Interesting problem. The old Unreal Engine was based on OpenGL and we were posting about how to rip the graphics from that the other day. In this particular case that might well be not allowed or condoned... serious frown etc. I think the new one is direct X tho which is harder. Also with the newer programmable graphics you can decrypt the map inside the video card which would make it quite a difficult job. Prob not worth going down that avenue.
The way you would make a map in real life given that you don't have GPS or any satellites or planes to make the job easy would be to use accurate measurements of distance and angles. So the old sextant or theodolite is used.
The first accurate maps of the UK were made by taking a pair of 50ft rods of glass onto Hamstead Heath which is a park in London and measuring them accurately and measuring how many were needed to get from one fixed location to another. From that and then by taking subsequent angles to other notable points and using a bit of fairly basic maths they measured out the whole of the UK. It turns out that by the time they got from Londons Hamstead Heath to the top of Scotland they were about 6 feet out. Thats not bad for about a thousand miles of very crinkly land to cover.
So, your right in that pacing out some starting point is probably the best thing you can do. Then I think the best would be to find out what angle the edge of the screen is in relation to the center forward travel. Then use some maths to figure out the distances to far off places like mountains or similar features. Do those from multiple directions, at least 2 but the more the better, and then average them all out reducing the error. Then you can plot those points down and find the land about them.
Its hard work to make a map like that. I think its an excellent ploy by the game makers not to have a map. That map presumably would be worth real money after you make it tho cos it allows you to go to the best places and get the best stuff. You might get more in virtual gold than real money for a similar commission !
Darn good example of why knowing trigonometry and geometry is useful too and should give you a bit of practice with it.
Oh yeah, the stars. Well its not possible to know which way is North or South any more than which end of a magnet is North or South but it should be possible to figure out a map based on the stars which some might draw upside down to others. What you need to do is figure out if this planet is round and if so then is it spinning. All assuming its somewhat earth like then you need to sit in one spot at night and take regular screen shots and look for the bit of the night sky that is stationary and all the other bits of the sky like the stars revolve around it. That will be one polar axis. The angle from a flat horizon like the ocean to that star is the tilt of the planet and the latitude. If you walk toward that point in the sky your heading polar and perpendicular is the planetary spin rotation direction - the Easts and Wests. If you assume that the sun rises in the East then you can figure out which way is North. Of course the planet may spin the other way and its actually South from an Earth like perspective.
If you have a shortest day and a longest day then the difference gives you, in a roundabout manner, the tilt of the planet. If you know the tilt then you know your latitude so you can now assign that. On Earth the Astro dudes at Greenwich arbitrarily decided that they were at 0 longitude and scaled off the rest of the planet accordingly. So you can do the same, get your lat and assign your long coord.
If you have some online or real friends in the world then you might also be able to put a known object in several places at once and see if you get a shadow. If that shadow is different lengths then that might give some clue as to the size of the planet. Whilst you have instant communications, in the old days people had to synchronize clocks to do that. Thats why ships clocks were so expensive and also why they used the moons of jupiter which could be seen from far different places on the planet at the same time.
Anyway, im going off on one a bit. Post how you get on.