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Farndon Village Features
During the Jacobite Rising of 1745, it was clear to the military authorities that there were major shortcomings regarding the accuracy and reliability of local maps. There was also a more general and nationwide need in light of the potential threat of invasion during the Napoleonic Wars which followed, and consequently mapping began in earnest by the end of the eighteenth century. This work was the starting point of the Principal Triangulation of Great Britain (1783–1853), and led to the creation of the Ordnance Survey. By 1840, the one-inch survey had covered all of Wales, and all but the six northern-most counties of England. In Farndon, there are a number of features which played a role in the survey and mapping of the locality, as well as being part of the national coverage. These features are visible reminders, known as trig points, benchmarks and flush plates, most of which can still be spotted today, despite falling out of use. They are explained below, with their precise locations listed.
Probably the most familiar site regarding horizontal positioning is the trig (triangulation) pillar. Represented today on an OS Explorer map by a small blue triangle with a dot in the middle, around 6,500 trig pillars were built between 1936 and 1962 to form a state-of-the-art network to re-map Britain. The trig pillar provided a solid base for the theodolites used by the survey teams engaged in the retriangulation of the country, the mathematical process that made accurate map-making possible. In the same way that earlier Benchmarks recorded an accurate height, trig pillars mark a point with an accurate horizontal position (eastings and northings coordinates). In addition, most, but not all, trig pillars also have a flush bracket, to define their height above sea level. (Right) The author's son Liam, standing on the highest point of the UK - the trig point at the summit of Ben Nevis Although there are no Triangulation Pillars in Farndon, St Chad's Church Tower is listed as a trig point and is shown on OS maps with the triangle with a dot in the middle. 
Survey of 1949 shows a larger triangle enclosing the church tower with the dot in the centre. Bench Mark Ordnance Survey Benchmarks (denoted by 'B.M.' with an arrow head on maps) are survey marks made by O.S. to record height above Ordnance Datum.If the exact height of one BM is known, the exact height of the next can be found by measuring the difference in heights through a process of spirit levelling. Some 750,000 BMs were established between 1840 and 1985. They usually comprise the familiar shape as shown in the figure (right), and a horizontal angled groove, the apex of which is a defined level above ODN. These are typically located on walls or building facades within a metre or two of ground level. Although the main network is no longer being updated, the record is still in existence and the markers will remain until they are eventually destroyed by redevelopment or erosion. Ordnance Datum (Great Britain) 
For this survey, height difference had to be consistent across the country, therefore a point of reference was needed from which to measure the height above sea level, just in a similar way time difference is measured from the Greenwich Observatory.
The 1840, levelling was carried out using a datum at St John’s Church in Liverpool (now demolished, it stood to the rear of St. George's Hall). From 1844 this was changed, and the datum was a tidal pole at Victoria Dock in Liverpool. There were other measuring points, but Liverpool was the data point until 1921.
Between 1915 and 1921, accurate measurements of tide heights were also made at Newlyn in Cornwall. From these measurements an average, or mean tide height was calculated - Ordnance Datum Newlyn (ODN) - and this is 'sea level'. Consequently, from 1921 Newlyn was then selected as the country-wide datum, superseding Liverpool. It is applicable to all locations in mainland Britain. All heights are measured relative to the mean tide level at Newlyn and can therefore be positive values, or negative values (when below ODN), for example deep basements.
The Newlyn Tidal Observatory (Source: Courtesy of Newlyn Archive. Image by Ron Hogg)
Benchmarks 
Bench marks are the visible manifestation of Ordnance Datum Newlyn (ODN). ODN is realised on the ground by a network of approximately 190 fundamental bench marks (FBMs). From these FBMs tens of thousands of lower-order BMs were established. The network has had little maintenance for 30 years, and in some areas (mining areas for example), subsidence has affected the levelling values. In these regions the BMs cannot be relied upon to accurately define ODN.
The benchmark originates from the chiseled horizontal marks that surveyors made in stone structures, into which an angle-iron could be placed to form a 'bench' for a leveling rod, thus ensuring that a leveling rod could be accurately repositioned in the same place in the future. These marks were usually indicated with a chiseled arrow below the horizontal line. Flush Bracket
Ordnance Survey in Britain today According to the Ordnance Survey, Global Positioning System (GPS) and the OSGM15 TM model (the height transformation between the European Terrestrial Reference System 1989 and the national height datums) is the preferred method of heighting used today. This method achieves the most accurate and uniform method of heighting. The modern equivalent to the network of trig points is the OS Net network of 110 Global Navigation Satellite System (GNSS) receivers. O.S. surveyors use OS Net and GNSS technology everyday to instantly position new map detail to within a few centimetres.
St Chad's Church 
Farndon Church Tower
Map of 1897 showing the bench mark (marked by an arrow and 'B.M.')
The cut bench mark can be seen at the bottom left of the porch
Close-up of the flush plate (photo: John S Turner)
Nag's Head 
Barnston Spire 
Map of 1897 showing the bench mark (marked by an arrow and 'B.M.')
Barnston Spire
The benchmark can be seen on the spire foundation base to the left
Parochial Infant Schoolhouse 
Map of 1897 showing the bench mark in the centre (marked by an arrow and 'B.M.')
Parochial Infant School House
Parochial Infant School House
Parochial Infant School House
Holly Cottage, Barnton Road
Map of 1897 showing the bench mark in the centre (marked by an arrow and 'B.M.')
Holly Cottage - the bench mark is at the base of the left hand side gate post
Bench mark is at the base of the left hand side gate post
Although the database records the benchmark as Holly Cottage, it is clearly on the gate post belonging to Glen Haven next door.
Farndon Bridge 
Map of 1897 showing the bench mark (marked by an arrow and 'B.M.') on the Holt side of the bridge (bottom left)
Farndon Bridge benchmark
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The flush bracket (right) is another way of accurately defining a height above sea level and used for the more important level control points. You will find flush brackets in most trig pillars – although not all – as well as set into walls and buildings (‘flush’ with the building).
