how to get the ground truth map from ORB_SLAM

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Hi,

I want to get the map scale form the map which is build by ORB_slam2. Example, there are two camera center kp1(x1-coordinate,y2-coordinate) , kp2(x2-coordinate,y2-coordinate), then i will know the distance between kp1 and kp2( in coordinate). But what's the real distance(m)(in the real word)? i think i need to get the scale between coordinate and meter. But i don't know where is the map scale in the ORB_slam2 code(now I am a rookie ...). Could you tell me how to get the real map?? thank you very much!!