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THR elevation and Pressure Difference

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On most LIDO charts, there tends to be a runway threshold, or TDZ, elevation shown.

For example, in EGLL, runway 09R it says: 79 / 3hPa (assumingly indicating runway threshold height)

So my question is, what does the Pressure after the elevation mean? Does this mean it Changes with 3ft every hPa change? So, let's say if you're entering the TDZ elevation in to your Head-up System, should you enter an elevation of 79ft Always, or correct it based on the Pressure difference?






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It's an info on how to convert between QFE and QNH. 

QNH = QFE + differential ( 3 hPa in your example )

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