Ordnance Datum Newlyn

Before satellite surveying methods became available in the 1970s, ellipsoid height was a rare and specialised coordinate type. The everyday vertical coordinate type was orthometric height, or more exactly mean sea level height relative to the national tide gauge datum, as realised in mainland Britain by the ODN heights of OS bench marks. Even though anybody with a GNSS receiver can now measure ellipsoid heights quite easily, Geoid‑related height coordinates such as ODN heights are much more useful for most applications because they are heights relative to a level surface.

The ODN datum

Like horizontal datums, vertical datums were ‘traditionally’ defined at a single initial point. In the case of horizontal datums, this is an astronomical observatory. In the case of vertical datums it is a coastal tide gauge. Tide gauge-based vertical datums are still used in most countries.

The tide gauge has a height reference benchmark, the height of which on the gauge scale is known. A long series of sea level records from the tide gauge is averaged to give the vertical offset of MSL from the bench mark. This is the MSL height of the bench mark.

In Britain, the MSL of the tide-gauge benchmark at Newlyn near Penzance in Cornwall is used – hence the name Ordnance Datum Newlyn. Its MSL height was established from continuous tide readings between 1915 and 1921. That adopted value has remained unchanged in the ODN system, even though MSL at the Newlyn gauge has slowly changed. Hence the height of a point above mean sea level given by OS is not affected by variations in mean sea level, and the proper description of an OS height is ‘orthometric height relative to Ordnance Datum Newlyn’.

The ODN TRF

A network of about 200 fundamental bench marks for height measurement was constructed across Britain in the first half of the twentieth century. These are underground chambers containing a height reference object, topped by a small pillar on which is another reference point. The idea is that the point on the top is easy to access for normal use, whereas the hidden underground mark is less susceptible to damage.

The orthometric height of each fundamental bench mark relative to Newlyn was determined by a network of precise levelling lines across Britain. You can find descriptions of the precise levelling technique in land surveying textbooks. This technique is capable of transferring orthometric height from one point to another with an error of less than 2 x √d millimetres, where d is the distance levelled in kilometres. Today, the accuracy of the precise levelling technique is rivalled by the combination of GNSS ellipsoid heighting with a precise gravimetric Geoid model that allows the ellipsoid height difference between two points to be easily converted to an orthometric height difference.

The ODN TRF was subsequently densified by less precise levelling to obtain the heights of about three-quarters of a million Ordnance Survey bench marks all over Britain. These lower-order bench marks are often seen cut into stone at the base of a building, church or bridge. About half a million of them are in existence and usable today. An important error source to bear in mind is possible subsidence of Ordnance Survey bench marks, especially in areas where mining has caused collapse of the ground. In these areas, cases of bench mark subsidence of several metres have been reported.

Relative accuracy of ODN bench marks

The following table is a statistical statement of ODN bench mark accuracy relative to other bench marks in the same area. The primary layer of the ODN TRF consists of the fundamental bench marks, which are considered error-free within the ODN coordinate system. The Ordnance Survey National Geoid Model (see section 6.4 below) relates ODN orthometric heights to the GNSS ellipsoid GRS80. Table 3 shows the expected maximum levelling error between bench marks on the same levelling line. This only describes the original measurement error. The Ordnance Survey bench mark network has not been maintained since the 1970s, so beware of possible bench mark subsidence.

Table 3. Accuracy of Ordnance Survey bench mark levelling, where d is distance levelled in km

Type of benchmark	Maximum error
Fundamental	Error free
Geodetic	±2mm x √d
Secondary	±5mm x √d
Tertiary	±12mm x √d