First off alll!!! Model rendering time is longer then for terrain and second if you really want use this model putting on terrain you should take out "ignore_models" part then model really should land on this terrain model!

Heres code what im using:
///////////////////////////////////////////////////////////////////////////
function poligoni
{my.polygon = on;
}

function gravito
{
poligoni();
vec_set (temp, my.pos); // kopē modeļa vietu uz temp
temp.z -= 10000; // sagatavo temp.z 10000 zem spēlātāja oriģinālaš vietas
trace_mode = ignore_me + ignore_sprites + ignore_models + use_box; // ignorējam visu lai tikai tiktu uz zemes
my.z -= trace (my.pos, temp); //
}
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And else in my case if you want model put on terrain nicely model center need be at bottom of model otherwise it sticks in terrain so deep till model center gets reached!

And if you also wishes nice angle for some bushes put this under "my.z -= trace (my.pos, temp);" line:
/////////////////////////////////////////////////
vec_to_angle (my.pan, normal); // un sakārtot to tieši pareizi


P.S. If you have more questions please sure ask!