In Blisworth Parish, there are several Ordinance Survey Bench Marks. These provide a reference for an accurate height above Sea Level, now ODN (Ordinance Datum Newlyn in Cornwall). There is a Bench Mark on the remains of the old railway bridge on the Blisworth Northampton line. BM583 is quoted as 90.6667m above ODN. Just think about that, the height is quoted to 0.1mm, or the thickness of a piece of paper over a distance of about 370km!
There are also Benchmarks, on WCML Railway Bridge, Candle Bridge and on the Tunnel entrance