Tunnel Datum is a datum based on Ordnance Datum used in designing tunnels which pass below sea level. By using a lower datum as the base point, negative numbers can be avoided in calculations, eliminating a possible source of mistakes.
- for the London Underground, a tunnel datum of ODN − 100m is used; thus a depth of -60m is 40 m ATD (above tunnel datum)
- for the Channel Tunnel, a tunnel datum of ODN − 200 m is used; thus a depth of -60 m is 140 m ATD
Read more about this topic: Ordnance Datum
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