What is an acre? The history of land surveying

Land is the basis of most agricultural production and is measured in area units, generally acres or hectares. As most modern agricultural CAP support systems are now area-based, the area ascribed to individual fields is of huge significance to farmers. Individuals have seen the area of some fields altered versus the original Ordnance Survey measurements, but which is the most accurate?

Past versus present

The word acre comes from an old English word that meant open field. During the Middle Ages, an acre was the amount of land that could be ploughed in one day with a yoke of oxen and measured by one chain in width (22 yards) and one furlong or 10 chains in length (220 yards), yielding 4,840 square yards.

Before measurements were more standardized, the size of an actual acre varied widely between different countries. Internationally, one acre is now considered to be equal to 4,046.8564224m2. The international symbol of the acre is ac, and it is defined as 1/640 of a square mile.

An acre has no prescribed shape and refers only to its plan dimensions. This means that on an acre that is sloped, the measured surface area (length by breath) is always greater than an acre on a map or an acre on the flat. A perfectly square acre would measure 63.8m by 63.8m.

The acre described previously is also called a statute acre. However, other acre denominations, such as the Irish acre and the Cunningham acre, were also used. These originate from a different unit of linear measurement based on the perch and there were 160 square perches in an acre. The statute perch was 5.5 yards while the Cunningham perch was 6.25 yards and the Irish perch was 7.0 yards. Multiplied up, this puts the Cunningham acre at 6,250 square yards (1.291 statute acres) and the Irish acre at 7840 square yards (1.6198 statute acres).

How was area originally measured for OS?

In 1824, the British government decided to initiate a comprehensive series of 6in maps of Ireland, the first of its kind. The project was entrusted to the British Board of Ordnance, a military body responsible for mapping, which authorised Col. Thomas Colby to supervise affairs. Since the maps were to be based on townland divisions, Colby ordered a study of existing boundaries.

Irish engineers, under the control of Richard Griffith, conducted this work, which was known as the ‘boundary survey’, using steel chains. This resulted in Ireland being the most accurately mapped country in the world at the time.

With the Ashbourne Act in 1885, the Land Commission developed into a tenant-purchasing commission and assisted in the agreed transfer of freehold farmland from landlord to tenant. This was a response to the turbulent land war that had started in 1879 and was a form of buyback from the English landlords. Because the entire townland was to be completely vested, there were to be no gaps or strips withheld so as to comply with this remit. This is the historic reason why rivers and roads are registered to the centreline.

How did anomalies arise in digitisation?

In 2010 the Property Registration Authority of Ireland (PRAI) embarked on a digitisation project which would result in every land registry folio in Ireland being converted from paper map to digital map format. An Indian contractor in Bangalore was awarded the contract to digitise our folios. This was done by effectively tracing the paper maps to produce the modern digital filed plans.

There are some clear discrepancies between the old paper folios and new digital filed plans and these arise directly due to this process. There is now a special unit within land registry to deal with queries where the land owner can clearly show a discrepancy by using a professional to overlay the old folio onto the new digital filed plan.

Does GPS measuring increase accuracy?

Measurement of area is unusual in that its accuracy has not improved dramatically over time. This is because errors in area measurement tend to have a compensating effect. However, our ability to position boundary fences has improved exponentially with the use of GPS.

GPS allows surveyors to map and set out boundaries to a very high degree of precision, which does show up discrepancies in Land Registry maps. The Land Registry takes the representation approach to mapping, which follows the principal that “no representation of geography is perfect” and the absolute mapped position of a boundary fence, compared to the actual position, is unimportant once the fence is reasonably represented on the map.

This is called the general boundaries rule and is adopted in the UK and its former territories. In Germany, America, Australia etc this is not the case. In those countries a more accurate approach to boundary mapping ensures that people’s boundaries carry a state guarantee, unlike Ireland.

Why were roads and rivers included in land registration? Are they still included?

The reason behind why land is registered to the centre of the road is so that the land Registry can avoid having loads of unregistered narrow slithers of land around the country. The rules set up within the PRAI computer mapping system do not allow slithers of overlap and gaps between adjoining ownership records.

As the same reasoning applies to rivers, the land owner should be aware that ownership is to the centre of the river, even if that river moved naturally the centre of the actual river takes precedence over the mapped position of the river centreline.

What is involved in modern surveying?

Surveying and mapping technology has exploded over the last 25 years. In the late 1980s, laser technology enabled surveyors to measure horizontal distances without the use of a tape measure. This made measuring faster as well as more reliable.

In the mid 1990s, GPS arrived on the mapping scene, enabling surveyors to position boundaries within a national mapping framework with precision and ease. Using GPS in conjunction with laser-based theodolites has been the de-facto boundary mapping standard method for the last 20 years.

This system is more reliable than tape measurements because when using a tape the surveyor is relying on one specific feature on the OSI map which could be inaccurately mapped in the first place. GPS avoids this issue because it ties in with the actual Ordnance Survey and Land Registry mapping coordinate system.

The latest advance in survey technology involves the use of drone aircraft. These are model aircraft with sophisticated GPS autopilots and mapping sensors. A drone can map one square kilometre in an hour.

The fly-over survey ensures that the boundary is accurately mapped. But the same process can also be used to pinpoint drainage issues, areas of diseased or malnourished crops, etc. It can even count the amount of trees and itemise each tree height to provide an accurate inventory of privately owned commercial forestry.