Adrian  wrote:
> 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.

The vertical gradient isn't so precisely fixed, it depends on the
temperature. Approximately varies between 11 and 13 Pa per metre at sea
level, which means the error might be more than 10% right from the start.
See https://en.wikipedia.org/wiki/Barometric_formula

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