RaycastHit.distance

Description

The distance from the ray's origin to the impact point.

	// Movable, levitating object.
	
	// This works by measuring the distance to ground with a
	// raycast then applying a force that decreases as the object
	// reaches the desired levitation height.
	
	// Vary the parameters below to
	// get different control effects. For example, reducing the
	// hover damping will tend to make the object bounce if it
	// passes over an object underneath.
	
	// Forward movement force.
	var moveForce = 1.0;
	
	// Torque for left/right rotation.
	var rotateTorque = 1.0;
	
	// Desired hovering height.
	var hoverHeight = 4.0;
	
	// The force applied per unit of distance below the desired height.
	var hoverForce = 5.0;
	
	// The amount that the lifting force is reduced per unit of upward speed.
	// This damping tends to stop the object from bouncing after passing over
	// something.
	var hoverDamp = 0.5;
	
	// Rigidbody component.
	var rb: Rigidbody;
	
	function Start () {
		rb = GetComponent.<Rigidbody>();
		
		// Fairly high drag makes the object easier to control.
		rb.drag = 0.5;
		rb.angularDrag = 0.5;
	}
	
	
	function FixedUpdate () {
		// Push/turn the object based on arrow key input.
		rb.AddForce(Input.GetAxis("Vertical") * moveForce * transform.forward);
		rb.AddTorque(Input.GetAxis("Horizontal") * rotateTorque * Vector3.up);
		
		var hit: RaycastHit;
		var downRay = new Ray(transform.position, -Vector3.up);
		
		// Cast a ray straight downwards.
		if (Physics.Raycast(downRay, hit)) {
			// The "error" in height is the difference between the desired height
			// and the height measured by the raycast distance.
			var hoverError = hoverHeight - hit.distance;
			
			// Only apply a lifting force if the object is too low (ie, let
			// gravity pull it downward if it is too high).
			if (hoverError > 0) {
				// Subtract the damping from the lifting force and apply it to
				// the rigidbody. 
				var upwardSpeed = rb.velocity.y;
				var lift = hoverError * hoverForce - upwardSpeed * hoverDamp;
				rb.AddForce(lift * Vector3.up);
			}
		}
	}

using UnityEngine;
using System.Collections;

public class ExampleClass : MonoBehaviour {
    public float moveForce = 1.0F;
    public float rotateTorque = 1.0F;
    public float hoverHeight = 4.0F;
    public float hoverForce = 5.0F;
    public float hoverDamp = 0.5F;
    public Rigidbody rb;
    void Start() {
        rb = GetComponent<Rigidbody>();
        rb.drag = 0.5F;
        rb.angularDrag = 0.5F;
    }
    void FixedUpdate() {
        rb.AddForce(Input.GetAxis("Vertical") * moveForce * transform.forward);
        rb.AddTorque(Input.GetAxis("Horizontal") * rotateTorque * Vector3.up);
        RaycastHit hit;
        Ray downRay = new Ray(transform.position, -Vector3.up);
        if (Physics.Raycast(downRay, out hit)) {
            float hoverError = hoverHeight - hit.distance;
            if (hoverError > 0) {
                float upwardSpeed = rb.velocity.y;
                float lift = hoverError * hoverForce - upwardSpeed * hoverDamp;
                rb.AddForce(lift * Vector3.up);
            }
        }
    }
}

See Also: Physics.Raycast, Physics.Linecast, Physics.RaycastAll.