Aire Mile 77 Temple Manor Tidal 14’ ODN

Aire Mile 77 Temple Manor Tidal 14’

Spot heights from the OS app have not always reflected that the River Aire flows mainly downhill. They do not consistently fall with the river so there is clearly some error. I’ve so far expressed spot heights as above sea level but now we’re in the tidal reaches, it’s time to be a bit geeky about the phrase.

What is this ‘sea level’? It is a mark on a sea wall in Newlyn, Cornwall. It is the mean tide height at Newlyn, calculated in 1921 using six years of data. All UK land heights are measured from this line that acts as Point Zero. You may see land heights followed by ‘ODN’ which stands for Ordnance Datum Newlyn.

Nautical charts or maps use a different ‘point zero’ or datum which is the depth of watermeasured at the lowest recorded tide. So when you see sea depth on a chart, it’s measured at or calculated from the very lowest tide. This makes sense as navigators need to be able to work out if their boats risk running aground.