# Great Circle Map Generator

For those of you who are interested in radio listening, particularly over long distances (AKA DXing) with even a moderately directional antenna, having a great circle map projection for your location is a necessity.  Looking at a Mercator Projection, the normal “flat” map most are familiar with, one might come to the conclusion that due west from upstate NY lies the Washington/Oregon state border.  Appearances can be deceptive, bearing away at 270 degrees true (due west) from upstate NY is the California/Mexico border.

This is because we live in a big sphere.  In this regard, the only place the Mercator Projection is accurate is around the equator unless one is going due north (0 degrees) or due south (180 degrees).  The further north or south from the equator, the less accurate a flat map is.   Therefore, having a Great Circle map projection based on your location is handy for choosing the right azimuth to listen along.

As with many things, the internet provides the required tools to generate a great circle map for any location in the world.  The first thing needed is an accurate fix of your location.  This can be obtained via GPS, or, if you know how to look at a map and or satellite picture, itouchmap can be very useful.  Once you know where you are,  you can plug that information into this Great Circle Map Generator.

I saved the image as a bit map and used it as the wallpaper on my computer.  That way, I just need to minimize any running programs and I can see what the correct azimuth is to any place in the world.  This is for upstate NY:

## 6 thoughts on “Great Circle Map Generator”

1. J. Aegerter says:

Spherical Trigonometry isn’t taught much in high-school these days, and there are many who may not even be aware of it. When I built a Heathkit H-89 back in 1981, the Heath Users Group (HUG) offered a program called “Great Circle”. Of course those were the days of the CPM operating system, but the program did run on DOS. It was a very handy program way before the FCC thought of publishing their own internal Distance/Bearing program. By entering two sets of geographical coordinates in degrees, minutes, and seconds, the distance in Nautical miles and the Great Circle bearing in degrees between the two sites was obtained.

2. Paul Thurst says:

It’s amazing what they don’t teach in high school or college these days.

3. J. Aegerter says:

Amen Brother!

4. I have personally-written programs for calculating azimuth and GC distance between two points in BASIC, Freemat / Matlab, HP RPN (for my 48GX and 50g) and (I think) C. I’ll have to post them some time. The distance boils down to a one line calculation (three cosines and two sines), but the azimuth is a bit trickier.

5. Bob Winn says:

A friend of mine would like to play with a Visual Basic program to calculate great circle distance and bearing. many years ago I wrote a Basic program to do this, but it is long gone. Would you sharing your Basic program?

Thanks.

Bob

6. Paul Thurst says:

You will have to ask Gary or Mark (WM7D) as I only link to their content.