As part of my latest research I spent a day walking the route of the Ordnance Survey levelling “Newlyn Circuit”.
The Ordnance Survey had decided to improve the accuracy of the “National Datum” (as used on their mapping of the UK) by relating it to the “Mean Sea Level”. They choose Newlyn out of Dunbar, Newlyn and Felixstowe.
The levels were taken during the second geodetic levelling survey (2GL) undertaken from 1912 to 1921 which covered the whole network (of England and Wales initially) all linked to the new National Datum at Newlyn.
The first national survey was based on tidal readings based at Liverpool during the Initial Levelling Survey taken from 1840 to 1860. This is now known as the first geodetic survey (1GL)
Many new types of levels were used, set up and used to three “classes” of level, 1st, 2nd and 3rd, later “Primary, Secondary and Tertiary” using deferent levels of accuracy with the primary being the most accurate.
To comply with the International mapping standards the Mean Sea Level (MSL) needed to be related to two of the new primary bench marks known as Fundamental Bench Marks (FBMs). These were at Newlyn (Tolcarne) and Paul FMBs.
To complete the Newlyn circuit the Ordnance Survey (OS) decided to add a further two FBMs in the area, that was unusual as the rest of the FBM Network the FBMs were positioned about thirty miles apart. I am not sure why the OS did this but it could have been used to add more accuracy to the checking of the “National Datum” and possibly give new OS staff surveyors some practice levelling on a smaller scale circuit that their work could then be checked for correct operation, that is setting up instruments, taking readings, booking and checking results for accuracy.
The whole circuit was measured at 15km but that was, as the crow flies, so I expected it to be actually longer on the ground (which it was!) as I would need to use current roads and foot paths available. I looked at various maps from the period that would have been available to use at the time (I expect that the OS surveyors would have better access to more and possibly different scale maps etc?).
The “Circuit” includes a further two FBMs at Sancreed and Madron plus a number of Secondary BMs known as Flush Brackets (FBs) spaced out between the FBMs most of which were still in place.
Having visited all the BMs, on my various previous visits to the area, I wanted to visit them in the order that they would be used by the OS surveyors (but without having to carry all the levelling equipment of course!). I elected to follow the BMs in the circuit in the order listed but the circuit could have equally be followed in the other direction and may well have been done if the whole survey needed to be double checked?
I was able to complete the circuit within one day but it did take me a bit more time than expected (probably due to my age as I am not as quick/fit? as I used to be!) to walk the approximate 15 miles of the circuit route. I did, however find it interesting to plot the possible route and then try to follow as near as I could.
I hope to report the walk and findings in a later update.