ar_track_alvar position output

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I'm detecting a pattern that is on the floor using my drone's bottom camera. The camera was calibrated as indicated in the ar_track_alvar ROS wiki website, but it's returning wrong values, and I can't find if it's OK that it returns some wrong values and I have to add a proportional constant to the x, y and z coordinates (multiplying the returned value so it gives me the output in my desired units (i.e, in meters)) or if the calibration or anything else is wrong.

The positive and negative sign of the returned values are OK.

For example, I placed my drone at 0.575 meters in the z coordinate and, posing the tag in 3 different parts of the FOV it returned the following values (in average):

• 0.932
• 0.906
• 0.950

What do you think I have to do?

Thanks :)