- The Earth is a spheroid, not flat, not a sphere.
- The platforms in our game do insane things like fly above the surface (more specifically above sea level) and sail below sea level (and if you get fancy some sail above sea level).
- Changing altitude uses or produces excess energy. Just something to think about.
In HC distances are calculated most often along Geodesic (which will stuff into the wrong but well known term Great Circle) routes that place the two platforms at an altitude of zero. When an aircraft at 40,000 feet is 150nm away from base the additional distance the 40,000 feet adds is negligible. However, when an SSM is at 100,000 feet and turns over for its dive to the target, 100k feet may well be a much greater distance than the HC distance (probably a distance of zero). Similarly ground troops 1nm away from each other at altitude of 0 is a very different environment than if one of those soldiers is at 15,000 feet and then other is at -100 feet.
A real thought provoker is the situation of a plane flying at 50,000 feet over thousands of miles, the difference between the zero altitude distance and the 50,000 feet distance is significant.
If your brain hasn't exploded, think about satellites
What do you expect from your Harpoon-style games in terms of distances? Should distances always be calculated as the actual number of meters (pick your scale) the platform would traverse? Should distances always be calculated like they are now, assuming height at sea level? Should there be crossover points where distance stops be calculated at sea level and starts being calculated as actual distance?
Most folks in GIS-land only need worry about the sea level distances, frankly they've got it easy!