When you want to calculate distance on a sphere or sphere-like shapes, there are three good options. The Cosine Law method is the easiest and can be done by hand. The Haversine formula isn’t too much rougher, but unless you really like math, you’re probably not going there. These two methods give remarkably similar results, and they’re great on a perfect sphere! So if you’re measuring things on a sphere, you’re done reading. No really, go. It’s ok. *Sniff*
But if you’re like me, you’re most likely dealing with these distance calculations because some awesome designer decided that you should be able to measure distances between two points on Earth. That’d be awesome! Of course, despite what all your globes teach you, the Earth isn’t a sphere. It’s squished. It’s called an oblique spheroid. Sounds dirty to me.
So if you want to measure distance on an oblique spheroid you need a more accurate formula. Please allow me to introduce you to your new friend, Vincenty’s Formula! Now, unless you are really, really (and I mean really) into math, this thing is going to suck. Luckily for you, I converted it into an AS3 class. See how nice I can be?
Have some lat/long coordinates you need to measure? Have a project where you can use this sort of info? Take and use. Let me know if you love me. I can use some love.
Source & Example