First: Where's your heightfield placed? Centered on the origin? Because,
without scalement or translation, it ranges from <0,0,0> to <1,1,1>.
Second: The script only covers the area from <-300,250,0> to <0,0,0>, not
more. If you want it to spread from <-300,250,0> to <300,0,0>, you should
modify this line:
#declare Start = x*(-300*rand(rd)) + z*(250*rand(rd));
to this:
#declare Start = x*(-300*(rand(rd)*2-1)) + z*(250*rand(rd));

That aside, random placement of course has some issues. Even distribution
with a little random noise might work better. But that's just my 2 cents.

Regards,
Tim

-- 
"Tim Nikias v2.0"
Homepage: <http://www.nolights.de>

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