I am playing around with spheric levels instead of flat ones, and am having a bit of trouble with player movement around the sphere. I have an almost working code, but it gets strange results at the poles (top & bottom).

Placing the player model on the sphere's surface is pretty straightforward. For a sphere with a radius of 500 for example I do the following:
place_on_surface(My_Planet, My_Vehicle, 500);
Code:
void place_on_surface(ENTITY* Planet, ENTITY* Vehicle, var planet_radius)
{
   VECTOR relative_pos;
   var desired_distance;
   
   //get vehicle position relative to the planet center
	vec_diff(relative_pos.x,Vehicle.x,Planet.x);
	
	//set desired distance from radius
	desired_distance=abs(Vehicle.min_z)+planet_radius;
	vec_normalize(relative_pos,desired_distance);
	vec_add(relative_pos,Planet.x);
	
	//place the vehicle on the planet surface
	vec_set(vehicle.x,relative_pos);
}



Now here comes the tricky part I am having trouble with, the player's rotations. So far this attempt is pretty close to what I want. but I get very strange results when the player reaches the top or bottom of the sphere:
rotate_on_surface(My_Planet,My_Vehicle,my_pan);
Code:
void rotate_on_surface(ENTITY* Planet, ENTITY* Vehicle, var desired_pan)
{
	VECTOR relative_pos;
	ANGLE relative_ang;
	ANGLE result_ang;
	
	//get vehicle position relative to the planet center
	vec_diff(relative_pos.x,Vehicle.x,Planet.x);
	
	//tranform relative position to angle
	vec_to_angle(relative_ang.pan,relative_pos.x);
	
	//build new angle
	vec_set(result_ang,vector(0,0,90));
	ang_rotate(result_ang,vector(0,-relative_ang.pan-90,0));
	ang_rotate(result_ang,vector(0,0,-relative_ang.tilt));
	ang_rotate(result_ang,vector(desired_pan,0,0));
	
	//rotate vehicle
	vec_set(vehicle.pan,result_ang);
}


I keep the desired absolute orientation (pan I want the vehicle to look at) in a separate variable so it dosn't get messsed up during the calculations.

The resuts are "almost" what I need, but even separating the pan like I do does not make my vehicle cross the planet surface in a straight line unless it is going straight across the equator... The farther it is from the middle (equator) the more it turns towards the poles (north/south), kind of like traveling in circles orbiting the poles and turning more and more towards them! And when walking directly on the north/south poles it just goes crazy! (it does stay aligned to the planet, but the pan just freaks out)
Movement Code:
Code:
while(1)
{
   c_move(My_Vehicle,vector(key_w-key_s,0,0),nullvector,IGNORE_MODELS);
   my_pan=cycle(my_pan+key_a-key_d,0,360);
   rotate_on_surface(My_Planet,My_Vehicle,my_pan);
   place_on_surface(My_Planet, My_Vehicle, 500);
   wait(1);
}


Has anyone tried something similar before?
Can you spot what I am doing wrong here?
Any advice would be great.

EDIT:
Uploaded an example video to youtube:
http://www.youtube.com/watch?v=KjgIwm75e8U
(keep in mind this is just placeholder art to test the code)

EDIT2:
Added a second video with a free camera further away so its easier to see whats going on with the vehicle withought getting dizzy:
http://www.youtube.com/watch?v=NXkbrUEMpfQ

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