How to measure distance with realsense

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I use realsense ROS wrapper from https://github.com/intel-ros/realsense . I want to measure the distance of some objects from depth image of /camera/aligned_depth_to_color/image_raw .

Subscribe /camera/aligned_depth_to_color/image_raw but data range 0~255. I can't know the distance from camera to object.

What is a good way to measure the distance from the depth image topic .

rostopic info is:

header: 
  seq: 0
  stamp: 
    secs: 1542785488
    nsecs: 734966992
  frame_id: "camera_color_optical_frame"
height: 480
width: 640
encoding: "16UC1"
is_bigendian: 0
step: 1280
data: [62, 7, 62, 7, 62, 7, 67, 7, 67, 7, 67, 7, 67, 7, 67, 7, 67, 7, 67, 7, 67, 7, 67, 7, 67, 7, 67, 7, 67, 7, 67, 7, 67, 7, 67, 7, 67, 7, 67, 7, 67, 7, 67, 7, 67, 7, 67, 7, 67, 7, 67, 7, 67, 7, 62, 7, 62, 7, 62, 7, 62, 7, 62, 7, 62, 7, 62, 7, 62, 7, 67, 7, 67, 7, 67, 7, 67, 7, 67, 7, 67, 7, 67, 7, 67, 7, 67, 7, 67, 7, 67, 7, 67, 7, 67, 7, 67, 7, 67, 7, 67, 7, 73, 7, 73, 7, 73, 7, 67, 7, 73, 7, 73, 7, 73, 7, 73, 7, 73, 7, 73, 7, 73, 7, 73, 7, 73, 7, 73, 7, 73, 7, 73, 7, 73, 7, 73, 7, 73, 7, 73, 7, 73, 7, 73, 7, 73, 7, 73, 7, 73, 7, 73, 7, 73, 7, 73, 7, 73, 7, 73, 7, 73, 7, 73, 7, 73, 7, 73, 7, 73, 7, 73, 7, 73, 7, 73, 7, 79, 7, 79, 7, 79, 7, 79, 7, 79, 7, 79, 7, 79, 7, 79, 7, 85, 7, 85, 7, 85, 7, 85, 7, 85, 7, 85, 7, 85, 7, 85, 7, 85, 7, 85, 7, 85, 7, 85, 7, 85, 7, 85, 7, 79, 7, 79, 7, 79, 7, 79, 7, 79, 7, 79, 7, 79, 7, 79, 7, 73, 7, 73, 7, 73, 7, 73, 7, 73, 7, 73, 7, 73, 7, 73, 7, 73, 7, 73, 7, 73, 7, 73, 7, 73, 7, 73, 7, 73, 7, 73, 7, 73, 7, 73, 7, 73, 7, 73, 7, 73, 7, 73, 7, 73, 7, 79, 7, 79, 7, 79, 7, 79, 7, 79, 7, 79, 7, 79, 7, 79, 7, 85, 7, 85, 7, 85, 7, 85, 7, 85, 7, ...]