Display the distance in meter real time obstacle avoidance.

How can i get the distance from the starting point to the highest point which i mark on the red line. I want to display the distance. When i make it in real time using mono vision camera,the distance will change everytime the camera move nearest to the point.

I just used Sobel Edge Detection,and lastly find the free path area for robot to go through it without obstacle.

Do I need just referring to camera Euclidean co-ordinate system?