Geomipmapping
Treats a terrain's height field like a mipmapped texture: fixed tiles, each with several precomputed levels of detail chosen by distance.
Willem de Boer described the method in 2000. The terrain is cut into square blocks, and each block gets a handful of index buffers — one per detail level, where coarser levels skip every other vertex just as a texture mipmap does. Distance, or a screen-space error metric, decides which level a block draws with.
The snag appears at the seam between two blocks on different levels, where the edges no longer line up and a hole gapes. It is fixed with "skirts" — a vertical strip hung along each block's edge — or by stitching the border. The technique was chosen over ROAM because it feeds the GPU whole, static buffers rather than fiddling one triangle at a time.