Talk:Under The Hood

For saved game file discussion, NKF's analysis of LOC.DAT "20 binary rows, 50 entries" contains Geoscape tokens

• Nothing = 0
• UFO = 1
• X-Com Craft = 2
• X-Com Base = 3
• Alien Base = 4
• Waypoint = 7

--JellyfishGreen 06:05, 23 Aug 2005 (PDT)

JFG or anybody, what's the Great Circle Route?

enquiring minds want to know!

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The best way to explain great circles is to first think of the old saying: "the shortest distance between two points is a straight line". For example, the shortest distance between the points (0,0) and (4,4) lies directly on the line y=x. To find distances in planer geometry it is as simple as finding a straight line which goes through those two points. But we have to remember that this is only true for Euclidean (or planer) geometry.

The Earth, however, is not a plane. It is a sphere. Spherical geometry is not the same as Euclidean geometry. A point-to-point distance is simply a line segment on a Euclidean graph, but when translated into spherical coordinates it is now an arc. (Case in point: the Euclidean distance formula is rather easy, whereas the spherical distance formula requires sines, cosines and the arctangent to compute). In the same manner as extending a line segment creates a line, extending an arc creates a circle. Navigational buffs out there call this circle: "the great circle". Catch is, in order for the circle to be considered "great" it must bisect the globe into two identical hemispheres.

If you ever see what the flight plan for transconinental airlines flying between London and New York is, it is an arc extending almost up to the Arctic Circle and then back down. That distance is shorter than flying on a direct line of latitude. Hope this helps. --Zombie 10:31, 5 Nov 2005 (PST)

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Thanks Z! Now I see. I gather that XCOM doesn't do it properly? And we should re-route with "waypoints" to save a little time?

---MikeTheRed 05:41, 7 Nov 2005 (PST)

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Well, I fooled around with this a bit today and for the most part X-COM does a pretty good job of finding the shortest distance. From all the tests I ran from the northern hemisphere to the southern hemisphere, the game always beat my time by a few minutes. Heck, I even went as far as calculating the anticipated "great circle" and following it as close as I could. Result: the game still put up better times than I did (though, not by much). I'm thinking that the problem may arise when the two points are near the equator or the poles, since those are by definition great circles. Gimme a chance to look at this closer.

--Zombie 21:36, 7 Nov 2005 (PST)

Suggestion

I'm not familiar with the great circle route myself, but you could try this experiment:

Set up a base in, say, Berlin. Station a Lightning there.

Now, plot to waypoint in Vancouver. See how far the Lightning gets.

Refill, rearm then make another trip to Vancouver, only this time plot a course through the polar cap, using short waypoints so that the Lightning doesn't default to moving around the curve of the globe.

You should, in theory, make it to Vancouver with a bit more fuel than when you set the one waypoint and let your ship do all the flying.

- NKF

Arctic Waypoints

It pretty much applies to manually routing your craft to the North Pole first, when going from North America to Europe/Russia/Siberia. The game's default route between any waypoints is to proceed north or south until it can follow a line of latitude, which takes longer than a pure diagonal (or great circle). Manually crossing the North Pole leaves out the latitude-following delay. Australia to Africa over the South Pole would be another application.