The north portal is 253.73 m above sea level .
The tunnels then initially ascend on a gradient of 0.1 % for about 2,433.5 m and towards the centre , the gradient rises at 0.54 % to the 3,280.8 m point .
The north portal is 253.73 m above sea level and the tunnels then initially ascend on a gradient of 0.1 % for about 2,433.5 m and towards the centre , the gradient rises at 0.54 % to the 3,280.8 m point .