hi friends

I use VS 2008 (C# & managed DX) for my 3d app, I'm rendering large terrains that are made of separated tiles (for instance a terrain can have up to 10x10 tiles), each of which has a large texture (sometimes up to 5000x5000) so rendering all tiles with high resolution textures is not ideal, it can take a huge amount of memory and can lead to a very bad performance, so I should manage my tiles based on camera angle & distance toward each tile (I'm making something like NASA world wind or Google earth, a 3d representation of earth).
I'm going to perform a mipmaping so I should now which tile is out of my frustum and which one is inside, then I can use low resolution textures for out-of-frustum tiles and high-res textures for inside-frustum tiles.

can you help me please? is there any sample? any easy way? any tutorial

thanks in advance