RaiseTerrain.cs

// RaiseTerrain.cs -- a simple script that raises (or lowers) Unity's built in terrain to be level
//                    with a GameObject's collider(s).
//
//   AUTHOR:  Martin "quill18" Glaude -- [email protected]
//   LICENSE:  CC0
//
//   The latest version will always be available at: https://gist.github.com/quill18/9becf56f685e60109551e034385fd8f9
//

using UnityEngine;
using System.Collections;

public class RaiseTerrain : MonoBehaviour
{

    // Use this for initialization
    void Start()
    {
        UpdateTerrain();    // NOTE: Start() may or may not be where you want this to happen.
    }

    public Terrain Terrain;

    [ContextMenu("Update Terrain")]    // In case you want to be able to update the terrain in the editor without running the game
    public void UpdateTerrain()
    {
        if(Terrain == null)
        {
            Debug.LogError("Please link TheTerrain to the appropriate terrain object.");
            return;
        }


        // Get all colliders on our object (this includes the current GameObject as well)
        Collider[] cols = GetComponentsInChildren<Collider>();

        if (cols.Length == 0)
        {
            Debug.LogError("No colliders on this object.");
            return;
        }

        // We need to determine our total bounding box
        Bounds bounds = new Bounds(cols[0].bounds.center, cols[0].bounds.size);
        for (int i = 1; i < cols.Length; i++)
        {
            bounds.Encapsulate(cols[i].bounds);
        }

        // The "near/left" corner of our object's area.
        Vector3 worldPos = bounds.min;

        Vector3 terrainLocalPos = worldPos - Terrain.transform.position;

        // Normalize the position into terrain "sample units"
        int indexX = (int)(terrainLocalPos.x / Terrain.terrainData.heightmapScale.x);
        int indexY = (int)(terrainLocalPos.z / Terrain.terrainData.heightmapScale.z);
        float newHeight = terrainLocalPos.y / Terrain.terrainData.heightmapScale.y;

        // The size of the area in number of samples
        int areaX = Mathf.CeilToInt(Mathf.Ceil((bounds.max.x) / Terrain.terrainData.heightmapScale.x) - indexX) + 1;
        int areaY = Mathf.CeilToInt(Mathf.Ceil((bounds.max.z) / Terrain.terrainData.heightmapScale.z) - indexY) + 1;

        float[,] heights = Terrain.terrainData.GetHeights((int)indexX, (int)indexY, areaX, areaY);

        for (int x = 0; x < areaX; x++)
        {
            for (int y = 0; y < areaY; y++)
            {
                heights[y, x] = newHeight;    // Why does x and y have to be inverted here?
            }
        }

        Terrain.terrainData.SetHeights((int)indexX, (int)indexY, heights);
    }

}