# 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) – HighVoltage Mar 3 '17 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 '17 at 9:43