Anisotropic Filtering with BC1

I’m writing a ray caster for a huge terrain and I need anisotropic filtering for displaying the texture of the terrain. The textures are stored in in the blockcompressed format BC1. What do I have to do to get anisotropic filtering to work correctly with the block compressed texture format BC1?

I’m using the tex2DGrad-function and the output looks good on detailed textures but with not so detailed textures you clearly see that it uses point not anisotropic or linear interpolation.
For creating a textureobject using the blockcompressed format I need to create a mipmap-Array, which is four times smaller and set the data type to two unsigned int. From this it follows that CUDA doesn’t allow to change the filter mode to “cudaFilterModeLinear” or the readMode “cudaReadModeNormalizedFloat” what I need for the displaying the texture correctly. Is it somehow possible to change this later or is it just not possible at all?

Thanks for your help.