BPM 085 Error.

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According this formula :

function get_altitude($station, $sea) {

//SEA is the QNH of metar of the ICAO station.

//Ive modified it to give me a difference of 8.9 hPa

//So sea is QNH of LFMP minus 8.9

global $sea;
global $uGC, $standardLapseRate,$gravity, $moleAir, $standardTempK;
$sea = 1013.25;
$pow = ($gravity * $moleAir) / ($uGC * $standardLapseRate); //5.25585
$halt = (1 – (exp((1 / $pow) * log($station / ($station + 8.9)/*$sea*/)))) * ($standardTempK / $standardLapseRate);

return $halt;
}

Altitude of my place is 73,98 meters. +- 0.25 cm

According the IGN cartographie :

My place is 54 m +- 20 m

ERROR 20 Meters !!!