The FS coordinate system is quite in tune with most of its other software
features, i.e totally weird.
The origo of the system can be found in the centre of the (then) SubLogic
office building in Champaign, IL. The x-axis runs NS, the y-axis EW.
One grid units equals 1/10 of a US mile (628 ft, I think) and remains the same
throughout the grid system.
The grid is overlaid on the official US ICAO sectionals, which use a Lambeth
conformal polyconic projection. That projection always renders meridians as
straight lines, meaning that one sectional can be joined to another along the
edge meridian, _provided_ the projection parallells remain the same - which
they do for Continental USA, as far as I can recall.
If you change the projection parallells, you will have to recompute the
projection plane, which is no trivial problem - you cannot do it with a pencil
on the back of an envelope in 15 minutes.
And the use of a (poly)conic projection is what causes some problems if you
want to cross the equator.
In practice, you place the scenery where you find it most convenient. The
computer believes, for example, that SD-EUR scernery actually represents the
nortern parts of Quebec and the Hudson Bay region. The (outstanding) Gemini
scenery of the British Isles use the British National Grid and places its origo
(which in real life is off Land's End) some 400 kms to the west to avoid
People get confused by SubLogic grid units. However, the grid system - by
relying on maps using the Lambeth project - does render the shape of the land
very accurately; the Gemini scenery achieves an average point placement error
of less than 30 meters (abt. 100 ft).
If you turn to FS5.x and later, it may seem very accurate, by using fractions
of arc seconds. However, the system does not recognise meridian convergence -
which means that in Scandinavia, where I live, what in real life is a perfect
square on the ground in Stockholm, appears in the simulator as a 2:1 east-west
rectangle... it is inherently extremely difficult to produce high-resolution/
high-definition scenery at high latitudes for FS 5.x/95/98.