- Q: How is a path specified?
- A: A path consists of two more morelocations,separated by dashes. For example,ORD-SFOspecifies a path fromORD (Chicago O'Hare International Airport)toSFO (San Francisco International Airport).
- Q: Can multiple paths be specified?
- A: Independent paths can be placed on separate lines or separatedby commas or semi-colons. A single path can consist of more than twolocations for multi-hop routings. For example,LAX-JFK,LAX-TUS-FTW-BNA-LGAcompares the contemporary non-stop flight from Los Angeles to New Yorkto the 1941 vintage route ofAmerican Airlines' Flight 004,the "Limited Mercury," which was flown using a Douglas DC-3.
- Q: Why do actual flight paths differ from what is shown on the maps?
- A: Eastbound flights often fly a path that's closer to the equator(more southerly in the northern hemisphere) than the great circle pathin order to hitch a ride on strong tailwinds. These tail winds resultin a shorter flying time for the longer ground path.
Various other factors may also dictate a flight path that is longerthan the great circle path. A track system is used across theNorth Atlantic to efficently and safely manage the larger number offlights, even though this may result in a less efficient path for agiven flight. There may also be a need to avoid certain areas,including areas too far from a suitable landing point(see the ETOPS section of the FAQ)or over unfriendly countries.
- Q: What does specifying the ground speed do?
- A: If a ground speed is specified the distance table willinclude a time computed from the distance and speed. This assumes theentire trip will occur at the selected speed, ignoring accelerationand deceleration.
- Q: How is Mach used to calculate trip time?
- A: A Mach number is a fraction of the speed of sound. The speed of soundvaries as a function of type of gas (dry air is different from humid air!),pressure, and temperature, so there isn't a straightforward conversionof Mach to distance/time.
The Great Circle Mapper assumes the speed of sound is 574knots(about 660.5miles per hour or 1063kilometers per hour),which is a close approximation for the lower stratospheric altitudesfrom 36,000feet to well above 65,000feet where transportjets usually fly, assuming a temperature of -70°F. More information isavailable from the following resources:
- Speed of Sound (NASA)
- Earth Atmosphere Model (NASA)
- Speed of Sound & Standard Atmosphere (Aerospaceweb.org)
- Q: Why are statute miles and miles/hour the default instead of nautical miles and knots?
- A: The Great Circle Mapper serves a diverse community of users. Some arepilots and would prefer nautical miles and knots. Others large groupsof users, frequent fliers being one example, are less likely tounderstand the distinction between nautical and statute miles and mightbe confused or annoyed if the default were different. Hence, the siteuses defaults which are least likely to cause confusion.
Perhaps a better answer would be if you could choose your own default.That capability will appear eventually.
- Q: What types of paths can be drawn?
- A: The default is to draw geodesic paths,commonly known as great circles. Alternatives includerhumb lines,Bézier curves,spline paths, andlongest paths.
- Q: What is a geodesic path?
- A: The default is to draw geodesic ("great circle") paths.A great circle path is the shortest path on the surface of a spherebetween two points on that sphere. Since Earth is not a true sphere,the proper term is a geodesic path though great circle iscommonly used.
- Q: What is a rhumb line?
- A: A rhumb line, also known as a loxodrome, is a path of constant heading.In almost all cases a rhumb line will be longer than a geodesic path,but it was far easier to navigate in a time when the primary tools werea magnetic compass, a sextant, and maybe a decent chronometer.
- Q: What is a Bézier curve?
- A: A Bézier curve is a parametric curve defined by its end points andone or more control points which tug the curve toward them.A good overview of these curves can be found atThe Birth of Bézier Curves & How It Shaped Graphic Design.
The Great Circle Mapper offers Bézier curves for creating network diagramsand similar "maps" where geodesic paths don't do a good job of illustration.Cubic Bézier curves are used, with two control points per path segment, defined relative to the end points of the segment. The user shouldconfigure these pointsto generate a suitable curve. No single configuration works for allcases so some experimentation may be necessary.
- Q: How are Bézier control points configured?
- A: The Map Conf tab of map pages includes a Bézier Configurationfield, which populates to PB parameter in a map URL. In its basic form,this option specifies a distance along the geodesic path (as a percentage)and an offset from the initial angle of the geodesic path. A simple example is
This says that the first control point should be 25% along the geodesic path,30 degrees to the left of the initial headingalong the geodesic path. The second control point is formed the same way,working backwards from the end of the geodesic path, except the sign ofthe offset is flipped, so in this example the second control point would be30 degrees to the right of the reversepath. Here's how they look for an SFO-ORD map:25%@-30°The % and ° can be omitted:
25@-30.Numbers are rounded to integers and the percentage is clamped to therange [-99% .. 99%].
A different configuration for the second control point can be specifiedif desired. This example specifies that the second point should be 45°to the right of the reverse geodesic path:
If there are multiple paths form the same origin, it may be desirableto adjust the angular offset from starting point for each path. Forexample,25@-30/25@45
places the control point for the first path 30° to the left of thegeodesic path, then 40° to the left for the second path, etc.This is illustrated in theFeatured Map for 24 March 2023.25@-30-10 - Q: What are good choices for Bézier configuration values?
- A: For the distance along the geodesic path 25% seems to be good,though for some cases 33% may be worth a try.
As for angles relative to the geodesic, both the magnitude and sign needto be considered. 30° or 45° seem like good starting value for the magnitudefor most cases though angles of 90º or more can be interesting for specialcases. Sign (negative is left, positive is right) depends on the path;there does not seem to be an easy rule.
- Q: What is a spline path?
- A: A spline path is a special case of aBézier curvefor paths with multiple segments. Bézier control points are calculatedto produce a smooth curve through intermediate points along the path.
- Q: What is a longest path?
- A: A longest path is like a geodesic pathexcept in reverse: the longest path starts 180 degrees opposite from thegeodesic path and continues the long way. It's not really the longestsince a zig-zag route with backtracks could be infinitely long but it'sthe longest route without a turn.
For paths that begin and end in the same location, there would be aninfiinite number of paths if Earth were a sphere. The Great Circle Mappermodels Earth as an oblate spheroid but for this corner case the longestpath is collapsed into a null path.
Back to main FAQ page
Information on this site may not be accurate or current and is not valid for flight planning or navigation. No warranty of fitness for any purpose is made or implied. Flight planning and navigation should only be done using official charts. Copyright©1996-2024Karl L. Swartz.All rights reserved. |    |
