Perhaps some pastel red text on the bottom like:
Sensor needs calibration. More info
Tapping "More info" in this case would show the same dialog as tapping the signal icon.
I'm imagining a scenario where you hand someone else your phone to let them use the compass, and they wouldn't know it's inaccurate because the signal icon (or any icon in the toolbar) isn't that obvious of an indicator.
Perhaps some pastel red text on the bottom like:
Sensor needs calibration. More info
Tapping "More info" in this case would show the same dialog as tapping the signal icon.
I'm imagining a scenario where you hand someone else your phone to let them use the compass, and they wouldn't know it's inaccurate because the signal icon (or any icon in the toolbar) isn't that obvious of an indicator.