VECTOR player_dist is for move the player along the x,y and z axis
and distance to the ground is
c_trace(vec1_from, vec1_to, IGNORE_ME|IGNORE_PASSENTS|IGNORE_PASSABLE|IGNORE_SPRITES|USE_BOX);

vec1 means
vec_set(vec1_from,my.x);
vec_set(vec1_to,my.x);
vec1_to.z -= 500;