5 Tides and Water Levels 



38 



Measurement Instrument 



From 1 978 to June 1 995 water level data were obtained from an 

 NOAA/NOS control tide station (sta 865-1370) located at the seaward end of the 

 research pier (Figure 2) by using a Leupold and Stevens, Inc. (Beaverton, OR), 

 digital tide gauge. This analog-to-digital recorder is a float-activated, negator- 

 spring, counterpoised instrument that mechanically converts the vertical motion of 

 a float into a coded, punched paper tape record. The below-deck installation at 

 pier sta 19+60 consisted of a 30.5-cm-diam stilling well with a 2.5-cm orifice and 

 a 2 1 .6-cm-diam float. 



Operation and tending of the tide gauge conformed to NOS standards. 

 The gauge was checked daily for proper operation of the punch mechanism and for 

 acciiracy of the time and water level information. The accuracy was 

 determined by comparing the gauge level reading with a level read from a refer- 

 ence electric tape gauge. Once a week, a heavy metal rod was lowered down the 

 stilling well and through the orifice to ensure free flow of water into the well. 

 During the summer months, when biological growth was most severe, divers in- 

 spected and cleaned the orifice opening as required. 



The tide station was inspected quarterly by an NOAA/NOS tide field 

 group. Tide gauge elevation was checked using existing NOS control 

 positions, and the equipment was checked and adjusted as needed. Both NOS and 

 FRF persoimel also reviewed procedures for tending the gauge and 

 handling the data. Any specific comments on the previous months of data were 

 discussed to ensure data accuracy. 



Digital paper tape records of tide heights taken every 6 min were analyzed 

 by the Tides Analysis Branch of NOS. An interpreter created a digital 

 magnetic computer tape from the punch paper tape, which was then processed on 

 a large computer. First, a listing of the instantaneous tidal height values was cre- 

 ated for visual inspection. If errors were encountered, a computer program was 

 used to fill in or recreate bad or missing data using correct values from the nearest 



Chapter 5 Tides and Water Levels 



