a hard rain's gonna fall

I've been wondering for a while what ODN stands for.  Should have guessed that uncle Wiki would know.

ODN - Ordnance Datum Newlyn - is the mean sea level originally recorded at Newlyn in Cornwall between 1915 and 1921.  So know you know.

I'm interested because I snapped this very gauge last week (see extra).  At first I wondered why it had been necessary to add a second scale.  The difference in level today is down to last night's rain.  

With Storms Dudley and Eunice on their way I assume the original is going to disappear below the water line in the next couple of days.  It's not often that you see a metric in the middle of a small town in the middle of the country that suggests that mean sea levels have risen in the past 100 years or so.