# Scaling monocular SLAM with another source?

I have implemented a stable monocular slam tracking system, based on ORBSLAM2. I am trying to find a way to add real-world distance/scale to this.

At the same time, I am running a (less stable) stereo slam system.

What i would like to do is:

Using the correct scale of the stereo data, figure out the scale factor that is need to correct the mono to real-world scale, and multiply the output by this factor.

I do not want to adjust the slam algorithm, I need to take the two output streams (currently an Eigen::Vector3f for each stream) and find an algorithm that I can run for 30 seconds or so to find the scale difference, and therefore the factor that I need to multiply the mono by.

I am currently:

//Start system.

//Store both values as Eigen::Vector3f;

Eigen::Vector3f initialWrong;
Eigen::Vector3f initialTruth;

//sleep the thread for a few seconds, and move the sensors.

// get the distances
float stopWrong = CurrentWrong - initialWrong;
float stopTruth = CurrentTruth - initialTruth;

scaleFactor = stopTruth / stopWrong ;


This works, within a margin of error, but is very manual. I am looking for a more automatic / iterative way to do this that will minimize the error as much as possible.

How can I use a ground truth to scale monocular slam in real time?

Any thought or tips here would be greatly appreciated. Thanks.

• I am not sure what you mean by 'is this a solved problem?'. If you're trying to initialize an environment with a known scale (from your stereo) and then keep track of your observations, do bundle adjustment etc. then perhaps something like PTAM (robots.ox.ac.uk/~gk/PTAM) will be of interest to you. The way PTAM initializes scale is by considering two viewpoints (which you probably wouldn't have to do as you have stereo slam already) Mar 3, 2017 at 4:54
• Hi, thank you for your reply. I have edited my question for clarity. What i need is to find a way, using the two xyz output streams, to find the scale factor, and then multiply the mono position by this factor, bringing it in line with the ground truth.
– anti
Mar 3, 2017 at 9:43