Waypoint.  NADMA is about 1,000NM north of Goose Bay and about 250NM south of Thule, i.e. it's up there.

 

As a pilot, the first thing that stands out is the line of Waypoints (BOPUT, ADSAM, NADMA and MEDPA) that are offset from the FIR boundary.  Waypoints are placed on FIR boundaries so ATC can swap you to the next radio frequency as you leave their airspace so why have a line of reporting Waypoints that are off the line?  Perhaps it's not surprising that when you plot a Great Circle line between the DAFIF corner points it perfectly matches the line of Waypoints:

How far away is the DAFIF line from the line of Waypoints? 

The JNC chart of the region depicts the FIR boundary following the Great Circle path and in case there's any doubt, the Waypoint information for NADMA makes it clear that the point is in the Edmonton FIR, not 27 NM inside Sondrestrom's:

So does this really matter?  In the case of NADMA it doesn't matter much.  Aircraft on an IFR clearance follow the route of flight they were given by ATC.  A FIR boundary is a "line on the map" and a reminder it's going to be time to change frequencies, but they also let you know who you should be trying to contact in an emergency and who's in charge of the airspace.  The line's precise location would matter a lot more if you were flying VFR and  maintaining a track next to (but not across) a "line".  If you believed you were on one side of a line and the other person thought you were on their side then mayhem (and potentially an international incident) could follow. 

Most examples aren't nearly as extreme as this one.  Several factors exacerbate the difference between Great Circle and Rhumb lines:

  • Extreme Latitude - The further N/S you go the more the Great Circle "swings"
  • Long Distance - Short lines don't have enough room to get separated from each other
  • E/W Component - The more E/W a line tracks the more a Great Circle path will diverge

Of course I wouldn't pick an example where the lines were two tenths of a mile apart would I?

Airways: IFR Airways have been in FalconView for many years and weren't revised when Great Circle and Rhumb lines were added in PFPS 3.2.  Instead of being depicted by a geographically accurate line, Airways are drawn as a straight line (on screen or on a printout) between the two route Waypoints.  Today's Airways example is J20 between Rock Springs and Pocatello:

 As you zoom in on the midpoint of the route you'll see the Great Circle line (the one your aircraft is likely to fly) diverges from the Airways line on the FalconView screen in Equal Arc Projection:

Although you can't see it, the Rhumb line between the two Navaids would fall roughly along the J20 line since a Rhumb line is straight on an Equal Arc map.  When you switch to Lambert Conformal projection the Great Circle line is straight and the Rhumb line remains where the Airway was previously depicted:

 

By now you may remember that the path between two Navaids on an airway is neither a Great Circle nor a single Rhumb line.  What you're supposed to do is fly along a radial from the outbound Navaid then switch at the crossover point to follow a second radial to the inbound Navaid.  What does that look like?

In the image above the green Great Circle line depicts the route you'd likely plan in PFPS, load into your aircraft and fly.  The two red lines depict the Navaid radials (Rhumb lines) that define the Airway.  The deviation is likely distressing the FAIP's out there, but it won't wake up ATC.   If you flew along the Radials you'd see contrails diverging as you approached the crossover point, then converging as you approach Pocatello. 

TaskView ACO's: As with FalconView's Airways, TaskView hasn't been updated to the newer line styles.  All TaskView lines are simple point to point depictions on screen and on printouts.  The images below show a segment of a TaskView line (in gray from the ACO sample installed with TaskView) along with the Great Circle and Rhumb lines.   You'll see that in Equal Arc the TaskView and Rhumb line nearly overlap.  When you switch to Lambert Conformal the TaskView line "jumps" to the Great Circle line - moving over a quarter of a mile:

I hope you can see that Great Circle, Rhumb and Simple line styles make a real difference.  If you find yourself working on an aircraft or C4 modernization program be sure the contractors and managers understand what this stuff means!  If you're not careful you'll end up with a system that depicts the information you need, but doesn't do it correctly.  (it's happened before)

Paul