Johnston Island (USA Trust) Site History

History Summary

Please note that the position information has been altered in this document - see document 83916

Reference Level

All heights have been referred to the station tide staff zero which is linked to fixed benchmarks.

Tide Gauge Benchmark (TGBM) Description

Benchmark 13(1955) is the primary benchmark (PBM) stamped 'NO 13 1955' set in a concrete bulkhead cap at the inshore end of the small boat pier.

Auxiliary Benchmarks

9 additional benchmarks are levelled to the TGBM and the Tide Station on a yearly basis.

Benchmark Relationships

TGBM is 9.35ft above Tide Gauge Zero (Station Datum)

NOAA Chart Datum (MLLW) is 1.74ft above Station Datum

Mean Sea Level (1960-1978) is 2.81ft above Station Datum

Quality Assessment

Comments

The timing accuracy was typically no better than ±5 min prior to 1976. Large fluctuations during November - April for most years are associated with meteorological events.

A reference level correction of +15 cm was applied to the original data during 11Z 03 November - 01Z 27 November 1980. Otherwise, the daily and monthly means reveal no apparent reference level changes or trends.

The data for 1996-2000 were supplied with heights relative to MLLW. Consequently 530mm was added to refer the data to tide gauge zero.

The data for 1998-2000 were supplied in non-GMT format and had to be shifted into GMT.

Notable episodic events:

• 23-25 May 1960: Tsunami from S.C. Chile
• 28 March 1964: Tsunami from Alaska

Gaps in the Data Record

The following gaps occur in the data set.