Levelling measurements from Auckland tide gauge to benchmarks found in non-reclaimed land, October 2001

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Beavan, R.J. 2002 Levelling measurements from Auckland tide gauge to benchmarks found in non-reclaimed land, October 2001 . Lower Hutt: Institute of Geological & Nuclear Sciences. Institute of Geological & Nuclear Sciences science report 2002/05 23 p.

Abstract: As part of a project to correct long-term trends in sea level for tectonic and other vertical motions near tide gauges, we have measured a levelling survey between the Ports of Auckland tide gauge, presently situated on Captain Cook Wharf, and several nearby benchmarks or survey marks set in non-reclaimed land. All of the benchmarks that have previously been used to check the height of the Auckland tide gauge have been located either on the wharf structure or on reclaimed land. While these benchmarks may be sufficiently stable because the wharf is founded in Waitemata sandstone [Edbrooke, 2001], they are nevertheless on built structures and potentially subject to cultural effects. We felt it was important for this project to also level from the tide gauge location to a number of benchmarks that are believed to be founded in original non-reclaimed land, and preferably in bedrock. We located four such marks to the south-east of the tide gauge site, and levelled from the tide gauge to these marks on 29 October 2001 using first-order levelling techniques. We also measured to two survey marks that had been incorporated in previous levelling runs [BCHF, 2001]. Adopting the published LINZ geodetic database value for the height of BM AK158 in the Auckland 1946 datum, we find good agreement between our work and previous work at the common survey marks. During the relocation of the Auckland tide gauge from Queen’s Wharf to Captain Cook Wharf in July 2000, the levelling connection between the two sites was undermined because a critical benchmark (CF46) had been moved without the height change being recorded [POAL, 200; BCHF, 2001]. The BCHF [2001] survey measured the height of the new tide gauge benchmark relative to the published height of survey mark SM 6177, while we measured it relative to the published height of BM AK158. We find that these values agree to 1 mm, which gives some confidence in the setting of the new tide gauge despite the problems with the levelling connection. We recommend that future levelling for checking the stability of the Auckland tide gauge includes measurements to first-order benchmark AK158, since it is the only high quality benchmark in the immediate vicinity (~500 m) of the tide gauge that may be founded in original non-reclaimed land close to bedrock. (auth)