losing Heightmap resolution when slicing terrain :?!

hey how do I create a large terrain without losing heightmap resolution. i have a terrain That is 20,000 X 20,000 large, I have split it up to 32x32 which is (1024 piece terrains) and the heightmap resolution is 4059. but when i have split up the terrain then on each individual terrains then the heightmap resolution is at 129 and I cannot change it because if i do then the terrain is going completely flat. I have also tried to take my heightmap picture and dividing it up to 1024 pictures instead and resize each piece of the image so I can get the heightmap resolution 2049 on the terrain, but then it just looks like minecraft on the terrain and the pictures is too pixelated and thats why the terrain is like that (minecraft Looks). and i can't smooth the image on PH i have tried. So what is the solution on this? i have my hight map picture that i want to use on the whole map, and i have all my chunks that i need but i need to get all chunks in to heightmap resolution 2049. :/ ThX Image may be NSFW.
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