I'd downloaded some free terrain props by 'PropSpinner' from 'BadKing'. That site now broken, but files are freely available else-where. I've broadband, so took me a while to notice the progress bar wasn't slow due their server, these were really, really big files...
Un-zipped, I found nice texture maps and...

Terrain Alien Glacier   OBJ 26.8 MB
Terrain Ring of Doom OBJ 26.8 MB (Looks like Meteor Crater...)
Terrain Rock Wall       OBJ 6 LODs, 141 KB to 184 MB ( Yes, a Hundred and Eighty Four MB !!)
Terrain The Mesa       OBJ 26.9 MB (Just a big corner, with river valley far, far beneath...)
Wooden Pier              OBJ 2.25 MB ( HiPoly ZTL ~200 MB) + 8k diff / normal maps

Ouch.

Okay, the middle pair of Rock Wall LODs were 2.50 MB and 10.3 MB OBJs, so usable as-is. I used free IrfanView to re-size pier's mega-maps to 1k JPGs.
Beyond that, I was stuck. Poser would not load the Glacier, Ring or Mesa. And, except for the classic PropViewer 3.2 by R. Kawecki (2011 !!), my other apps just 'hung' or 'fell over'...
D'uh...

Some months later, while looking for an UV-Unwrapper that was 'comprehensible', I followed a link to 'Atangeo'. Google / Bing will find them.

They'd a utility called 'Balancer' which briskly shrinks mega-poly OBJ files to LOD of your choice. There are neat 'tweak' tools I've not tried as I don't need game-grade 'Super-Lo-Poly' or eg delicate facial details preserved. Basically, you drag in OBJ, move slider until shows eg 25% of original complexity, save out the now~10 MB Mesa under modified name...

Note this is NOT a free product, hence lack of link. It requires purchase and registration. They currently offer a discounted 'Non-Commercial' license (~$30) for my budget version. YMMV...

I've just used it to salvage those three mega-poly terrains, and I hope to use it on a bunch of other models. There's several 'game' terrains / maps I'd sorta-salvaged, by using their tweaked 'albedo' maps as bump/displacement maps for ground-plane. This tool should let me wrangle their original, impossibly high-poly meshes, plus a raft of other terrain / forest models...

You could try free beta of eposer. It has a poly reducer, thou not very nice to use. I got it from sharecg or RR freestuff. I don't remember, but one of those. A sleek, simple poly squasher, especially one that preserves the mapping, for free would be nice. Can't be too complicated and might even be possible in Pyton, but not for me that is. I'm no coder

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