Relevant wiki information:
"math.noise[edit]
math.noise (x [, y [, z]])
The math.noise function can be called with up to three argument and returns a value between -1 and 1. If you leave arguments out, they will be interpreted as zero, so math.noise(1.158) is equivalent to math.noise(1.158, 0, 0) and math.noise(1.158, 5.723) is equivalent to math.noise(1.158, 5.723, 0).
The function uses a perlin noise algorithm to assign fixed values to coordinates. For example, math.noise(1.158, 5.723) will always return 0.48397532105446 and math.noise(1.158, 6) will always return 0.15315161645412."
As stated, the function returns fixed values and fluctuates between -1 and 1. It will give you a continuous and smooth height map that you can define however you would like for whatever region of space you want. We can't decide how to determine that for you. The easiest method is probably just doing what you stated which is making it so that certain materials are used at certain heights. |