Today’s guest blog is by Nick Hancock
In June 2014, I landed for the second time on the remote Isle of Rockall which lies around 250 miles off the Outer Hebrides in the North Atlantic. I had already landed on Rockall in 2012 on a reconnaissance for Rockall Solo. The challenge was now for me to survive on the rock on my own in order to set two new endurance records: THE LONGEST SOLO OCCUPATION OF ROCKALL and THE LONGEST OCCUPATION OF ROCKALL IN HISTORY.
During my 45-day record-breaking occupation of the remote Isle of Rockall in order to help pass the extended time alone I measured and mapped the summit of Rockall and Hall’s Ledge, the only vaguely level area on the rock, where my shelter, the ‘RockPod’, was secured. In addition, in an attempt to update the current United Kingdom Hydrographical Office (UKHO) 1977 Doppler sourced records relating to the position and height of Rockall, I installed a fixed permanent survey marker on the summit plateau of the rock, from which on 13 June and 14 July I ran two 24-hour data collection sessions using the Leica GS10 GNSS receiver with AS10 antenna which had been loaned to me for the expedition by Leica Geosystems.
The data sets were initially processed by Leica Geosystems and then passed on to Ordnance Survey (OS) for further processing. Data from Ordnance Survey OS Net stations at Arisaig, Barra, Benbecula, Inverness, Kinlochbervie, Lochcarron, Stornoway and Tiree were included, as well as data from EUREF Permanent Network stations at Argir (ARGI) in the Faroes, Londonderry (FOYL) in Northern Ireland and Hoefn (HOFN) in Iceland. This gave a good spread of data around Rockall which helped get the best estimation of atmospheric errors, especially those from the Troposphere.
A map of OS stations can be found here: http://www.ordnancesurvey.co.uk/gps/os-net-rinex-data/ and a map showing the other stations can be seen here: http://www.epncb.oma.be/_networkdata/stationmaps.php.
OS then used the same processing software and methods that they use to determine new coordinates for their own stations. The results from the four data sessions showed very good precision, with a comparison of coordinates from each session agreeing at the sub 5mm level. A “coordinate recovery” quality check was then performed on the final completed data set spanning all four data sessions. This is where all the computed baselines radiate out from Rockall itself, so that the data collected there has an influence on all the other coordinates. The coordinate recovery check indicated that the solution gave coordinates accurate to better than 2cm in all dimensions!
The UKHO coordinates for Rockall are N 57 35 46.686, W 13 41 14.226; my new coordinates (to same precision) are N 57 35 46.695, W 13 41 14.308. Simply differencing the UKHO coordinates and the new coordinates gives a difference of only about 1.3m in an east/west direction and 0.3m in a north/south direction. Given the small amount of noise in the comparison of the two different datums (about a metre or so) and also the likely accuracy of the UKHO survey point then it is clear that both the UKHO and my new survey arrived at much the same answer, which is testament to the quality of the 1977 Doppler survey, but does mean that I can’t claim to have significantly ‘moved’ Rockall.
However, the height of Rockall was previously estimated at circa 18 metres above sea level, after the summit, which was estimated by the Royal Geographical Society in their ‘First Map of Rockall’ as 19.2 metres, was removed by 39 Regiment, Royal Engineers in 1971 during ‘Operation Tophat’. The height of Rockall was recalculated by OS using my GNSS data, relative to an approximation of a height above mean sea level determined using a geoid model. The OS geoid model OSGM02 does not unfortunately cover Rockall so they referred to several online models. These models gave results of 17.10, 17.22 and 17.85 metres, depending on the geoid model used. The OS then decided to ignore the 17.85 metre result as it was from an old model and taking the results from the more recent 1996 and 2008 models, it became clear that the mean sea level height of Rockall is in the region of 17.15m, some 0.85 metres lower than the previous estimate. I had also physically measured the height difference between the summit plateau and Hall’s Ledge, where my RockPod shelter was located during the occupation, during my stay on the rock as 3.13 metres, meaning that the ledge is approximately 14 metres above mean sea level.
I had hoped that my survey results might significantly relocate Rockall, but being able to finally establish the true height of the rock means that my time spent there has even greater long-term significance than just breaking the previous occupation records and raising funds for Help for Heroes.
‘The First Map of Rockall’, G. S. Holland and R. A. Gardiner, The Geographical Journal Vol. 141, No. 1 (Mar., 1975), pp. 94-98