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"The airport elevation at Rocky Mountain House, Alberta is 3,242 feet."

 

Which would decrease refraction values (absolute values) for all altitudes below 45 degrees by more than a tenth of a minute of arc --and increasing to 0.5 minutes of arc at 10 degrees altitude (10% of the standard sea level values). Did Thompson correct for this? Most accounts of lunars were written for observers at sea level, literally. Of course, if he measured barometric pressure at all, I suppose it would necessarily have been with a barometer calibrated at low altitude which would account for refraction at mountain altitudes automatically (assuming he had tables that covered such low pressures).