For each higher terrain setting, OFP halves the terrain grid size (doubles the vertices). The engine interpolates the height of the additional data points, based on the cell they are in, adjacent cells, plus noise. I don't know of anybody working out what equation it uses though.
Cells with a slope less than a certain amount are always flat.
See the BI wiki:
http://community.bistudio.com/wiki/setTerrainGrid
This is from the "commented config" :
// Fractal and white noise random offset are added
// White noise generating more "wild" terrain
// Fractal tends to generate smooth, varied terrain
class Subdivision
{
// Fractal component of subdivision
// Changes are smaller for smaller rectangles
class Fractal
{
// Texture roughness factor
rougness = 10;
// Max. value for squares containing road
maxRoad = 0.2;
// Max. value for squares containing track
maxTrack = 1.0;
// Max. coefficient depending on slope
maxSlopeFactor = 0.05;
};
// White noise component of subdivision
// Change size is independent of rectangle size
class WhiteNoise
{
rougness = 5;
// Max. value for squares containing road
maxRoad = 0.1;
// Max. value for squares containing track
maxTrack = 0.5;
// Max. coeficient depending on slope
maxSlopeFactor = 0.025;
};
// Do not divide surfaces that are under given limit
minY = -0.0;
// Do not divide flat surfaces
minSlope = 0.02;
};
};
good luck : )