## Why are we measuring gravity?

A ground-breaking project is underway to measure absolute gravity across the entire island of Ireland, involving not only world-leading scientists, but also priests and judges. Discover why measuring gravity is both incredibly challenging and absolutely necessary.

Here is something you may not know.  Gravity – measured by the acceleration given to objects falling to the ground – is not the same everywhere.  In fact, gravity changes as you go up hills and down valleys, depending on the bulk of the earth underneath you. Mountains have a higher value of gravity than flat land and lakes, so if you were to hang a plumb line down beside a mountain, it would be pulled ever so slightly in that direction.

National Convention Centre and Samuel Beckett Bridge on River Liffey, Dublin, Ireland

Do these small gravitational variations really matter?

For most people, maybe they don’t.  However, in some industry sectors, gravity readings are vitally important as they are used to calculate accurate height measurements.  They can be used by civil engineers to measure the exact height of new bridges or quantify the precise amount of soil that would need to be removed when excavating a road or rail cut through a hill.  They can be used by environmentalists and planners to better understand flood risks and ensure buildings are built at the correct height above flood plains.  Gravity readings are also used by meteorologists to help them accurately forecast the weather.  All these professionals need definitive, scientific measurements of gravity, called absolute gravity – but Ireland has none.

Relative gravity is not enough

At present, the only gravity data available in Ireland is relative gravity, which is the difference in gravity between two different locations.  In contrast, absolute gravity measurements provide a stand-alone gravity value for each discrete location.  This means an absolute gravity value at one site can be accurately compared, not only with any other location in Ireland, but with any other location in the UK, Europe or indeed the world.

With relative gravity data it can be very hard to make accurate comparisons between two sets of readings in two different countries.  In the 1970s the Dublin Institute of Advanced Studies (DIAS) tried to tie Irish relative gravity measurements to those from Cambridge in the UK, but the initiative proved highly complex and difficult to accomplish.  Even today, it is challenging to incorporate Irish relative gravity data into EU gravity models, because it is statistically difficult to compare relative gravity measurements taken on an island with those taken on mainland Europe.

A step towards removing a step

Recognising the pressing need for absolute gravity data for Ireland, OSi launched a project to establish a modern gravity control network using state-of-the-art absolute gravity measurement techniques.  This ground-breaking initiative is being undertaken in collaboration with Land and Property Services (LPS) Northern Ireland, to generate consistent, accurate gravity readings for the whole island of Ireland for the first time.

The relative gravity measurements currently used in Northern Ireland were calculated by the Geological Survey of Northern Ireland (GSNI) in around the 1950s and 1960s.  In Ireland, relative gravity was calculated by DIAS between the 1940s and 1980s. The two organisations used slightly different methods and applied different corrections, so there are variations in height calculations, which have resulted in a digital ‘step’ at the border.  While this step is less than 10cm in height and can be smoothed to a certain extent, it is impossible to know which side is more accurate without local knowledge.  By calculating absolute gravity – rather than relative gravity – and by using the same equipment and scientific method across the whole island, the team will be able to eliminate the digital step completely.

Highly sensitive, scientific equipment

OSi and LPS appointed the Institute of Geodesy and Cartography from Warsaw in Poland to help them carry out this specialised scientific survey.  First, the team had to find suitable locations for over 50 gravity control stations across the island of Ireland, as well as six indoor stations from north to south, through the centre of the island, for calibrating the gravimetric baseline.  Six locations were chosen close to existing DIAS sites to allow the new data to be linked with historical data.  Other sites were chosen for their proximity to partner sites, such as Met Eireann and the UK Met Office.

Then, in three survey missions, in 2018, 2019 and 2021, scientists from the Institute of Geodesy and Cartography transported highly sensitive gravimeters to Ireland, travelling through Germany and France.  In one surveying expedition, the survey team travelled 3,630 km within the island of Ireland alone, visiting 50+ locations.  A huge amount of planning was required, as the equipment had to be charged overnight, and cables had to be passed through hotel bedroom windows to the trucks parked outside.  Such is the sensitivity of the equipment that if someone walks past it, or enters a building where it is set up, the movement shows up in readings back at the institute’s main observatory in Poland.

The goodwill of strangers

The success of the project can largely be attributed to the goodwill of strangers, who recognised the importance of the survey and helped OSi and LPS to gain access to the most suitable locations for taking absolute gravity readings.  Baseline surveys had to be taken inside house-locked, internal rooms that had at least another room and a door between them and the outside, as well as solid stone floors.  For this reason, the survey team often found itself in churches and, on one occasion, took up the carpet in a priest’s dressing room, with the priest’s enthusiastic support.

For consistency and comparison, OSi wanted to reuse specific locations where relative gravity had been measured in previous studies.  This meant gaining access to judges’ chambers, and permission was cordially granted, so long as OSi avoided times when the court was in session.  Similarly, the survey team wanted to take absolute gravity readings in an old observatory at Cork University.  When the stone plinth used for the previous survey was found to have been covered with a new wooden floor, university staff cheerfully lifted the floorboards.  Without the support of all these people, the survey project may not have achieved its goals.

Killary Harbour fjord in northern Connemara, Ireland

Accurate height data, everywhere

OSi have recently received the final results of the absolute gravity survey.   The organisation will then use these scientific measurements in aerial surveys and improve the accuracy of height data in its geoid models.  Significantly, the use of absolute gravity measurements will enable OSi to reduce its reliance on GPS data which only estimates heights as mathematically-calculated ellipsoid shapes.  While roughly right, these ellipsoids don’t take into account humps and hollows in the landscape, so people can be walking downhill while the height data is going up.  Through the use of absolute gravity measurements, OSi’s geoid model will more accurately represent the actual surface of Ireland and give professionals like engineers, developers, planners and environmentalists more accurate height data to work from.

OSi’s new height data will be particularly beneficial in western Ireland, where height levels in most locations have not been re-measured since the 1820s.  When height levels in Ireland were originally surveyed, a century ago, the process started in Dublin and small errors in calculation occurred as the surveyors worked from east to west.  Consequently, by the time the surveyors got to Galway, the margin of error had widened.  Over the years, height data for the west coast has been converted from feet to metres and corrections have been applied, but many inaccuracies still remain.  OSi will be able to use its absolute gravity measurements to provide accurate height levels for the whole of this region, for the first time.

The project to measure absolute gravity has been undeniably challenging, not least because it was hampered by COVID-19 restrictions.  However, it is now on track to deliver the precise, scientific data that OSi needs to increase the accuracy of its height data.  With better quality base data sets, OSi can then improve all of the products and services it delivers, from the digital mapping services it provides for government and businesses to the leisure maps it produces for the general public.

So does measuring absolute gravity matter?

Absolutely.

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