Adrian,

> My local airfield is giving 1004hPa (at 29m altitude), my BME280 is giving
> 994, so using the 1hPa/27ft quoted by Tauno Voipio, that suggests a 270ft
> (~93m) difference in altitude, whilst the map suggests 71m.  Which doesn't
> sound right, but the trend is right.

I did some more googeling, and found an Arduino project* for the BMP180
which incuded a formule for altitude correction (and vise-verse).   The
result was that I'm now only 4...5 full points away from what it should be.

*https://www.circuitbasics.com/set-bmp180-barometric-pressure-sensor-arduino/

Going the other way around it results me being at about 50 meters over sea
level, instead of about 20 - and that doesn't seem right (I don't think my
ceiling is 7 meters up. :-) ).

Regards,
Rudy Wieser

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