# Presentation: 670024

## Tidal Datumsand TheirApplications

- Tidal Datumsand TheirApplications

*Thomas F. Landon, NOAA                                                            Center for Operational Oceanographic Products and Services (CO-OPS)*

## http://co-ops.nos.noaa.gov

## DATUM

**DATUM**
**(VERTICAL)**

**For marine applications: **
** ****a base elevation used as a reference from which to reckon heights or depths.                                                                                                                                               ****	**
**TIDAL DATUM: **
** ****defined by a certain phase of the tide (ex. MHW) **
** ****are local datums **
** ****in order that they may be recovered when needed, such ****	****datums are referred to fixed points known as BENCH ****	****MARKS**

**CHART DATUM**

**The tidal datum to which soundings on a nautical chart are ****	****referred.**
** ****USA chart datum = Mean Lower Low Water (MLLW)**
** ****For non tidal waters (including Great Lakes) chart datum is Low Water Datum (LWD)**

**Notes:**
Note: local datums should not be extended into areas which have differing hydrographic or topographic features without substantiating measurements
#= mouse click on all note pages
#=chart datum

## Types of Tides

**Types of Tides**

**Mixed Tide**

**Semidiurnal Tide**

**Diurnal Tide**

**Notes:**
These types of tides are what makes up a datum.  For instance, if you were to take the all the HHW’s for a month and calculate their mean you would get the MHHW monthly mean [once referenced to a corresponding control stn with a 19 yr mean you will get the datum.]  Say you want to know what the inequality is  for the high waters: subtract the MHHW from the MHW.  The next slde will show these kinds of relationships.

Todd’s stuff:
Semi Diurnal – Two high and two low tides per day.  Heights of the highs and
lows are relatively equal.  Occurs along the US East Coast

Diurnal – One high and one low tide per day.  

Mixed – A combination of the other two types of tides.  Will show a significant
difference in the heights of successive highs and lows.  

US West Coast is Mixed (Primarily Semi-Diurnal)  - Normally two highs and
lows per day, but occasionally can be only one.
US Gulf Coast is Mixed (Primarily Diurnal) – Normally one high and low per
day.  During Spring tides will become Semi-Diurnal.

## MHHW

**MHHW**

**Notes:**
Highest and Lowest Observed are not a tidal datum or a computation.
The acronyms can be found in our reference Glossary

Standard Method (MTL, MSL, Mn, DHQ & DLG are known):
MLW= MTL-(0.5*Mn)
MHW= MLW+Mn
MLLW= MLW-DLQ
MHHW= MHW-DHQ
DTL= 0.5*(MHHW+MLLW)
GT= MHHW-MLLW

MRR Method (MTL, DTL, MSL, Mn & GT are known):
MLW= MTL-(0.5*Mn)
MHW= MLW+Mn
MLLW= MLW-0.5*GT
MHHW= MHW+GT
DHQ= MHHW-MHW
DLQ= MLW-MLLW

GT is known as the spring or diurnal range depending on what coast it is on.

## Slide 6

## Function	Application

**THE NATIONAL WATER LEVEL AND TIDAL CURRENT PROGRAMS**

_***Function***_***	***_***Application***_
**Navigation/Safety****	****Real-Time Navigation,**
**	****Hydrographic Surveys, **
**	****Airborne Hydrography, ****	****	****HAZMAT Response ...etc.**
**Environmental Assessment/****	****Wetland Delineation, Coastal**
**Monitoring****	**** Zone Management; Enforcement,**
**	****Insurance, Storm Surge, **
**	****Tsunami Warning...etc.**
**Basic & Applied Research****	****Modeling, Wetland Studies, ****	****Global Warming Studies, **
**	****Sea Level  Change, Estuarine/**
**	****Hydrodynamic Studies...etc.**

**Notes:**
This slide explains itself...

## Datum’s Boundary Applications

**Datum’s Boundary Applications**

**Notes:**
Boundaries to talk about in order of graphic enhancements: 
#MLW 
#MHW
A coastal home owner would have more land if living in VA compared to MD owners
#MLLW (chart datum)
#territorial sea related to MLLW (12 n. mi.)
#exclusive economic zone related to MLLW (200 n. mi.)

## CORRECTIONS TO ECHO SOUNDINGS

**(HYDRO)**

**Notes:**
Tide correction is used to put echo soundings on the correct reference datum (MLLW).
These soundings are then used to update Notice to Mariners and chart products.

## TIDES SUPPORT TO NAUTICAL CHARTING

**TIDES SUPPORT TO NAUTICAL CHARTING**
**HYDROGRAPHY APPLICATIONS**

**HEIGHTS ARE REFERRED TO MHW**

**SOUNDINGS ARE REFERRED TO MLLW**

## Slide 11

## Slide 12

## National Water Level Observation Network (NWLON)

**175 Permanent Stations**

**Notes:**
All datums are based on this network of stations.
As Todd stated earlier, most of these stns have at least 19 yrs of data.
It is these stations that are used to calculate a Tidal Datum Epoch.
What is a ...?  Next slide...

## NATIONAL TIDAL DATUM EPOCH

**A specific 19 year period that includes the longest periodic tidal****	****	****	****	****	****variations caused by the astronomic tide-producing forces.**
**Averages out long term seasonal meteorological, hydrologic, ****	****and ****	****oceanographic fluctuations.**
** ****Provides a nationally consistent tidal datum network (bench ****	****marks) by accounting for seasonal and apparent environmental ****	****trends in sea level that affects the accuracy of tidal datums.**
**The NWLON provides the data required to maintain the epoch                                                                                                                                                                                        ****	****and make primary and secondary determinations of tidal ****	****datums.**

**NATIONAL TIDAL DATUM EPOCH**

**Notes:**
To begin to understand datums, you need to understand what an epoch is and why we have them.  
Epoch is how long it takes for the Sun, Moon, & Earth complete epoch tidal cycle
This takes 18.6 yrs, so we use 19 yrs (because statistically the 0.4 is not significant out of the 19 yr avg.)
What this means is that the NOS has a standard for creating 19 year means.

## SEATTLE, PUGET SOUND, WA

**VARIATIONS IN MEAN RANGE OF TIDE: 1900 - 1996**

**Notes:**
 MN= MHW - MLW diff
this is the response of tide at Seattle to the 19 yr avg (i.e. notice the recurring frequency throughout the years)
note the scales on the Y-axis.
Monthly declination changes higher or lower depending on the position of the moon; we call this the precession of the moons nodes.
Reason epoch’s were created was to compensate for the precession

## IDEALIZED CHANGE OF TIDAL EPOCH

**ACTUAL**

**Notes:**
We need a change in Epoch to keep water levels and in turn depths up to date.... If we had continued on the 41-59 epoch we would be nearly half a foot off or more in certain areas such as Louisiana, Texas, and Alaska

## RELATIVE SEA  LEVEL CHANGE AT SEVERAL LOCATIONS IN THE U.S.

**Notes:**
AK MSL’s are dropping because the land is tectonically active and rising
TX MSL’s are rising because the land is sinking due to oil drilling and such
this will be discussed over the next few slides.

## Slide 18

**Notes:**
#rising 1-3
#rising 3-5&5-10
#falling all
What do we see nationally?  
MSL on east coast is rising while west is mix depending on the tectonic location.  
Note, almost everywhere in CA is rising with the exception of LA 
remember falling MSL= land rising
                    rising MSL= land falling
AK is falling at least 5mm/yr more like 8 or more/yr

## Slide 19

## 60-78 Epoch

**60-78 Epoch**

**83-01 Epoch**

- MSL Linear Trend

**MSL= 1.542**

**MSL= 1.490**

**0.051 m**

**Epoch Change**

## 0.000m

**0.000m**

**0.016m**

**0.038m**

**0.042m**

**0.044m**

**0.057m**

**0.051m**

**0.050m**

## NORTH

**SOUTH**

## Slide 23

**Notes:**
What do we see in the Ches. Bay? 
MSL is rising (land is sinking) therefore leaving more water for navigation.  
Slide is of Tangier Island, VA.

## National Water Level Observation Network (NWLON)

**175 Permanent Stations**

**Notes:**
All datums are based on this network of stations.
As Todd stated earlier, most of these stns have at least 19 yrs of data.
It is these stations that are used to calculate a Tidal Datum Epoch.
What is a ...?  Next slide...

## STATION HIERARCHY

**STATION HIERARCHY**

**SECONDARY TIDE STATION: **
** ****minimum of 1 yr but less than 19 yrs**

**PRIMARY TIDE STATION: **
** ****minimum of 19 years **
** ****allows statistical and datum control for short term stations**

**TERTIARY TIDE STATION:**
** ****more than 30 days but less than a year**

**Notes:**
#primary stn
#secondary stn
#tertiary stn
The main thing to remember here is that all other stations are based on the primary
secondary’s are usually long-term hydro or special project stations that could eventually become a primary
tertiary are mostly short-term hydro stations that are in and out in one season
datum’s are computed for the short term stns using the primary as the control.

## Slide 26

**Notes:**
#Alameda datum MTL 
By doing a simultaneous comparison of the phase of tide (MTL in this case) between the primary (SF) and the secondary (Alameda) we can apply a 19yr mean (epoch) to the secondary.
The red line shows SF’s 19 year MTL
Now, to relate the red to the blue line you take the differences between the two
By applying this difference to the one year datum of Alameda (the black line) we get the purple line as the result
The purple line is Alameda’s one year datum for MTL as referenced to SF

## GENERALIZED ACCURACY OF TIDAL DATUMS 

**FOR SHORT SERIES (BASED ON ONE STD DEV)**
**       ****Series****	****       East****	****       Gulf****	****      West**
**       ****Length****	****       Coast****	****       Coast****	****      Coast**
**        **_**# Mo**__**	**__**         FT**__**	**__**         FT**__**	**__**        FT**_
**          ****1****	****	****        0.13****	****        0.18****	****       0.13**
**          ****3****	****	****        0.10****	****        0.15****	****       0.11**
**          ****6****	****	****        0.07****	****        0.12****	****       0.08**
**         ****12****	****	****        0.05****	****        0.09****	****       0.06**

**Notes:**
Sigma= std dev/statistics (standard deviation is a measure of how widely values are dispersed from the mean) 
basically these numbers tell us how close the MLLW of a short series is to the MLLW of a 19 yr series
diffs are due to shallow waters and meteorological events.

## Leveling and Datum’s

**Leveling and Datum’s**

**Notes:**
All tide stns have a benchmark network associated with them.  
The reason being that they are a stability check for the stn and the bm’s themselves. 
benchmarks are all related to where the acoustic sensor sits on the well.  
Some of the bm’s are part of NGS’s National Spatial Reference System (NSRS).  
From these bm’s we can derive a land based datum (NAVD88) that the tide can be related to. 
 BM’s are also related to CO-OPS’s final water level product called the “Published Benchmark Sheet.”  

## NOS BENCHMARK LEVELING

- Distances vary but usually several hundred meters.

**Notes:**
NOS uses a double run method
we basically hit each benchmark twice this way the mark is double checked for stability (a.k.a. closure).
Once per year during annual inspection for primary and secondary or before installation and after removal for tertiary.
We’re more interested in the vertical stability than the horizontal.

**Go to PC’s to show how to find this on the net**

## Relative to NAVD 88

**Relative to NAVD 88**

## Slide 31

## Summary

_**Tidal datums**_** are based on a phase of the tide (ex. MHW, MLLW).**
**Station datums are based on a 19 year mean called an National Tidal Datum Epoch (NTDE).**
**Tide stations are referenced to bench mark system for stability checks & long term maintenance.**
**Datum applications: PORTS, chart depths , marine boundaries, hydrographic surveys, dredging, storm surge, modeling, wetland restoration, etc.**

## Why Are We Here...?

## ships out there...

- There are REALLY

**BIG**

## QUESTIONS?