Dimensions and measurement accuracy are a function of several factors:

Image resolution - More pixels means more accuracy.
Lens height measurement accuracy (the distance between the ground and the nodal point of your camera's lens) - You can also calibrate the measurements against a real world measurement. As an example, in a rectangle shaped room of 200sqft, we normally achieve 98% or better accuracy.
Room size - Dimensions are triangulated from the lens height, and so larger rooms have more oblique angles for triangulation, leading to less precision.
Image distortion - Sometimes panoramas are stitched inaccurately with distortions, which in turn affects the measurement accuracy. Using a better camera and stitching software will increase accuracy.

Generally speaking, customers report accuracy around 96% to 98% for residential locations.
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