Frequently Asked Questions (FAQ)


What does this web site do?
What else does it do?
What is a great circle path?


How is a path specified?
Can multiple paths be specified?
Why do actual flight paths differ from what is shown on the maps?
What does specifying the ground speed do?
How is Mach used to calculate trip time?
Why are statute miles and miles/hour the default instead of nautical miles and knots?


How are locations specified?
How do I enter a location by latitude and longitude?
Can locations be specified by city or airport name?
How can I find an airport's code?
Why do some airports have such goofy codes?
What are the differences between the ICAO, IATA, and FAA location codes?
What happens if the same code appears in several sources?
What if I want the FAA meaning of a code instead of the IATA meaning?
Why are IATA codes preferred over ICAO codes in many parts of the site?
What point is the location of an airport?
What does Part 139 certification for an airport mean?
What does "customs landing rights" mean?

Airport Database

Where did the airport data come from?
Why isn't data from IATA used?
What is DAFIF and what happened to it?
How much authoritative airport data is used?
Why isn't authoritative data from AIP Australia used?
Why isn't authoritative data from AIP China used?
Why isn't official data from other countries used?
What does AIP mean?
I found a problem in the data. Will you fix it?
Can I have a copy of the database?
How many locations are in the database?


What is ETOPS?
What is the humorous expansion of ETOPS?
What does rule-time mean?
How did someone come up with a 138 minute rule-time?
What is an aircraft's engine-out speed?
What are the sources for the single-engine speeds for the listed aircraft?
What is a "no-go" area?
Why are there "no-go" areas over land areas?
Why is Narsarsuaq not listed as an ETOPS alternate?
Why are so many alternates missing?

Map Options and Features

Where did the Blue Marble maps come from?
What is the difference between Blue Marble and Blue Marble: Next Generation?
How can I make different paths use different colors?
How many points/paths can be on a map?
What map projections are available?
Why use the rectangular (Plate Carrée) projection?
How does the "best" projection work?

Great Circle Calculations

How can I calculate great circle distances?
Does the Great Circle Mapper assume Earth is a sphere?
How does the Great Circle Mapper calculate distances and paths?
Why does the example in Ed Williams' Aviation Formulary calculate a different distance for LAX-JFK?

Using The Great Circle Mapper's Maps

May I use the Great Circle Mapper's maps on my web page?
May I use the Great Circle Mapper's maps in printed material?
May I use the Great Circle Mapper's maps on Wikipedia?
May I use the Great Circle Mapper's maps for commercial sites or publications?

Other Questions

What software does the Great Circle Mapper use?
Why not use Google Maps?
Can I get the source code for the Great Circle Mapper?
How can I buy advertising on the Great Circle Mapper?
What browsers does the Great Circle Mapper work with?
I found a bug in the Great Circle Mapper. How can I report it?
Does the Great Circle Mapper have a Facebook page?
Is the Great Circle Mapper on Twitter?
How can I make a donation to help keep the site going?

Copyright © 1996-2020 Karl L. Swartz. All rights reserved.
The Great Circle Mapper name and logo are trademarks of the Great Circle Mapper.
All other trademarks mentioned herein belong to their respective owners.
Please see credits for attibutions and further copyright information.

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