mouse clicking on the plane and drag to other position

Hello, this question is quite similar to the existed questions we can serach, I have searched for many related answers, but I am still stuck.

This question is something different from others.

I have a fixed plane, plane will never change its position,it look like a map. Also there is a camera in the top view, i want to click on the plane and drag to other position. In this drag process, i want the point i click on is follow the mouse move.

I want only the camera to move, not the plane.

ps: now, i already know how to get the coordinate of point which i touch on the screen by using following code

Vector3 startpos=new Vector3();
  if (Input.GetMouseButtonDown(0)){ // if click...
    // create a logical plane perpendicular to Y and at Y=0:
    Plane horPlane = new Plane(Vector3.up,;
    // get the ray from the camera...
 Ray ray =Camera.main.ScreenPointToRay(Input.mousePosition);
    float distance1 =0;
    // if the ray intersects the logical plane...
    if (horPlane.Raycast(ray, out distance1)){
      cube.transform.position = ray.GetPoint(distance1);
      print("cube position="+cube.transform.position);

the effect i want is look like a map.
Thanks in advance!!

Using a mathematical plane is a good way to go if the camera is at an angle with respect to the plane. But it reading between the lines, I get the impression that you have a map on the XZ plane at the origin of ‘Y’ with a camera looking directly down. With that, you can simplify things a bit and use Camera.ScreenToWorldPoint(). Example:

using UnityEngine;
using System.Collections;

public class DragMap : MonoBehaviour {
	private float dist;
	private Vector3 v3OrgMouse;
	void Start () {
		dist = transform.position.y;  // Distance camera is above map
	void Update () {
		if (Input.GetMouseButtonDown (0)) {
			v3OrgMouse = new Vector3(Input.mousePosition.x, Input.mousePosition.y, dist);
			v3OrgMouse = Camera.main.ScreenToWorldPoint (v3OrgMouse);
			v3OrgMouse.y = transform.position.y;
		else if (Input.GetMouseButton (0)) {
			var v3Pos = new Vector3(Input.mousePosition.x, Input.mousePosition.y, dist);
			v3Pos = Camera.main.ScreenToWorldPoint (v3Pos);
			v3Pos.y = transform.position.y;
			transform.position -= (v3Pos - v3OrgMouse);