NOAA Technical Memorandum NOS CO-OPS 0048

A Network Gaps Analysis For The National Water 

Level Observation Network 

 

noaa National Oceanic and Atmospheric Administration 

U.S. DEPARTMENT OF COMMERCE 

National Ocean Service 

Center for Operational Oceanographic Products and Services 
Center for Operational Oceanographic Products and Services 

National Ocean Service 

National Oceanic and Atmospheric Administration 

U.S. Department of Commerce

The National Ocean Service (NOS) Center for Operational Oceanographic Products and Services 

(CO-OPS) provides the National infrastructure, science, and technical expertise to collect and 

distribute observations and predictions of water levels and currents to ensure safe, efficient and 

environmentally sound maritime commerce. The Center provides the set of water level and tidal 

current products required to support NOS’ Strategic Plan mission requirements, and to assist in 

providing operational oceanographic data/products required by NOAA’s other Strategic Plan 

themes. For example, CO-OPS provides data and products required by the National Weather 

Service to meet its flood and tsunami warning responsibilities. The Center manages the National 

Water Level Observation Network (NWLON), a national network of Physical Oceanographic 

Real-Time Systems (PORTS) in major U. S. harbors, and the National Current Observation 

Program consisting of current surveys in near shore and coastal areas utilizing bottom mounted 

platforms, subsurface buoys, horizontal sensors and quick response real time buoys. The Center: 

establishes standards for the collection and processing of water level and current data; collects 

and documents user requirements which serve as the foundation for all resulting program 

activities; designs new and/or improved oceanographic observing systems; designs software to 

improve CO-OPS’ data processing capabilities; maintains and operates oceanographic observing 

systems; performs operational data analysis/quality control; and produces/disseminates 

oceanographic products.
NOAA Technical Memorandum NOS CO-OPS 0048

A Network Gaps Analysis For The National Water 

Level Observation Network 

Stephen K. Gill 

Kathleen M. Fisher 

March 2008 

 

 

 

National Oceanic and Atmospheric Administration

 

U.S. DEPARTMENT OF National Oceanic and Atmospheric National Ocean Service 

COMMERCE Administration John H. Dunnigan, 

Carlos M. Gutierrez, Vice Admiral Conrad C. Lautenbacher, Jr., Assistant Administrator

Secretary., U.S. Navy (Ret.), Administrator 

Center for Operational Oceanographic Products and Services 

Michael Szabados, Director 
NOTICE 

Mention of a commercial company or product does not constitute 

an endorsement by NOAA. Use for publicity or advertising 

purposes of information from this publication concerning 

proprietary products or the tests of such products is not authorized. 
i 

TABLE OF CONTENTS 

Purpose............................................................................................................................................ 1 

Introduction..................................................................................................................................... 1 

Network Design .............................................................................................................................. 2 

Layer 1) Tide Control ............................................................................................................ 3 

Layer 2) Relative Mean Sea Level Trends ............................................................................ 4 

Role of Error Budgets ..................................................................................................................... 6 

The Swanson Error Analysis Report............................................................................................... 7 

The Bodnar Report.......................................................................................................................... 8 

Technical Approach........................................................................................................................ 9 

Limitations .................................................................................................................................... 13 

The Results.................................................................................................................................... 14 

East Coast............................................................................................................................. 14

Caribbean ............................................................................................................................. 22

Gulf Coast ............................................................................................................................ 23

West Coast ........................................................................................................................... 28 

Alaska .................................................................................................................................. 32 

Hawaii.................................................................................................................................. 36 

Summary....................................................................................................................................... 37 

Acknowledgments......................................................................................................................... 37 

References..................................................................................................................................... 39

Appendix 1. National Water Level Observation Network Station Maps as of February 2008 .... 41

Appendix 2. National Water Level Observation Network Station Configuration........................ 49
ii 

LIST OF FIGURES 

Figure 1. The cotidal lines for Diurnal Range (GT) and Tropic Lower Low Water Interval 

(TcLLWI) in the Gulf of Mexico. ................................................................................... 4 

Figure 2. Estimated contours for subsidence in Louisiana. ............................................................ 5 

Figure 3. Relative mean sea level trends at NWLON stations in Southeast Alaska....................... 6 

Figure 4. The locations of the historical tide stations and offshore points used in the Bodnar 

equation analysis for Grand Isle.................................................................................... 11 

Figure 5. The GIS error polygon for Grand Isle, Louisiana depicting the geographic area of 

coverage for datum determination at short-term tide station. ....................................... 12 

Figure 6. Example of the GIS analysis results for nearby stations. . ............................................ 12 

Figure 7. NWLON gaps analysis for southeast Florida................................................................ 14 

Figure 8. NWLON gaps analysis for northeast Florida and southern South Carolina. ................ 15 

Figure 9. NWLON gaps analysis for South Carolina. .................................................................. 16 

Figure 10. NWLON gaps analysis for North Carolina. ................................................................ 17

Figure 11. NWLON gaps analysis for the Chesapeake Bay......................................................... 18 

Figure 12. NWLON gaps analysis for Delaware Bay and New Jersey. ....................................... 19 

Figure 13. NWLON gaps analysis for New York and southern New England. ........................... 20 

Figure 14. NWLON gaps analysis for northern New England..................................................... 21 

Figure 15. NWLON gaps analysis for the Caribbean.................................................................. 22 

Figure 16. NWLON gaps analysis for Texas................................................................................ 23 

Figure 17. NWLON gaps analysis for Louisiana.......................................................................... 24 

Figure 18. NWLON gaps analysis for the Mississippi and Alabama coasts. ............................... 25 

Figure 19. NWLON gaps analysis for northwest Florida............................................................. 26 

Figure 20. NWLON gaps analysis for southwest Florida............................................................. 27 

Figure 21. NWLON gaps analysis for southern California. ......................................................... 28 

Figure 22. NWLON gaps analysis for central California. ............................................................ 29 

Figure 23. NWLON gaps analysis for northern California and southern Oregon........................ 30 

Figure 24. NWLON gaps analysis for the northwest coast and Puget Sound. ............................. 31 

Figure 25. NWLON gaps analysis for southeast Alaska. ............................................................. 32 

Figure 26. NWLON gaps analysis for the northern Gulf of Alaska. ............................................ 33 

Figure 27. NWLON gaps analysis for the eastern Aleutian Islands, Bristol Bay and Alaskan 

Peninsula..................................................................................................................... 34 

Figure 28. NWLON gaps analysis for the Chukchi Sea and North Slope, Alaska....................... 35 

Figure 29. NWLON gaps analysis for Hawaii.............................................................................. 36 
iii 

LIST OF TABLES

Table 1. Generalized accuracy of tidal datums from short series of observation; based on one 

standard deviation (one-sigma) uncertainty level (from Swanson 1974)........................ 7

Table 2. The regression equations and parameters for estimating uncertainties in tidal datums for 

Mean Low Water (from Bodnar, 1981) .......................................................................... 8

Table 3. An example of a Bodnar Equation error analysis spreadsheet for a three-month datum 

comparison using Grand Isle, LA as control................................................................. 10 
iv 
1 

A NETWORK GAPS ANALYSIS FOR THE NATIONAL 

WATER LEVEL OBSERVATION NETWORK

PURPOSE 

The purpose of this report is to provide a deterministic assessment of the size and geospatial 

density of water level stations for the National Water Level Observation Network (NWLON). It 

provides a rationale for the number of and location of NWLON stations that is required to 

support NOAA Missions and Goals. The report identifies specific locations where network gaps 

exist. 

INTRODUCTION

An observing system network can be a system of interconnected measurement points that 

provides information over a desired geographical area, such that variations in the desired 

observational parameters can be fully described and understood and the information can be 

continuously obtained and applied. For good network design, considerations must be made for 

redundancy and backup at each of the measurement points, and the design must consider 

overlap and backup coverage for a particular measurement point if it stops operating or data are 

lost. 

The Center for Operational Oceanographic Products and Services (CO-OPS) is responsible for 

managing the National Water Level Program (NWLP) to meet NOAA’s present and future 

mission requirements for national tide and water level data. The NWLP is recognized as a major 

component of the federal ocean observing system backbone for the nation, providing a baseline 

network of water level measurements and a national reference system for water level derived 

vertical datums that can be used to dovetail with other Federal, State, academic, and private and 

public sector water level requirements. The NWLP must provide for the density of information 

required to describe water level variations at all appropriate time and geographic scales. This 

density is described in terms of geographic coverage of a particular measurement location. Time 

scales range from short-term (real-time) data for navigation to long term (decades to centuries) 

data for estimation of relative sea level trends and for updates to National Tidal Datum Epochs 

(NTDE). The geographic scales required include the need for regional coverage of significant 

variations in tidal characteristics and gradients in relative sea level trends. Depending upon 

specific requirements, water level stations can be either short-term (one year or less) or long term 

(one-year to several decades). The overall NWLP network design is a blend of long-term 

National Water Level Observation Network (NWLON) stations and shorter-term (subordinate) 

stations (one-month to several year occupations) that provide localized information to specific 

areas for specific applications. 

Based on operational experience in planning of hydrographic, shoreline and coastal engineering 

projects and cumulative knowledge from project analyses, there is empirical knowledge of 

coastal areas that lack or have inadequate control for datum determination. Several qualitative 

assessments of NWLON coverage have also been completed over the years (NOS, 1986). This 
2 

gaps analysis report provides a quantitative scientific assessment of the geographic gaps in 

WLON coverage using a quantified error analyses approach to identify the gaps. 

NETWORK DESIGN

The requirements of the NWLP can be thought of as a set of layers, with each layer having its 

own needs for coverage; that is the number and placement of water level stations. These layers 

of requirements can be described as two fundamental information layers followed by five 

application layers: 

Fundamental layers, determination of: 

Layer 1) Tidal and Water Level Datums 

 Layer 2) Relative Sea Level Trends 

Applications layers, support for: 

Layer 3) Marine Transportation System Operations 

Layer 4) Nautical Charting Program: Hydrographic Surveying and Shoreline Mapping 

Layer 5) Storm Surge Inundation, Emergency Evacuation, and Tsunami Warning 

Layer 6) Habitat Restoration, Coastal Resource Management, Coastal Engineering 

Layer 7) Climate Change, Environmental Monitoring 

CO-OPS Special Publication Tidal Datums and Their Applications (NOS, 2001) provides 

summaries of the major applications. Layers 1) and 2) are the focus of this report as the 

determination of tide and water level datums and sea level trends are fundamental to the 

successful application of the data to Layers 3) through 7). The NWLON is supplemented by 

short-term station to provide more precise and accurate coverage in between NWLON stations to 

meet the needs of these application layers. Marine Transportation System Operations (Layer 3) 

requirements are met through programs such as the Physical Oceanographic Real Time System 

(PORTS), a localized real-time observing system designed and established in partnership with 

local maritime constituencies. Layers 3 and 4 are interconnected through the Nautical Charting 

Program. The NWLON has been increasingly enhanced to meet the needs of the hazards warning 

and response community (Layer 5) both through improved communications and high rate 

measurement capability, new sensors, and with new station placement. This has been 

accomplished in partnership with NOAA’s National Weather Service (NWS) and Tsunami 

Warning Centers (TWC’s). CO-OPS established the Coastal Oceanographic Applications and 

Services to Tides and Lakes (COASTAL) program to deliver tailored products to the nonnavigation community (Layer 6) such as the need for habitat restoration and planning and for 

environmental monitoring. CO-OPS maintains a set of long-term remote ocean island stations 

and key shore stations that contribute to the Global Sea Level Observing System (GLOSS) and 

works in partnership with the NOAA Climate Program Office to enhance products and services 

for climate monitoring and research (Layer 7) and to integrate our long-term data sets with 

geodesy to support determination of global sea level change. 

` 
3 

Layer 1) Tide Control This layer describes the coverage necessary to meet requirements for a national vertical reference 

system for tide and water level datums. The network must be able to provide reliable, accurate 

information on tide and water level datums for all areas of the nation’s coasts. As mentioned 

before, the NWLON is a network of long-term continuously operating water level stations that 

provides the fundamental baseline of the national reference system for tide and water level 

datums. The overall NWLP network design is a blend of long-term NWLON stations and 

shorter-term stations (operational one-month to several years) that provide project specific 

localized information. Together, these long-term and short-term stations and the derived 

products comprise a tidal datum reference system for the nation. Appendix 2 provides 

information on the physical configuration of an NWLON station. 

The time period necessary to incorporate all of the major astronomical tide producing cycles into 

the computation of a tidal datum is 19-years. All tidal datums are referenced to specific 19-year 

National Tidal Datum Epochs (NTDE). The present NTDE is 1983-2001. NTDE datums for 

short-term stations are computed and adjusted to the NTDE using simultaneous comparison with 

an appropriate nearby control station (NOS, 2003 and Marmer, 1951). The NWLON stations 

generally serve as the control stations for tidal datum determination at nearby short-term stations. 

The accuracy of tidal datum computations at these subordinate stations depends upon an 

NWLON with proper geographic spacing and density. This report does not cover the remote 

ocean island tide stations, as the cotidal lines for the open ocean have not yet been mapped with 

sufficient precision. These remote regions will be covered in a subsequent report. Requirements 

for these stations are highly driven by climate needs (monitoring global climate oscillations such 

as ENSO) and international Global Sea level Observing Systems (GLOSS). The technique for 

determining gaps used in this report does not apply in the Great Lakes because they are 

essentially non-tidal. The NWLON stations in the Great Lakes provide coverage for water level 

datum transfers to short-term stations, for hydraulic corrector calculation, and for seasonal water 

level forecasts (Coordinating Committee, 1995). 

Appendix 1 shows the location of the 200 NWLON stations as of 2007. An analysis of gaps in 

the Great Lakes portion of the NWLON will be presented in a subsequent report. The question 

becomes “What is the distribution and number of NWLON stations required to provide 

continuous and overlapping coverage of the coast?” For this study, coverage is defined by the 

geographic area in which a particular station can provide accurate control for the determination 

of tidal datums. 

This NWLON network design is driven by an error analyses for the uncertainty in knowing tidal 

datum elevations to the confidence level demanded by the multiple user applications expressed 

by Layers 3 -7 described earlier in Network Design. These errors are in turn driven by the 

geospatial changes in tidal characteristics (differences in time and range of tide) and changes in 

relative sea level trends. The NWLON network must be designed to provide sufficient coverage 

for these changes. Figure 1 displays the area wide complexity of the tide, exhibited by the corange (GT, or Diurnal Range) and co-time lines (TcLLWI, or Tropic Lower Low Water Interval) 

for the northern Gulf of Mexico. In addition, the network must be designed to provide 
4 

appropriate levels of redundancy or overlap. This provides a nearby backup station for datum 

determination without a severe loss in accuracy if a station goes down. Complete loss of data 

from a NWLON station is rare, as backup sensors, redundant data collection paths, and real-time 

quality control systems are in place. However, unavoidable losses sometimes occur when 

stations are destroyed when in the eye-paths of hurricanes or damaged by nor’easters. 

Figure 1. The cotidal lines for Diurnal Range (GT) and Tropic Lower Low Water Interval 

(TcLLWI) in the Gulf of Mexico. 

Layer 2) Relative Mean Sea Level Trends

The NWLON design must also account for variations in relative trends in mean sea level along 

the coast. Even though tidal characteristics may vary similarly across a particular region, if the 

sea level trends change significantly, then the extrapolation of the tidal datums will still be 

limited. Inferring similar relative mean sea level trends at control/subordinate station pairs 

during the datum computation process will bias the computed datum at the subordinate station if 

the trends are not the same. One example of this variation is in Louisiana (see Figure 2) where 

the diurnal tide dominates. Even though the tidal characteristics are similar, there is a strong 

gradient in relative mean sea level trends due to local and regional subsidence. Because of the 

disparity in relative mean sea level trends (Zervas, 2001) the NWLON station at Grand Isle, LA 

(+9.85mm/yr) and Dauphin Island, AL (2.93mm/yr) for instance, cannot be used to provide 

datum control in Lake Pontchartrain (~ 7.0mm/yr (USACE, 2006)). Another example is in 

Southeast Alaska. Both Juneau (inside) and Sitka (outside) have similar tidal characteristics 

(Figure 3). However, Juneau (-12.69 mm/yr) has a much higher rate of relative sea level fall 

than Sitka (-2.17 mm/yr). Both have very similar ranges and times of tide; however neither tide 
5 

station should be used to provide datum control for subordinate stations that would be 

established near the other station due to the bias introduced by such a large difference in relative 

sea level trend. 

Figure 2. Estimated contours for subsidence in Louisiana. 
6 

Figure 3. Relative mean sea level trends at NWLON stations in Southeast Alaska. 

ROLE OF ERROR BUDGETS

The density of stations in the network design is driven by error budgets of the applications. Tidal 

datums are local in nature and determined at specific locations by water level measurements at 

tide stations. Geographic extrapolation and/or interpolation of datum information from an 

NWLON station are constrained by errors in the technique for simultaneous comparison. These 

limits are driven by tolerable uncertainties for the desired application. NOAA has traditionally 

used the error analysis approach in Swanson (1974) to estimate errors in tidal datums when 

computed at short-term stations located between NWLON stations. Errors in determination of 

tidal datums at short-term stations through the method of simultaneous comparison (NOS, 2003) 

are known to be generally correlated with geographic distance from the control station and with 

difference in range of tide and time of tide between control and subordinate stations. There are 

other important considerations necessary for specific project implementation that are discussed 

later in this report in the section on Limitations. Operationally for hydrographic surveys, NOAA 

uses the International Hydrographic Organization (IHO) error budget (see NOS Specifications 

and Deliverables, 2007) to construct error budget analyses for the tides component to the total 

error. These analyses are used to determine the number and location of subordinate stations 

required to obtain tide reducers for survey operations for specific areas. The error budget 

considers measurement error, datum computation error, and tidal zoning (extrapolation) error 

sub-components. Datum uncertainties need to be on the order of 0.10 ft. for the error budget to 

be within desired surveying specifications. The 0.10 ft. is driven by marine boundary and coastal 

engineering applications and other applications, such has hydrography that have less stringent 

needs. The NWLP strives for the highest accuracy because of the multiple applications. 
7 

The estimates of relative sea level trends found in Zervas (2001) are operationally used to 

estimate errors in tidal datum elevations if sea level trends are not properly taken into account 

and are used to assess the need to update to a new National Tidal Datum Epoch (NTDE) time 

period. NTDE periods are assessed for potential update on a nation-wide basis every few 

decades based on analyses of relative sea level trends. For marine boundary purposes and 

delineation of the location of the mean high water line, the surveying user community desires 

known elevation points to the 0.10 ft. uncertainty. This is because a very slight slope in the 

beach or water front surface will result in a boundary line location different by several feet in the 

horizontal if the vertical elevation is in error or not precise. Real-time navigation users are now 

interested in more accurate water levels relative to accurate chart datum and channel depth 

reference systems because larger and larger vessels with deeper drafts are now coming into most 

ports. Elevations previously at the several tenths of a foot uncertainty are now desired to the 

nearest 0.10 ft. for marine operations. 

THE SWANSON ERROR ANALYSIS REPORT 

Swanson (1974) performed an error analysis for determining tidal datums from short-term 

observations. Using the comparison of simultaneous observations method, Swanson developed 

datum uncertainties at 1, 3, 6, and 12 month time periods based on comparisons between 

NWLON station pairs proceeding along the coast. One NWLON station was selected as control, 

the other as subordinate. The resulting datums for the shorter time periods were compared to the 

accepted values based on a NTDE. His analyses of these differences resulted in the generalized 

accuracy estimates for tidal datums determined at short-term stations for the East Coast, West 

Coast, and Gulf Coast (see Table 1) 

Table 1. Generalized accuracy of tidal datums from short series of observation; based on one 

standard deviation (one-sigma) uncertainty level (from Swanson 1974). 

Series Length 

(months) 

East Coast Gulf Coast West Coast 

1 0.13 ft 0.18 ft 0.13 ft 

2 0.10 ft 0.15 ft 0.11 ft 

3 0.07ft 0.12 ft 0.08 ft 

4 0.05 ft 0.09 ft 0.06 ft 

The uncertainties of datums for Gulf Coast stations are generally higher because of the low 

amplitude tidal signal in that area and the relatively larger effects of weather on the water levels 

than the East and West Coasts. These generalized accuracy estimates have been used 

operationally for error budgets and error estimates for CO-OPS tidal datum products since the 

report was issued in 1974. It is recognized that these are regional in nature and are also 

expressions of maximum errors as subordinate stations are typically installed between NWLON 

stations, thus shortening the geographic and tidal distances between control and subordinate 

pairs. Because of these constraints, the Swanson regional pooled analysis does not provide a 

good technique for operational purposes to estimate errors at the resolution needed for precise 
8 

locations of interest. The Swanson estimates cannot be easily used to describe a radius or extent 

of coverage of a particular NWLON station. 

THE BODNAR REPORT

In applied research performed by Bodnar (1981), multiple curvilinear regressions equations 

estimating the accuracy of computed datums were developed using a regression analysis of the 

standard deviations found in the Swanson (1974) report. Bodnar’s analyses effectively 

determined which independent variables related to differences in tidal characteristics might 

explain the variations in the Swanson standard deviations using the standard deviations as the 

dependent variables. Table 2 summarizes the independent variables that proved to be highly 

significant and displays them in equation form with the slope coefficients for each variable 

produced by the regression model. Bodnar noted deficiencies of his approach in the sample 

size, interdependence of station pairs, and statistical population representation. Bodnar also 

developed formulas for Mean High Water (MHW). For purposes of this study, the formulas for 

Mean Low Water were adopted for use because the low water differences express the effects of 

shallow water and bottom friction better than MHW. 

Table 2. The regression equations and parameters for estimating uncertainties in tidal datums 

for Mean Low Water (from Bodnar, 1981)

 S1M = 0.0068 ADLWI + 0.0053 SRGDIST + 0.0302 MNR + 0.029 

 S3M = 0.0043 ADLWI + 0.0036 SRGDIST + 0.0255 MNR + 0.029 

 S6M = 0.0019 ADLWI + 0.0023 SRGDIST + 0.0207 MNR + 0.030 

 S12M = 0.0045 SRSMN + 0.128 MNR + 0.025 

Where: 

S is the standard deviation (in feet), 

M is the number of months of subordinate station observation, 

ADLWI is the absolute time difference of the Low Water Intervals between control and 

subordinate stations (in hours), 

SRGDIST is the square root of the geographic distance between control and subordinate stations 

(in nautical miles), 

MNR is a mean range ratio that is defined as the absolute value of the difference in mean range 

between control and subordinate stations divided by the mean range of tide at the control station 

(using range values in feet), and 

SRSMN is the square root of the sum of the mean ranges at the control and subordinate stations 

(in feet). 
9 

TECHNICAL APPROACH 

For purposes of this NWLON study, the target value of datum uncertainty of 0.12 ft (95% 

confidence interval) has been selected for determination of the extent of coverage for datum 

determination for each NWLON station. This target value would ensure the accuracy of datum 

determination at subordinate locations will meet most user requirements. The study identifies the 

geographic region for each NWLON station within which a datum computation at a subordinate 

station with a 3-month time series will be accurate to less than or equal to 0.12 ft. Using GIS 

derived polygons, areas determined to contain no NWLON coverage are identified as gaps for 

consideration of new priority NWLON station requirements. Error analysis using a 3-month 

time series was selected as it is the typical length of time a subordinate station is operational for 

NOAA shoreline and hydrographic surveys and for outside users such as the US Army Corps of 

Engineers. 

Table 3 is an example of the error calculation for the NWLON station at Grand Isle, Louisiana. 

Each of the parameters required are entered into the spreadsheet for a set of stations near the 

location of the NWLON station. They are obtained from the CO-OPS GIS historical tide station 

data table (Allstations), which also provides latitude and longitude information. The Allstations 

table has been populated in support of using GIS tools for tidal zoning and reflects the most 

recent record of operational and historic water level stations. Geographic distance is estimated 

using the GIS measure tool. 

The basic equation being used in the spreadsheet calculation is: 

 1) S3M = 0.0043 ADLWI + 0.0036 SRGDIST + 0.0255 MNR + 0.029. 

Equation 1 shows that the error in a datum computation at a 3-month long subordinate station is 

dependent upon the difference in time of low waters between control and subordinate (first 

term), geographic distance from the control to the subordinate (second term), and ratio of the 

mean ranges of tide (third term) (see table 2). The values of the coefficients for each term show 

the relative weight of each of the terms. The coefficients for time difference and geographic 

distance are about the same and both are much less that the coefficient for the range ratio term. 

Thus differences in range of tide between control and subordinate contribute more to the error 

than time difference or geographic distance. The last term in equation 1 is a constant of 0.29 ft. 

It represents the error in the datum because the time series at the subordinate is only 3-months 

long instead of 19-years. It is the remaining error if the difference in time of low waters was 

zero and the subordinate station was co-located with the control station. 

For Gulf Coast tide stations the tides are predominantly diurnal (one high tide and one low tide 

per day). Values for Greenwich high and low water intervals are only computed when there are 

two high and two low waters each tidal day, and are therefore not computed for diurnal stations. 

A substitute for difference in time of tide is derived from the difference in tropic time intervals 

(TcLLWI) derived from harmonic analyses at each of the stations. The values for diurnal range 

of tide (GT) are used as a substitute for mean range of tide (MN) in the Gulf as well because GT 

is a much better technical description of the full daily range of tide for diurnal tides. These 

values are readily available from the CO-OPS historical tide station GIS table as these values are 

used for tidal zoning and planning tide support for hydrographic and shoreline surveys. 
10 

Figure 4 is a map showing the location of the NWLON station at Grand Isle, LA, the locations of 

historical stations used in the datum error analyses, and the locations of the stations for which the 

error equation results in a value of <=0.06 ft.(one standard deviation) or <= 0.12 ft. 95% 

confidence level. Offshore data points are located at the intersection of the co-range and co-time 

lines, thus providing data values and a distance to be entered into the Bodnar equation 

spreadsheet (Table 3). Co-range lines (GT) are shown in blue and co-time lines (TcLLWI) are 

shown in green. 

Figure 5 is a GIS polygon drawn using the data points in Figure 4 as a guide. This polygon is the 

estimated spatial representation of datum coverage for Grand Isle, LA for datum computation at 

nearby subordinate stations. Grand Isle would provide less accurate control for datum 

determination for stations outside this polygon, unless the subordinate stations were left in for 

one-year or longer. The polygon was manually constructed using a GIS drawing tool and visual 

interpolation between the numerical value of the error assigned to each location. 

Table 3. An example of a Bodnar Equation error analysis spreadsheet for a three-month datum 

comparison using Grand Isle, LA as control.

Grand Isle Control Accepted Values TCLLWI 14.327 hrs. GT 1.06 ft. 

 

 TCLLWI ADLWI DIST SQRDIST 

Sub. 

Range MNR S3M 

 hrs. n. miles ratio ft. LAT LONG 

Southwest Pass 13.16 1.17 35.30 5.94 1.30 0.23 0.061 28.93 -89.42 

Pelican Island 14.30 0.03 19.00 4.36 1.12 0.06 0.046 29.27 -89.60 

Caminada Pass 14.54 0.21 6.33 2.52 0.99 0.07 0.041 29.21 -90.04 

Mendicant Island 15.57 1.24 3.80 1.95 1.00 0.06 0.043 29.32 -89.98 

Billet Bay 16.33 2.00 12.60 3.55 1.02 0.04 0.051 29.37 -89.75 

Hackberry Bay 16.49 2.16 9.20 3.03 0.90 0.15 0.053 29.41 -90.04 

St Marys Point 15.94 1.61 10.20 3.19 1.00 0.06 0.049 29.43 -89.93 

Bay Rambo 18.37 4.04 11.30 3.36 0.73 0.31 0.066 29.36 -90.15 

Bayou St. Dennis 19.98 5.65 14.50 3.81 0.80 0.25 0.073 29.50 -90.02 

Port Fourchon 14.32 0.01 16.30 4.04 1.26 0.19 0.048 29.12 -90.21 

Shell Oil, East Bay 13.64 0.69 36.40 6.03 1.32 0.25 0.060 29.06 -89.04 

East Timbalier Is. 15.20 0.87 20.90 4.57 1.33 0.25 0.056 29.08 -90.29 
11 

(Table 3 continued) 

 TCLLWI ADLWI DIST SQRDIST 

Sub. 

Range MNR S3M 

 hrs. n. miles ratio ft. LAT LONG 

Leeville 17.31 2.98 13.80 3.71 0.88 0.17 0.060 29.56 -90.21 

Golden Meadow 21.05 6.72 17.60 4.20 0.56 0.47 0.085 29.38 -90.27 

Bayou Petit Caillou 15.21 0.88 37.50 6.12 1.29 0.22 0.060 29.19 -90.66 

Cocodrie 16.05 1.72 36.80 6.07 1.05 0.01 0.058 29.24 -90.66 

Four Island Bayou 17.90 3.57 43.10 6.57 1.06 0.00 0.068 29.24 -90.78 

Bayou Dulac 19.58 5.25 39.60 6.29 0.38 0.64 0.091 29.46 -89.79 

Pointe Au Chien 19.38 5.05 26.80 5.18 0.45 0.58 0.084 29.42 -90.45 

South Pass 12.91 1.42 45.60 6.75 1.22 0.15 0.063 28.99 -89.14 

Pilot Town 15.13 0.80 38.10 6.17 1.00 0.06 0.056 29.18 -89.26 

Venice 21.50 7.17 31.90 5.65 0.98 0.08 0.082 29.27 -89.36 

North Pass 13.58 0.75 47.70 6.91 1.10 0.04 0.058 29.21 -89.04 

Offshore Point 1 13.80 0.53 12.00 3.46 1.40 0.32 0.052 29.10 -90.10 

2 13.90 0.43 34.20 5.85 1.40 0.32 0.060 29.00 -90.53 

3 13.90 0.43 56.20 7.50 1.40 0.32 0.066 28.83 -90.91 

4 13.70 0.63 23.30 4.83 1.30 0.23 0.055 28.93 -90.20 

5 13.50 0.83 19.30 4.39 1.30 0.23 0.054 28.94 -89.91 

6 13.20 1.13 26.80 5.18 1.30 0.23 0.058 28.94 -89.60 

7 13.10 1.23 68.40 8.27 1.20 0.13 0.067 28.24 -89.40 

8 13.90 0.43 48.60 6.97 1.50 0.42 0.067 29.02 -90.85 

Figure 4. The locations of the historical tide stations and offshore points used in the Bodnar 

equation analysis for Grand Isle. The locations within the 0.060 ft. cutoff are highlighted in blue.
12 

Figure 5. The GIS error polygon for Grand Isle, Louisiana depicting the geographic area of 

coverage for datum determination at short-term tide station.

Figure 6 below, shows the polygon analysis for other stations in the region near Grand Isle. The 

overlap and the lack of overlap in coverage polygons is readily apparent. This analysis has been 

repeated around the U.S. coastline, excluding the remote pacific ocean islands and the U.S. Great 

Lakes. 

Figure 6. Example of the GIS analysis results for nearby stations. 
13 

LIMITATIONS

There are limitations to the broad assumptions of the methodology used in this study. While this 

study assumes that the error will be mainly driven by the factored geographic distance, range 

ratio, and time difference determined by the Bodnar analysis, those assumptions are an over 

simplification in areas with extremely fasting changing tidal characteristics become extremely 

complicated within small distances. Examples are in a closely knit amphidromic point in which 

co tidal lines and the interplay of semidiurnal and diurnal tidal constituents and changes in tide 

types are not smoothly varying; in areas of tidal rivers where the effects of river flow need to be 

factored into error budget considerations; and in areas where the effects of meteorological 

forcing dominate the tidal signal (Parker, 2007). The actual planning of survey or project control 

requires a more detailed analysis for each project area than the broader analysis used for this 

gaps identification. 
14 

NWLON GAPS ANALYSIS FOR 

SOUTHEAST FLORIDA

THE RESULTS 

East Coast

1) Southern Biscayne Bay 

2) Outer Coast, Vicinity of Lakeworth and North Palm Beach, FL: There is difficulty in 

 maintaining a station in this gap. Former long-term stations at Lakeworth, Haulover Pier, 

 and Miami Beach Pier were all destroyed by storms or construction. 

3) Inner Bays, Indian River, FL: This is an area of transition from tidal on the outer coast to 

 non-tidal in the inner bays. 

Figure 7. NWLON gaps analysis for southeast Florida.
15 

 

4) Outer Coast, Vicinity of Flagler Beach, FL: The former NWLON station at St. Augustine 

 Beach filled this gap, but the pier no longer extends to deep water due to beach re-

 nourishment. 

5) Upper St. Johns River, FL 

6) Upper Satilla River, GA 

Figure 8. NWLON gaps analysis for northeast Florida and southern South Carolina.

NWLON GAPS ANALYSIS FOR 

NORTHEAST FLORIDA AND 

SOUTHERN SOUTH CAROLINA
16 

 7) Upper Edisto River, SC 

 8) Upper Cooper River, SC 

 9) South Santee River, SC 

10) Winyah Bay, SC

Figure 9. NWLON gaps analysis for South Carolina.

NWLON GAPS ANALYSIS 

FOR SOUTH CAROLINA
17 

11) Upper Cape Fear River, NC 

12) New River, NC 

13) Bogue Inlet/Sound, NC 

14) Cedar Island, Southern Pamlico Sound, NC 

15) Western Pamlico Sound, NC 

16) Albemarle Sound, NC 

Note that for gap areas 12 though 16 represent areas of transition from tidal on the outer coast to 

non-tidal in the inner bays. 

Figure 10. NWLON gaps analysis for North Carolina.

NWLON GAPS ANALYSIS 

FOR NORTH CAROLINA
18 

17) Upper James River, VA 

18) Upper York River, VA 

19) Lower Chesapeake Bay Vicinity of Rappahannock Shoal, VA 

20) Upper Rappahannock River, VA 

21) Potomac River, MD/VA 

22) Upper Wicomico River, MD 

23) Upper Nanticoke River, MD 

24) Vicinity of Wye River, Eastern Bay, MD 

25) Upper Chester River, MD 

26) Havre de Grace, Upper Chesapeake Bay, MD 

Note gaps are generally at the upper ends of the tidal rivers in this region. 

Figure 11. NWLON gaps analysis for the Chesapeake Bay.

NWLON GAPS ANALYSIS 

FOR CHESAPEAKE BAY
19 

27) Chincoteague Bay, MD 

28) Isle of Wight and Assawoman Bays, MD 

29) Indian River, DE 

30) Maurice River, NJ 

31) Great Egg Harbor, NJ 

32) Barnegat Bay, NJ 

Figure 12. NWLON gaps analysis for Delaware Bay and New Jersey.

NWLON GAPS ANALYSIS 

FOR DELAWARE BAY AND 

NEW JERSEY
20 

33) Mid-Hudson River, NY 

34) Great South Bay, NY 

35) Inside Shinnecock/Moriches Bay, NY 

36) Southern Shore, Outer Coast, Long Island 

37) Western Peconic Bays, NY 

38) Eastern Long Island Sound, CT/NY 

39) Upper Connecticut River, CT 

40) Outer Coast, Cape Cod, MA 

Note: EC gaps 36 and 40 are problematic to try to establish new stations due to the high energy 

beaches and lack of existing infrastructure in the form of ocean piers. 

Figure 13. NWLON gaps analysis for New York and southern New England.

NWLON GAPS ANALYSIS FOR 

NEW YORK BIGHT AND SOUTHERN NEW ENGLAND 
21 

41) Upper Merrimack River 

42) Vicinity of Bellamy River, NH 

43) Upper Kennebec River, ME 

 

Figure 14. NWLON gaps analysis for northern New England.

NWLON GAPS ANALYSIS FOR 

NORTHERN NEW ENGLAND 
22 

Caribbean

The gaps analysis for the Caribbean (Figure 13) shows that with the addition of the recent 

NWLON station established as part of the tsunami warning system upgrade in the region, there 

are no gaps in NWLON coverage for Puerto Rico and the U.S. Virgin islands. The addition of 

the new NWLON stations established through the tsunami warning system upgrade results in 

several layers of redundancy in coverage for tidal datum purposes in the Virgin Island region. 

 

Figure 15. NWLON gaps analysis for the Caribbean exhibiting no existing gaps in coverage.

NWLON GAPS ANALYSIS FOR 

PUERTO RICO AND THE U.S. VIRGIN ISLANDS
23 

Gulf Coast

1) Laguna Madre, TX: This is an area of transition from tidal on the outer coast to non-tidal in 

 the inner bays. 

2) Corpus Christi Bay; Aransas Pass Inside, TX 

3) Outer Coast, Pass Cavallo 

4) Lavaca, Keller, Carancahua, Tres Palacios Bays, TX 

5) Matagorda, East Matagorda Bays, TX 

6) West Bay,TX 

7) Houston Ship Channel, TX: This is an area showing high rates of land subsidence. 

8) Upper Galveston Bay, TX 

9) East Bay, TX 

10) Upper Neches and Sabine Rivers, TX/LA 

Figure 16. NWLON gaps analysis for Texas.

NWLON GAPS ANALYSIS FOR TEXAS
24 

10) Upper Neches and Sabine Rivers, TX/LA 

11) Upper Vermillion and West Cote Blanche Bays, LA 

12) Upper Atchafalaya River Region, LA 

13) Houma Ship Canal, LA 

14) Entrance to Terrebonne Bay, LA 

15) Lake Salvador : This is an area of transition of tidal to non-tidal. 

16) Lower Mississippi River, LA: The river transitions from tidal to non-tidal in this region. 

17) Lake Borgne, LA 

18) Western Breton ; Chandeleur Sound. LA 

Figure 17. NWLON gaps analysis for Louisiana.

NWLON GAPS ANALYSIS FOR LOUISIANA
25 

19) Lower Pearl River and Vicinity, MS 

20) Upper Bay St. Louis, MS 

21) Upper Biloxi Bay, MS 

22) Pascagoula River, MS 

23) Offshore Ocean Springs, Vicinity of Horn Island, MS 

24) Weeks Bay, AL 

25) Wolf Bay, AL and Perdido Bay, AL/FL 

Figure 18. NWLON gaps analysis for the Mississippi and Alabama coasts.

NWLON GAPS ANALYSIS FOR 

MISSISSIPPI AND ALABAMA COASTS
26 

25) Wolf Bay, AL and Perdido Bay, AL/FL 

26) Choctawhatchee Bay, FL 

27) Apalachee Bay, St. George Sound, FL and Vicinity 

Figure 19. NWLON gaps analysis for northwest Florida.

NWLON GAPS ANALYSIS FOR NORTHWEST FLORIDA
27 

28) Charlotte Harbor, FL 

29) Vicinity and Outer Coast of Venice, FL 

30) Chokoloskee, FL 

31) Cape Sable, FL 

32) Lower Keys (Gulf of Mexico side) and Vicinity, FL 

33) Northern Florida Bay, FL: This bay is a region of transition form tidal to non-tidal. 

Figure 20. NWLON gaps analysis for southwest Florida. 

NWLON GAPS ANALYSIS FOR 

SOUTHWEST FLORIDA
28 

West Coast

1) Tijuana Slough NERRS, CA 

Figure 21. NWLON gaps analysis for southern California.

NWLON GAPS ANALYSIS FOR 

SOUTHERN CALIFORNIA
29 

2) Southern south San Francisco Bay,CA 

3) Stockton River Delta, CA 

4) Sacramento River Delta, CA: This portion of the river transitions from tidal to non-tidal. 

Note: The Lower Sacramento River Delta and the Stockton River Delta, CA are both areas of 

land subsidence. 

Figure 22. NWLON gaps analysis for central California.

NWLON GAPS ANALYSIS FOR 

CENTRAL CALIFORNIA
30 

NWLON GAPS ANALYSIS FOR 

NORTHERN CALIFORNIA 

AND SOUTHERN OREGON

Figure 23. NWLON gaps analysis for northern California and southern Oregon 

exhibiting no existing gaps in coverage.
31 

5) Upper Columbia River, OR/WA 

6) Olympia, Budd Inlet, WA 

Note: The upper Columbia River uses a different Chart Datum than MLLW called Columbia 

River Datum (CRD) 

Figure 24. NWLON gaps analysis for the northwest coast and Puget Sound.

NWLON GAPS 

ANALYSIS FOR 

THE NORTHWEST 

COAST AND PUGET 

SOUND
32 

NWLON GAPS ANALYSIS FOR 

SOUTHEAST ALASKA

Alaska

1) Craig, Bucareli Bay, AK 

2) Snow Passage, AK 

3) Frederick Sound, AK 

4) Glacier Bay, AK 

5) Entrance to Dry Bay, AK 

Note: AK gaps 2,3, and 4 are in areas undergoing rapid uplift due to post-glacial rebound. 

Figure 25. NWLON gaps analysis for southeast Alaska.
33 

NWLON GAPS ANALYSIS FOR

THE NORTHERN GULF OF ALASKA

6) Cape St. Elias, Controller Bay, AK 

7) Cook Inlet Entrance, AK 

8) Tuxedni Bay, AK 

9) Kamishak Bay, AK 

10) Shelikof Straits, AK 

Figure 26. NWLON gaps analysis for the northern Gulf of Alaska.
34 

NWLON GAPS 

ANALYSIS FOR THE 

EASTERN ALEUTIAN 

ISLANDS, ALASKAN 

PENINSULA, AND 

BRISTOL BAY

11) Port Wrangell to Chignik Bay, Alaska Peninsula 

12) Aleutian Islands, South Side, Ugamak Strait (Unimak Island) to Unalaska Island 

13) Aleutian Islands, South Side, Unalaska Island to Atka Island 

14) North Side Unimak Island 

15) Port Moller, Bristol Bay 

16) Kvichak Bay Vicinity 

17) Nushagak Bay 

18) Hagemeister Island Vicinity 

19) Pribilof Islands and Vicinity 

20) Kuskokwim Bay 

Not shown in Figure – western Aleutian Islands 

21) Amchitka Island 

22) Attu Island 

Figure 27. NWLON gaps analysis for the eastern Aleutian Islands, Bristol Bay and Alaskan 

Peninsula. 
35 

NWLON GAPS ANALYSIS FOR 

THE CHUKCHI SEA AND NORTH SLOPE, ALASKA 

23) Yukon River Delta 

24) Eastern Norton Sound 

25) Eastern Kotzebue Sound: This area transitions form tidal to non-tidal. 

26) Bering Straits 

27) Chukchi Sea, Pt Hope to Pt. Barrow 

28) Pt. Barrow to Prudhoe Bay 

29) Prudhoe Bay to Canadian Border 

Figure 28. NWLON gaps analysis for the Chukchi Sea and North Slope, Alaska.
36 

NWLON GAPS ANALYSIS FOR HAWAII

Hawaii

1) Southeast Point of Hawaii Island, HA 

2) South shore of Kaho O’Lowe Island, HA 

Figure 29. NWLON gaps analysis for Hawaii.
37

SUMMARY

A deterministic approach to estimating the areas of NWLON coverage for datum determination 

at nearby subordinate tide stations has been developed. The approach uses the basic error 

analyses of Swanson (1974) and the regression error analyses of Bodnar (1981) to estimate 

regions of coverage for each individual NWLON station. Using GIS tools, the information is 

displayed on maps of coverage polygons. The GIS output is then used to identify geographic 

areas that represent gaps in the NWLON. The datum error polygons can be used for multiple 

purposes for short-term and long-term management of the NWLP. This analysis is being used 

for strategic planning and prioritization of locations to establish new NWLON stations as the 

network grows towards the optimum number of stations. It is being used to make decisions 

regarding utilization of resources for the importance of bringing an NWLON station back on line 

immediately or if a nearby station can be used effectively as a back-up until reconstruction can 

take place. The analysis results identified approximately 113 gaps in NWLON coverage beyond 

the 200 station deployed as of FY2007. Forty-three (43) gaps are located along the east coast, 33 

in the gulf coast, 6 gaps on the west coast, 29 gaps in Alaska, and 2 in Hawaii. 

The technical approach is also being used to make operational decisions for optimal locations to 

establish stations for hydrographic and shoreline survey support. A significant advancement is 

that this approach will now be used as a replacement for the broad regional generalized 

accuracies of Swanson with location specific estimates of errors for datum determination at 

subordinate stations. This effort will allow for more precise error estimates to be input to the 

total error budgets for all applications. 

The 113 NWLON gaps, added to the existing 200 NWLON stations, results in an approximate 

target NWLON of 313 stations (excluding any Great Lakes analysis that would identify new 

stations). As a next step towards adding additional stations to the NWLON, the GIS layers 

created by this effort will be used to evaluate the locations of other Federal, state, and regional 

water level observation networks for possible leveraging and partnership opportunities. NOAA 

has some key State and federal partners who are or have worked with CO-OPS to establish and 

operate their own local networks that closely match NOS operating standards. Examples are the 

Florida Department of Environmental Protection (FLDEP) and the Texas Coastal Ocean 

Observing Network (TCOON). In some instances, stations independently operated by these 

partners are located within NWLON gaps and CO-OPS will work with them to integrate those 

stations into the NWLON. Other potential partners include the US Army Corps of Engineers 

(USACE) and the US Geological Survey (USGS). 

ACKNOWLEDGMENTS

The authors wish to thank Allison Allen and Monica Cisternelli of CO-OPS and former CO-OPS 

employee, Kelly Stumbaugh, in the compilation of information for this report and appreciate all 

of the dedication and hard work of CO-OPS and predecessor organization employees that has 

resulted in a National Water Level Program that continues to serve the nation every day. 
a
39 

REFERENCES

Bodnar, A. Nicholas 1981. Estimating accuracies of tidal datums from short term observations, 

draft report, Tides and Water Levels Division, National Ocean Survey, 44 pp. 

Coordinating Committee, 1995. Establishment of International Great Lakes Datum (1985), by 

The Coordinating Committee on Great lakes Basic Hydraulic and Hydrologic Data, December 

1995, 17 pp. 

Shinkle, K.D. and Dr. R. K. Dokka (2004). Rates of vertical displacement at benchmarks in the 

lower Mississippi valley and the northern gulf coast, NOAA Technical Report NOS/NGS 50, 

U.S Department of Commerce, NOAA, National Ocean Service, 135pp. 

NOS, 1986. A review and analysis of operational stations of the national water level observation 

network, internal NOS report to Paul M. Wolff, Assistant Administrator for Ocean Services and 

Coastal Zone Management, 14 pp plus appendices. 

NOS, 2001. Tidal datums and their applications, NOAA Special Publication, NOS CO-OPS 1, 

National Ocean Service, NOAA, 111 pp. 

NOS, 2003. Computational techniques for tidal datums handbook, NOAA Special Publication 

NOS CO-OPS 2, National Ocean Service, NOAA, 90 pp. 

NOS, 2003. NOS Hydrographic survey specifications and deliverables, March 2003 edition, 

Office of Coast Survey, NOS, NOAA, 102 pp. 

Marmer, H.A., 1951. Tidal datum planes, NOAA National Ocean Service Special Publication 

No. 135, U.S. Coast and Geodetic Survey, U.S. Govt. Printing Office, revised ed., 1951 

Parker, Bruce P., 2007. Tidal Analysis and Prediction, NOAA Special Publication NOS COOPS 3, U.S. Department of Commerce, NOAA, NOS, CO-OPS, Chapter 2, p 48-69. 

Swanson, Robert L., 1974. Variability of tidal datums and accuracy in determining datums from 

short series of observations, NOAA Technical Report NOS 64, National Ocean Survey, NOAA, 

41 pp. 

USACE, 2006. Performance Evaluation of the New Orleans and Southeast Louisiana Hurricane 

Protection System, Draft Final Report of the Interagency Performance Evaluation Task Force, 

Volume II – Geodetic, Vertical and Water Level Datums, U.S. Army Corps of Engineers 

Headquarters, 2006. 

Zervas, Chris E., 2001. Sea level variations of the United States 1854-1999, NOAA Tech. Rep. 

NOS CO-OPS 36, National Ocean Service, NOAA, 186 pp. 
40 
41 

NATIONAL WATER LEVEL 

OBSERVATION NETWORK

-MAINE TO NORTHERN NEW JERSEYAPPENDIX 1. National Water Level Observation Network (NWLON) Station 

Maps as of February 2008 
42 

NATIONAL WATER LEVEL 

OBSERVATION NETWORK

-SOUTHERN NEW JERSEY TO GEORGIA- 
43 

NATIONAL WATER LEVEL 

OBSERVATION NETWORK

-FLORIDA AND ALABAMANATIONAL WATER LEVEL 

OBSERVATION NETWORK

--TEXAS, LOUISIANA AND MISSISSIPPI- 
44 

-PUERTO RICO AND THE U.S. VIRGIN ISLANDSNATIONAL WATER LEVEL 

OBSERVATION NETWORK

NATIONAL WATER LEVEL 

OBSERVATION NETWORK

-HAWAIIAN ISLANDS- 
45 

NATIONAL WATER LEVEL 

OBSERVATION NETWORK

-CALIFORNIA- 
46 

NATIONAL WATER LEVEL 

OBSERVATION NETWORK

-WASHINGTON AND OREGON- 
47 

NATIONAL WATER LEVEL 

OBSERVATION NETWORK

-ALASKAN PENINSULA 

AND

ALEUTIAN ISLANDSNATIONAL WATER LEVEL 

OBSERVATION NETWORK

-SOUTHEAST ALASKA- 
48 
49 

APPENDIX 2. National Water Level Observation Network (NWLON) Station 

Configuration 

All NWLON stations maintain a high degree of accuracy and reliability, and are considered 

“multipurpose”, providing both high rate, real-time data, and long-term sea level trends. 

NWLON station construction is quite robust, and great care is taken in obtaining long-term 

continuous and valid water level data. Many stations have now been operating continuously for 

over 100 years. The tide station at San Francisco has been continuously operating for over 150 

years. The tide houses used to house the equipment are designed to last 30-40 years and 

underwater components are designed to withstand harsh coastal wave and current environments. 

Tide station platforms may be elevated to withstand storm surge. In colder regions, station 

configurations are designed to withstand ice conditions. All stations have an associated network 

of bench marks that are surveyed annually to ensure vertical stability of the gauge, and preserve 

a consistent data record in case of slow or sudden vertical movement due to pier deterioration, 

earthquakes, glacial rebound, ship/dock collisions, or station destruction by coastal storms. If 

destroyed, a new station can be established relative to the same vertical reference datum using 

the established bench mark elevations. Differential leveling is done on a yearly basis between 

the water level sensor and the bench marks to ensure vertical stability of the sensor relative to the 

land and to ensure the bench marks are vertically stable among themselves. Efforts are also 

underway to systematically connect all long-term tide stations to the National Spatial Reference 

System (maintained by NOAA’s National Geodetic Survey) where relationships do not currently 

exist between local tidal datums and geodetic vertical datums, such as the North American 

Vertical Datum of 1988 (NAVD88). Annual routine Operations and Maintenance (O&M) is also 

performed at each NWLON station to make any minor necessary repairs, upgrades, and/or to 

recalibrate the sensors as necessary. Figure A2-1 illustrates a common NWLON station 

configuration that features both acoustic primary and pressure backup sensors, which is found in 

many locations, particularly along the East and Gulf coasts, and in the Caribbean. Many Alaska 

NWLON primary gauge configurations feature a dual orifice pressure sensor configuration 

because the severe effects of the ice can destroy the acoustic sensors’ protective wells and also to 

account for the fact that downward looking acoustic sensors cannot measure the correct water 

levels when the water is frozen or ice accumulates in the sounding tube. At each station one 

data collection platform (DCP) is designated as a primary, and the other as a backup. At some 

very remote locations a redundant set of DCPs and sensors are installed to reduce data loss. 

Lower precision strain-gauge pressure sensors are typically used for the backup system. This 

two-DCP configuration has several applications. Not only do the redundant observations limit 

the potential for data gaps associated with equipment malfunction, but in tsunami station 

configurations, the primary DCP records both 1 and 6 minute averaged water level values, and 

the backup DCP records 6-minute and 15-second averaged water level data which can be 

accessed following an event for modeling applications. The backup transducer may also be used 

to record extreme high water level events, which may exceed the height of the acoustic sensor. 
50 

Figure A2 -1. One common NWLON station configuration including acoustic 

and pressure water level sensors with a primary and backup DCP. (NOAA, 2001) 
51