=========================== FOLLOWING PSMSL DOCUMENTATION ADDED 11-Jun-1991
Holyhead            170/191 RLR(1988) is 12.1m below BM SH2520 8296
=========================== FOLLOWING PSMSL DOCUMENTATION ADDED 20-Jul-1994
Between 1926-1950 gauge was a Harrison type, replaced by a Lea in 1958, and a Munro/Ott combination in July 1978. From August 1984 the gauge has been an "A Class" bubbler. Gauge destroyed by a ship in 1991 and reinstalled in 1995
=========================== FOLLOWING PSMSL DOCUMENTATION ADDED 30-Sep-1996
Data for 1938-59 are mean tide levels, whereas data for 1960-62 are MSL based on 8 measurements per day, and then MSL based on 24 per day. At Holyhead, MTL will be approximately 3.2cm BELOW MSL, based on the contribution from the M4 tidal constituent only. The PSMSL data have NOT been corrected for this systematic offset. It can be seen that, even with this correction, there is still an apparent datum shift around 1960 (see report of Lennon and Rossiter) Data prior to 1960 came from the LMS Railway Company and was considerably noisier, compared to data from Newlyn, for example than data from 1960 onwards which was computed at POL. (Superseded by new entry on 26-Jan-2016)
=========================== FOLLOWING PSMSL DOCUMENTATION ADDED 02-Jan-2001
Recent work on vertical land movements in the UK can be found from the University of Nottingham web site:http://ukcogr.iessg.nottingham.ac.uk/~eosi/
=========================== FOLLOWING PSMSL DOCUMENTATION ADDED 13-Feb-2001
Holyhead channel 2 - Spare pressure point fitted 22/Dec/97, subsequently data reading 2cms higher.  Also, pneumatics becoming blocked with marine growth, not cleared until 6/Feb/99.  Thereafter data assumed to be to previous datum.  Data December 1997 - January 1999 inclusive not used.
=========================== FOLLOWING PSMSL DOCUMENTATION ADDED 28-Jan-2009
Treat data 1960 onwards with caution.
=========================== FOLLOWING PSMSL DOCUMENTATION ADDED 03-Jul-2014
TGI visited the site a number of times during 2013. on the 13.03.2013 site was levelled and calibration checks made. 11.06.2013 General maintenance was carried and and the compressor changed. 03.07.2013 The gauge was serviced to improve data quality. 18.11.2013 The radar gauge was removed for testing and on 17.12.2013 testing was carried out on the installed radar gauge.
=========================== FOLLOWING PSMSL DOCUMENTATION ADDED 18-May-2015
TGI visited the site 6 times in total during 2014. There were intermittant issues with channels blocking during August and October on 18.08.14 the TGI visited to resolve the problem with channel 2. While there they removed the radar from channel 1 and re-instated the float gauge.On 12.10.14 TGI again visited to clear the blocked bubbler channel and the float gauge was serviced.
=========================== FOLLOWING PSMSL DOCUMENTATION ADDED 26-Jan-2016
A value of MTL-MSL=-32 mm has been applied to the RLR data for the period 1938-1959. The values were derived using the GESLA2 (high-frequency tide gauge) dataset using harmonic constituents. The value also agrees with the estimate published in Woodworth et al., GJI, 136, 651–670, 1999.