Camera Movement issue

hey guys. i have a camera movement c# script that follows the player and also has mouse orbit. however, the camera goes through the terrain and buildings and i want to fix that. here's the script:

 using UnityEngine;

using System.Collections;

public class CameraMovement : MonoBehaviour { public Transform target; public float walkDistance; public float runDistance; public float height; public float xSpeed = 250.0f; public float ySpeed = 120.0f; public float heightDamping = 2.0f; public float rotationDamping = 3.0f;

 private Transform _myTransform;
 private float _x;
 private float _y;
 private bool _camButtonDown = false;

 void Awake() {
     _myTransform = transform;    
 }
 
 // Use this for initialization
 void Start () {
     if(target == null)
         Debug.LogWarning("there is no target assigned to the camera");
     else {
         CameraSetUp();
     }


 }
 
 void Update() {
     if(Input.GetMouseButtonDown(1)) {   //Use the Input Manager to make this user selectable button
         _camButtonDown = true;
     }
     if(Input.GetMouseButtonUp(1)) {
         _camButtonDown = false;
     }
 }
 
 void LateUpdate() {

// _myTransform.position = new Vector3(target.position.x, target.position.y + height, target.position.z - walkDistance); // _myTransform.LookAt(target);
if(target != null) { if(_camButtonDown) {
_x += Input.GetAxis("Mouse X") xSpeed 0.02f; _y -= Input.GetAxis("Mouse Y") ySpeed 0.02f;

 //             y = ClampAngle(y, yMinLimit, yMaxLimit);
          
 
                
             Quaternion rotation = Quaternion.Euler(_y, _x, 0);
             Vector3 position = rotation * new Vector3(0.0f, 0.0f, -walkDistance) + target.position;
         
             _myTransform.rotation = rotation;
             _myTransform.position = position;
         }
         else {

// _myTransform.position = new Vector3(target.position.x, target.position.y + height, target.position.z - walkDistance); // _myTransform.LookAt(target); _x = 0; _y = 0;

             // Calculate the current rotation angles
             float wantedRotationAngle = target.eulerAngles.y;
             float wantedHeight = target.position.y + height;
     
             float currentRotationAngle = _myTransform.eulerAngles.y;
             float currentHeight = _myTransform.position.y;
 
             // Damp the rotation around the y-axis
             currentRotationAngle = Mathf.LerpAngle(currentRotationAngle, wantedRotationAngle, rotationDamping * Time.deltaTime);

             // Damp the height
             currentHeight = Mathf.Lerp(currentHeight, wantedHeight, heightDamping * Time.deltaTime);

             // Convert the angle into a rotation
             Quaternion currentRotation = Quaternion.Euler(0, currentRotationAngle, 0);
     
             // Set the position of the camera on the x-z plane to:
             // distance meters behind the target
             _myTransform.position = target.position;
             _myTransform.position -= currentRotation * Vector3.forward * walkDistance;

             // Set the height of the camera
             _myTransform.position = new Vector3(_myTransform.position.x, currentHeight, _myTransform.position.z);
 
             // Always look at the target
             _myTransform.LookAt (target);            
         }           
     
     }

 }
 
 public void CameraSetUp() {
     _myTransform.position = new Vector3(target.position.x, target.position.y + height, target.position.z - walkDistance);
     _myTransform.LookAt(target);    
 }

}

i know its a long script, but maybe someone can give some code in c# that keeps the camera from going through the terrain. thanks.