I did something similar a few years back with that 22gb displacement map, although much more accurately and easily: it's a piece of cake to directly ray trace a

I did something similar a few years back with that 22gb displacement map, although much more accurately and easily: it's a piece of cake to directly ray trace a displaced sphere: you basically intersect two spheres (the outer and inner radius), which gives you a ray interval that you can "march" through. At each step while marching, you look at the surface height at the current point, compare it to your point's radius, and if it's greater then you've intersected the surface since the last step (and can perform bisection method to refine the intersection distance to machine precision).