Only Spawn tree on flat ground - performance

Question

What I'm trying to do
I'm trying to spawn a tree only on flat ground but GetSteepness() isn't working and GetInterpolatedNormal() also doesn't seem to be working
The Problem
the trees always seem to spawn on places I don't want them to like on really steep hills they do also spawn on flat ground but i would rather them not spawn on hills at all
I would also like to eventually
use this method whenever it gets solved to place grass only on flat grounds
but I don't know how to spawn grass and I don't think instantiate will be good for performance I would be using billboard grass for performance reasons
What I've tried
GetInterpolatedNormal system doesn't work FYI I've tried playing around with Value.x/y/z and switching the Steepness.x/y/z > Value.x/y/z to Steepness.x/y/z < Value.x/y/z
void PopulateTrees(TerrainData terrainData) {
var ray = new Ray(new Vector3(Random.Range(-100, 100), 1000 f, Random.Range(-100, 100)), -this.transform.up);
RaycastHit hit;
if (Physics.Raycast(ray, out hit)) {
Vector3 terrainSize = Terrain.activeTerrain.terrainData.size;
Steepness = Terrain.activeTerrain.terrainData.GetInterpolatedNormal(hit.point.x / terrainSize.x, hit.point.y / terrainSize.z);
if (Steepness.x < Value.x && Steepness.y < Value.y && Steepness.z < Value.z) {
Instantiate(PineTree, new Vector3(hit.point.x, hit.point.y - .1 f, hit.point.z), transform.rotation = new Quaternion(0, Random.Range(-1000, 1000), 0, 100));
}
}
}
GetSteepness() method I don't know what system will be more performant this way seemed to only put trees in weird areas
void PopulateTrees(TerrainData terrainData) {
var ray = new Ray(new Vector3(Random.Range(-100, 100), 1000 f, Random.Range(-100, 100)), -this.transform.up);
RaycastHit hit;
if (Physics.Raycast(ray, out hit)) {
Steepness = terrainData.GetSteepness(hit.point.x, hit.point.z);
if (Steepness > Value) {
Instantiate(PineTree, new Vector3(hit.point.x, hit.point.y - .1f, hit.point.z), transform.rotation = new Quaternion(0, Random.Range(-1000, 1000), 0, 100));
}
}
}
This is a photo of the problem

Answer 1

Try this:
void PopulateTrees(TerrainData terrainData) {
var ray = new Ray(new Vector3(Random.Range(-100, 100), 1000 f, Random.Range(-100, 100)), -this.transform.up);
RaycastHit hit;
if (Physics.Raycast(ray, out hit)) {
Steepness = terrainData.GetSteepness(hit.point.x / terrainData.size.x, hit.point.z / terrainData.size.z);
if (Steepness > Value) {
Instantiate(PineTree, new Vector3(hit.point.x, hit.point.y - .1f, hit.point.z), transform.rotation = new Quaternion(0, Random.Range(-1000, 1000), 0, 100));
}
}
}
GetSteepness() should return a value in the range of 0...90