Rail gage is the measurement between the inner surfaces of the rail tracks.  This distance is critical for safe operation of a locomotive on the tracks.  This inspection algorithm measures the gage from a drone flying at 250 ft above the track with a zoom lens and accurate telemetry data.  

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The gage is independently calculated every few inches using photogrammetric techniques.  The measurments are heavily filtered to reduce noise and can provide accuracy of about 1/8".

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Note: Yes, it is really spelled "gage" when dealing with railroad tracks...

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