Dr. Laramie V. Potts Mr. Stephen Farrell...2.4 Fundamentals of Ocean Tides 2.5 Methods to Establish Local Datums for Engineering Projects 3 Depth Determination 3.1 Conventional (Manual)

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Dr. Laramie V. Potts

&

Mr. Stephen Farrell

New Jersey’s Science & Technology University NEWARK COLLEGE OF ENGINEERING

February 3, 2011

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COURSE DESCRIPTION This workshop is an introductory course in Hydrographic surveying. It is designed for surveyors, engineers, survey technicians, dredge operators, and hydrographers. The course focuses on theoretical principles of hydrographic surveying, project description, operation, and map production. Attendees will be able to 1) demonstrate a working knowledge of the principles and limitations of hydrographic surveying, 2) understand the fundamentals of project specifications, and 3) be able to execute a simple site survey from the conceptual stage to project implementation. The workshop is divided into four parts. Part 1 introduces the fundamentals of hydrographic surveying. Part II includes concepts of offshore positioning and geodetic control in the marine environment. Part III considers depth determination and errors in measurements. Part IV covers the design principles of a hydrographic survey campaign and data processing. Contents

1 Fundamentals of Hydrographic Surveying 1.1 Introduction 1.2 Disciplines Associated with Hydrography Surveying 1.3 Basic Measurements and Survey Equipment 1.4 Survey Standards/Specification er

2 Geodetic Control and Tidal Effects 2.1 Geodetic Reference and Coordinate Systems 2.2 Horizontal Positioning 2.3 Vertical Datum and Positioning 2.4 Fundamentals of Ocean Tides 2.5 Methods to Establish Local Datums for Engineering Projects 3 Depth Determination 3.1 Conventional (Manual) Methods 3.2 Acoustic Depth Determination Methods 3.3 Real-time Hydrographic Mapping

4 Field Operations and Surveying Procedures 4.1 Field Operations 4.2 Data Processing 4.3 Coverage and Resolution

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Fundamentals of Hydrographic Surveying Hydrography is the branch of applied science which deals with the measurement and description of the physical features of oceans, seas, coastal areas, lakes and rivers, as well as with the prediction of their change over time, for the primary purpose of safety of navigation and in support of all other marine activities, including economic development, security and defense, scientific research, and environmental protection.” International Hydrographic Organization – June 2009 1.1 Introduction More than half of the world’s population lives within 100 km of its shores. The effects of denser coastal population and accelerating climate change can be seen in degraded (and even disappearance of) ecosystems, coastal erosion, over-fishing, marine pollution, and higher vulnerability to marine disasters such as tsunami or volcanic activity. Marine environments (oceans, lake, rivers, swamps, wetlands) cover more than two-thirds of the Earth’s surface, and are not easily accessible to direct observations. In the past 20 to 30 years technological advances have allowed us to discover and map much more detailed coastal and ocean bathymetry and to delineate shore boundaries , mostly through acoustic remote sensing. Hydrography is that branch of physical oceanography that deals with measurement and definition of the configuration of the bottoms and adjacent land area of oceans, lakes, harbors, and other water bodies on Earth. Hydrographic surveying, in the strictest sense, is defined merely as the surveying of a water area; however, in modern usage it may include a wide variety of other objectives such as measurements of tides, currents, gravity, and the determination of physical and chemical properties of water. The principal objective of most hydrographic surveys that are conducted by large government agencies like the National Oceanic and Atmospheric Administration (NOAA) is to produce nautical charts and mapping. NOAA uses very large vessels to obtain basic data for the compilation of nautical charts with emphasis on features that affect safe navigation. Other objectives of NOAA include acquiring the information necessary to produce related marine navigational products for coastal zone management, engineering, and scientific investigations. Other government agencies such as the US Army Corp of Engineers (USACE), the Naval Oceanographic Office (NAVO), the US Geological Survey (USGS), are tasked with hydrographic surveys for a variety of purposes Some state and local agencies as well as the private sector also have hydrographic survey capabilities The US Army Corps of Engineers (USACE) is responsible to collect, process, and map hydrographic survey data for federally authorized civil and military navigation channels and shore protection projects throughout the US including Puerto Rico and the Virgin Islands. The main purpose of collecting hydrographic survey data is to be used by engineers and scientists to monitor channel shoaling conditions. Survey results in the form of a bathymetric map become a decision making tool for channel maintenance operations, channel deepening contracts, planning studies,

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environmental monitoring, near shore engineering designs, location ( and sometimes removal of obstructions such as sunken vessels, sediment transport modeling, and beach nourishment projects. Other objectives include volume computations for fair and equitable payment on dredging contracts. The overarching reason to perform hydrographic surveys is to ensure safe navigation conditions for all commercial and public users within the limits of the federal waterways. Hydrographic surveys are very complex in terms of (electronic) equipment integration, logistics on field operations and costs. On smaller scale local marine environments, the survey operations can be far less complex. Surveys conducted in shallow waters, lakes and rivers may invoke conventional (manual) surveying procedures.

Nautical Charting: Periodic hydrographic surveys must be performed to determine shipping channel conditions. Minimum controlling depths along with location of shoals and other critical information regarding safe navigation gets documented. Reports of Channel Conditions are accessible to waterway users.

Port and Harbor Operations: Survey data are required for effective management of water resources and harbor estuaries. Operations include maintenance dredging, debris removal for clear passage of vessels, environmental restoration, marine structural design, and many others. Coastal Geomorphology: Hydrographic surveys provide data for morphodynamic classification of coastal areas from sea state (breaking wave heights), bathymetry, tide regimes (F-factor computed from tide constituents). Coastal Engineering: Coastal mapping data is required for civil works projects such as revetments, jetties, and beach nourishments. Hydrographic survey data is used to understand various processes that shape the coastlines and human interaction with these processes. Coastal Zone Management: Hydrographic surveys provide data for coastal hazards and vulnerability assessment of coastal landscapes in relation to climate change, subsidence, glacial rebound, and others. Bathymetric data provide ancillary information on indicators that capture the biophysical conditions and morphodynamic classification. Offshore Resource Mapping: Offshore energy resources include wind, wave, and geologic mineral (oil, natural gas etc) deposits. Surveys and Geographic Information Systems are invaluable tools to identify the exploitation of these energy resources.

Hydrographic surveys support a variety of activities including:

• nautical charting, • port and harbor operations

(maintenance & dredging),

• coastal engineering (beach erosion and replenishment studies)

• coastal zone management • offshore resource mapping

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1.2 Disciplines Associated with Hydrographic Surveying Hydrography relies on a variety of scientific and engineering disciplines. Figure 1 illustrates the core disciplines like Geodesy, Photogrammetry, Cartography, Global Positioning System, Oceanography, Tides, Physics and Mathematics. These are the various disciplines that influence the science and products delivered by hydrographic survey.

Hydrography Geodesy Photogrammetry

Dive operations

Cartography

Oceanography

Survey Planning

Tides

AcousticsGlobal Positioning System

Figure 1. Disciplines that Influence the Science of Hydrography Geodesy is an interdisciplinary science which uses space-borne and airborne remotely sensed, and ground-based measurements to study the shape and size of the Earth, the planets and their satellites, and their changes; to precisely determine position and velocity of points or objects at the surface or orbiting the planet, within a realized terrestrial reference system, and to apply these knowledge to a variety of scientific and engineering applications, using mathematics, physics, astronomy, and computer science.

Oceanography is the scientific discipline concerned with all aspects of the world’s oceans and seas, including their physical and chemical properties, their origin and geologic framework, and the life forms that inhabit the marine environment. Traditionally, oceanography has been divided into four separate but related branches: physical oceanography, chemical oceanography, marine geology, and marine ecology. Physical oceanography deals with the properties of seawater (temperature, density, pressure, and so on), its movement (i.e., waves, currents, tides), and the interactions between the ocean waters and land surface waters (rivers and streams).

Remote Sensing: is the process of detecting and monitoring the physical characteristics of an area of the earth’s surface by measuring its reflected and emitted radiation at a distance from the targeted

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area. Various remote sensing techniques of remote sensing include photogrammetry, laser-ranging such as LIDAR (LIght Detection and Ranging), and RADAR (RAdio Detection And Ranging) commonly used for acquiring high-resolution land cover topographic data. Photogrammetry has two distinct branches of application including the metric branch which involves precise measurements and computations regarding the size, shape, and position of photographic features. The interpretive branch deals only with recognition and identification of the photographic features.

Cartography is the art, science and technology of making maps. The process of map-making often involves five steps, that is, selection of a number of features in the real world, classification of selected features into groups, generalization (including simplification) of jagged coastlines, exaggeration of features that are too small to show at the scale of the map, and symbolization to present the different classes of features chosen. The cartographic process greatly enhances the presentation of geographical information in graphic format. Digital cartography combines with GIS as an effective tool for coherent data analysis such as examining the relationship between two or more distributions - analogous to map interpretation from transparent overlays of conventional map separates.

Acoustics: is the study of the behavior of sound in water. Mapping of submerged topography (bathymetry) was made easier with the advent of electronic depth sounding equipment (echo sounder). The echo sounder works by transmitting sound waves toward the submerged topography (or ocean floor). A delicate receiver interprets the wave reflected from the bottom and a clock precisely measures the pulse travel time to a fraction of a second. Depth measurement by SOund Navigation And Ranging (SONAR) is modulated by environmental and chemical conditions of the water column.

1.3 Basic Measurement and Survey Equipment

The basic measurement for hydrographic surveys is depth measurements. Depth measurement during the pre-1920’s was very rudimentary. The photograph shows a surveyor who is handling a depth measurement or sample sound calibration measurements.

Planning and design of the hydrographic survey must produce an accurate and reliable chart

derived from sufficient data coverage. Figure 1.1 illustrates the basic elements for designing a survey. The design of the survey must produce an accurate and reliable chart derived from sufficient data coverage. For example, the production of the bathymetric contour depends on spatial resolution. By definition, the resolution (S) describes how close two objects can be and still be determined unambiguously.

Hydrographic surveying (for dredging operations or bathymetric mapping) involves synergy of three major surveying units. Three major components of hydrographic surveying include the marine vessel that carries the crew and supplies, the geodetic positioning technology, and the depth

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measuring equipment. 1.3.1 Marine Vessel The size and payload of the marine vessel depends on the extent of the survey project requirements. Surveys can be classified by vessel size -small scale (from wading to small boats), medium scale (using medium size boats and acoustic methods), and regional scale surveys using deep sea research vessels with state-of-the art multi-disciplinary data collection systems. Essential equipment list for each survey is as follows; A) Small Surveys:

1. Vessel: Oars, Life jackets, Gas tanks (minimum 2), extra oil, and 10 HP engine 2. Depth and Position: 50’ leadline, range poles, and plans. Survey equipment may include

Total Station Instrument (TSI), compensating level as required, prism pole with extension rods. Deeper water requires a fathometer and transducer installation.

3. Miscellaneous: Radio, 300 ft tape, Navigation chart, staff sheets, Batteries (2), repair kit, tool box

B) Medium Scale Surveys:

1. Vessel: 25-65 ft vessel, licensed operator. 2. Depth and Position: Echosounder with Transducer and aedquate power from batteries or

generator, tool box, transducers , GPS or TSI positioning, motion reference units (MRU) 3. Miscellaneous: A small vessel for the near-shore shallow water survey system to perform

as rover platform. C) Regional Scale Surveys:

1. Vehicle: 65 ft and larger research vessels , with competent crew and equipment. 2. Depth and Position: Multi-beam transducer and GPS. 3. Other Equipment: Cameras for stereo imaging (require positioning of frames) Integrated

multi-disciplinary data collection systems (e.g., gravity, magnetics), requires accurate in-ship surveys for sensor integration, calibration, and synchronization.

4. , 1.3.2 Positioning equipment Offshore positioning equipment has been revolutionized due to dramatic evolution in sensor technology and computer science. Traditional offshore equipment includes a sextant, transit, stadia, and an electronic distance measuring (EDM) device. Nowadays, several methods for horizontal positioning include optical, land-based electronic ranging, and space-based positioning. A basic method of positioning is the resection. However, the positioning methodology employed on any project will be evaluated based on site-specific conditions and project specification.

Figure 1.2: Acoustic depth measurement

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The preferred method of positioning for-offshore surveys is GPS. Wide- and narrow-lane GPS observations have proven to be the most efficient and cost effective for offshore hydrographic surveys. 1.3.3 Depth Sounding Equipment

A transducer initiates a sonic pulse. The sound wave propagates through the water and a receiver detects the return pulse. A basic technique of depth measurement is using a Single Beam Echo Sounder (SBES). An echo sounder performs the following operations;

• Transmit Sound • Measure round trip travel time. • Use sound speed to get distance

The depth (or distance) is computed from the two-way travel time as

The transducer interfaces with the depth sounder which outputs a profile of the bathymetry or “bottom return”. Two important components of depth sounding equipment include the frequency and the beam divergence (or cone angle).

Frequency

Most single frequency sonar units operate in the range of about 24 - 210 kHz (kilohertz). A few are dual frequency capable, meaning they can use both 50 and 200 kHz transducers. Typically, high frequency (192 or 200 kHz) sonar units provide the best resolution and definition of submerged structures and targets. 50 kHz units have much greater depth penetration capability, but show less definition. 24 kHz transducers also have a much wider cone angle than 192 or 210 kHz transducers.

It is critical to match the transducer's frequency to that of the sonar unit. For example, a 192 kHz sonar unit requires a 192 kHz transducer.

Acoustic Parameters (Instrument Specific)

Characteristics of echo sounders are determined by transducer; 1) Directivity 2) Beam width 3) Beam steering and side lobes

Table 1.0 lists the depth versus frequency for standard hydrographic sounding surveys. The signal frequency determines the range (distance) and sound penetration depth into sediments. The range is inversely proportional to the frequency:

2speedtimecenDista ×

=

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fRIMAGE

1∝ ,

and the penetration depth into sediment is inversely proportional to the frequency:

fPSEDIMENT

1∝ .

For bathymetric Echo Sounders (Phillipe, 2002) we Table 1:0

Property Depth (m) Frequency (kHz) Other <100 f ≥ 200 <1500 12 < f < 50 >1500 12 < f < 50 Sediment f < 8

Beam width depends on the acoustic wave and the size of the transducer

Cone Angle

A transducer's cone angle determines its footprint size and coverage area of the underwater world. The wider the cone angle the greater the footprint. For example, a 200 kHz transducer can have either a wide (20°) or narrow (3°) cone angle. A 24 kHz transducer may come with a standard 19° cone angle. Manufacturers also produce dual frequency echosounders that use 2 separate transducers for high or low frequency operations.. Generally, it is better to use a wide cone angle for finding minimum soundings in shallow to medium depths, since the echosounder will record the first return. The narrow cone will show greater structural detail (spatial resolution) due to its narrow beam.

The depth capability of a sonar unit depends on its transmitter power, receiver, sensitivity, frequency, transducer, and transducer installation. Other things that effect depth capability include:

1. water conditions and type (in general, SONAR will show greater depth readings in fresh water than in salt water), and

2. bottom conditions (mud, gravel, morphology, etc.).

The vertical structure of the (oceanic) water column is variable. The water column is stratified into various layers of various chemical composition. Accordingly, the sound velocity will

Water depth is the one data type that is common to all hydrographic surveys.

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change based on these varying conditions. Hydrographic surveying requires specific knowledge of the physical oceanography Seawater (or fresh water lakes and rivers) is a medium in which many hydrographic measurements take place for various engineering projects. Knowledge of seawater’s physical properties and of acoustic wave characteristics is important. Electromagnetic radiation (EMR) waves are excellent for propagation through the atmosphere or vacuum but hardly penetrate nor propagate in a liquid. Acoustic waves (sonic or ultra-sonic) achieve good penetration and propagation in a liquid. Factors affecting wave propagation include:

2. Temperature (T): sound velocity is a function of temperature field distribution. Variation of 1ºC causes a 4.5 m/s variation in velocity.

3. Salinity (Sy): is the measure of % of dissolved salts and other minerals (= ppt by volume) 4. Pressure (P) is a function of depth. The rate of change of velocity with depth is

approximately 1.6 m/2 per 10 atmosphere 5. Density (ρ) is a function of both temperature and pressure. The largest influence on density

is water compressibility with depth.

Other environmental and hydrodynamic processes effect the characteristics of the water column. Figure 1.3 illustrates most of the major processes that affect the water temperature and salinity variability of the water column.

• Precipitation and

evaporation • Longshore currents • Glacial melting • Surface water run-off • Water circulation • Solar heating (Steric) • Wind-induced Upwelling • Tide-induced mixing All these processes may not occur simultaneously although a few of them can have an appreciable affect on the acoustic depth measurements. Because of this variability, the sound velocity variability must be check frequency as the survey progress and the sea state changes. Several sound velocity calibrations measurements must be performed using a CTD

Figure 1.3 Processes that affect water temperature and salinity

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1.4 Survey Standards and Procedures

Traditional hydrographic survey, in its most basic form, is an open-ended traverse. Horizontal positioning has no independent check – therefore the survey precision depends on the measuring method used. The vertical accuracy is more uncontrolled due to variable physical properties of the water column and the characteristic (morphology and type) of thebottom.

Figure 1.4 shows the corrections that should be applied to acoustic depth measurements.

Sound Velocity Correction

SEDIMENT

Instantaneous Sea Surface

Transducer

Observed Depth

Actual Depth Chart

Depth

Tide Corrn

Dynamic Transducer Draft Correction

Figure 1.4: Corrections to acoustic depth measurements

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Table 1.0 (taken from Table 3-1 of the USACE Survey Manual EM 1110-2-1003) shows the technical performance standards for hydrographic surveying activities of the Corps. These standards are mandatory for navigation and dredging support Surveys. The standards for "Other Surveys and Studies" are recommended. These standards are designed to reflect current survey instrumentation, practices, and capabilities; however, it is fully recognized that exceptions to these standards will exist for some applications, or as technological advances occur. Table 1.0. Minimum Performance Standards for Corps of Engineers Hydrographic Surveys (Mandatory) PROJECT CLASSIFICATION Navigation & Dredging Support Surveys

Bottom Material Classification Other General Surveys & Studies (Recommended Standards)

Hard Soft RESULTANT ELEVATION/DEPTH ACCURACY (95%) System Depth (d) Manual d<15 ±0.25 ± 0.25 ± 0.5 Acoustic d<15 ±0.5 ± 0.5 ± 1.0 Acoustic 40>d>15 ±1.0 ± 1.0 ± 2.0 Acoustic d>40 ± 1.0 ± 2.0 ± 2.0 OBJECT/SHOAL DETECTION CAPABILITY Minimum object size (95% confidence)

>0.5 cube >1 m cube n/a

Minimum # of acoustic hits >3 3 n/a HORIZONTAL POSITIONING SYSTEM ACCURACY (95%)

2m (6ft) 5m (16 ft) 5m (16 ft)

REPORTED FEATURE HORIZONTAL LOCATION ACCURACY (95%) Plotted depth location 2m (6ft) 5m (16 ft) 5m (16 ft) Fixed planimetric features 3m (10ft) 3m (10ft) 3m (10ft) Fixed navigation aids 3m (10ft) 3m (10ft) 3m (10ft) Floating navigation aids 10m (30ft) 10m (30ft) 10m (30ft) SUPPLEMENTAL CONTROL ACCURACY Horizontal control 3rd Order (I) 3rd Order (I) 3rd Order (I) Vertical control 3rd Order 3rd Order 3rd Order WATER SURFACE MODEL ACCURACY

0.5 depth accuracy standard

0.5 depth accuracy

MINIMUM SURVEY CONVERAGE DENSITY

100% Sweep NTE 200ft (60m) NTE 500 ft (150m)

QUALITY CONTROL & ASSURANCE CRITERIA Sound velocity QC calibration >2/day 2/day 1/day Position calibration QC check 1/day 1/project 1/project QA performance test Mandatory Required (multibeam) Optional Maximum allowable bias ± 0.1 ft ± 0.2 ft ± 0.5 ft

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2 Geodetic Control and Tidal Effects

2.1 Geodetic Reference and Coordinate Systems

Geocentric Cartesian coordinates, illustrated in Figure 2.1 (A), of a point located by GPS on the surface of the Earth is based on the WGS84 reference ellipsoid and defined as follows;

λϕ coscos)( hNX R += λϕ sincos)( hNY R += ϕsin])1([ 2 heNZ R +−=

where the geometric quantity RN is the radius of curvature at the point and e is the eccentricity of the WGS84 ellipsoid. The GPS height of the point is given by h=H+N; the sum of the orthometric height (H) and the geoid undulation (N). On the open sea we can assume that H is zero. The topocentric coordinates of a point (shown as a plane perpendicular to the ellipsoidal normal) are described by East, North, and Up (E, N, U) axes. The conversion from geocentric to topocentric coordinates involves a coordinate transformation. The magnitude of the Up coordinate can be set to zero in most marine environments.

The chart coordinate system, shown in Figure 2.1 (B), consists of Eastings and Northings (E, N). The coordinates are derived from rigorous application of the sound principles of map

λ

N

Z

X

Y

φ

Equatorial plane

Greenwich Meridian

Ellipsoid

Geoid

N U

E

COM

Y( N)

X(E)

SHORE

Vessel

A) Geodetic Reference System B) Chart Coordinate System

Figure 2.1: Geodetic and Chart Coordinate Systems

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projections. The map projection is constrained by conditions (length-preserving, area- and shape-preserving, or conformal) and purpose of the chart. The cylinder is the intermediate mapping surface for the Universal Transverse Mercator (UTM) projection. The UTM coordinate axes are orthogonal. This projection is preferred by the NATO Armies, Air Forces and Navies for charting applications. 2.2 Horizontal Positioning A) Conventional (Manual) Positioning

Pre-electronic positioning includes visual and optical methods of positioning.

Visual methods (obsolete) include Stadia/Azimuth, Sextant Resection, Triangulation, and Tag line procedures. For example, a three-point hydrographic Sextant fix was obtained from two measured angles between three points of known geographic position (i.e., onshore monuments). Other optical methods include shore-based theodolite intersection methods. A minimum of two control stations must be occupied. This method is labor-intensive and weather-dependent. On the other hand, this method is advantageous in harbors, rivers and other restricted areas where electronic measurements are impractical.

Another approach is the Baseline Range Alignment method which involves setting a baseline

along the shore. Stakes are set at even stations along the baseline. Perpendicular offset visual ranges are placed at even stationing to form an offshore imaginary survey grid. Survey vessel positioning is by intersecting point projected from the baseline range alignment stations that were previously set on the shore. This method of positioning is relatively inexpensive, perhaps more economical than GPS on small projects, and is reliable since no electronics are required. It may be still useful on small dredging projects. Optical methods include Triangulation methods. B) Electronic Positioning: Electronic Distance measurements include short range, medium range, and long-range in the Range-Range positioning include

• Intersection method • Resection Method Range-Azimuth positioning may be still in use on some projects using a total station instrument. Automatic extracting azimuth and range updates facilitate dynamic computing of the vessel’s offshore position using the resection method.

Differential Satellite Positioning (DGPS) involves code or phase range measurement. For example, the pseudorange measurement from one receiver at Station A observing GPS satellite S described as

( ) ( ) ( ) SA

SAION

SATROP

SAATM

SA

SA CCTTTcR ++++⋅+= δδδρ

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where SAρ is the geometric range, ( )S

AATMTc δ⋅ is the atmospheric correction, ( )SATROPTc δ⋅ is the

tropospheric delay, ( )SAIONTc δ⋅ is the ionospheric delay, AC is the receiver clock error, and SC is

the satellite clock error.

Observation differencing techniques reduced the correction terms (biases) for high accuracy position fixes. Furthermore, linear combinations of GPS phase measurements like wide and narrow land observations. 2.3 Vertical Datum and Positioning

• Geodetic Vertical Datum • Tidal Datums • Sea Surface, Lakes, and River Levels In Figure 2.2, the elevation and state of the Instantaneous Sea Surface Topography (ISST) is

induced by various factors including wind, tides, and currents. Hydrographic surveys provide depth from ISST and bathymetric contours relative to a vertical datum. The bathymetric survey datum is the North American Vertical Datum. A vertical datum is a geodetic datum - established by adjustment of orthometric leveling nets. It is a reference (surface) for national vertical control networks. Vertical datums include the NGVD of 1929 called the NGVD29.

NGVD29 is derived from adjustment of 1st order level nets of US and Canada – 21Tidegagues (TGs) (US) and 5 TG’s (Canada) held fixed. Comparison between the NAVD29 and MSL reveals spatio-temporal variations due to

1. many unaccounted for physical variables affecting sea level 2. MSL is hourly average height over 19yr period of observations 3. non-linear relationship between mean tide level (MTL) and NGVD29 4. monthly MTL (planes) changes due to major seasonal changes resulting from barometric

pressure, steric level, river discharge and wind effects

Figure 2.2: Geodetic Datums and Sea Level

Ellipsoid

ISST Surface

Geoid NAVD

Chart Datum

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Tidal Datum: Definition: A plane of reference derived from rise/fall of oceanic tides. Various datum planes: Mean Higher High Water (MHHW), Mean High Water (MHW), Mean Sea Level (MSL), Mean Tidal Level (MTL), Mean Lower Water (MLW), and Mean Lower Low Water (MLLW).

Datums relative to a specific time (i.e., Epoch) can be determined; located on the ground and mapped. Datums can be determined by observations when needed (i.e., settle dispute, engineering projects, or scientific investigation). Tidal Datum (TD) are used for engineering projects, coastal boundary delineation, and nautical charting (Q. What is the relationship of a TD to the NAVD?) 1) Engineering Design

• TD defined by the low water - for safe under keel clearance for safe ship navigation in harbors

• TD required for design of structures in coastal regions including jetty reconstruction, dredging for under keel clearance, ship navigation, and beach replenishment etc.,

2) Seaward boundary mapping

• Establish Seaward Boundary for 1. Offshore oil industry required definition of federal boundary for tax claim revenue

purpose 2. Private-State boundary delineation definition due to coastline variations

Shoreline: is the intersection of the TD plane with respect to the coast (beach topography). A Tidal boundary is defined by local TD. The amount of error in the Tidal datum (eTD) determination and the slope angle (α ) of beach have considerable influence in delineating the true location of the shoreline boundary (See Figure 2.3). The relationship of the error in the shore boundary error line (SBL) and the beach topography is described as

)cot(αTDSBL ee =

Example: Compute the error in the seaward boundary line (SBL) due to TD error of 1 ft on a beach slope of 5 degrees

Tidal Datum

Beach Topography

α

Shore LineBoundary

Sea Surface

Figure 2.3: Seaward Boundary and Tidal Datum

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Solution: )5cot(0.1 0×=SBLe = 11.4 ft

3) Nautical Charting

Tide determination by GPS observations is not straightforward. However, RTK provides convenience in the form of

aFhb GPS −+∆=

where b is the height different between the BM and the Chart datum, a is the GPS antenna height, is the GPSh∆ is the geometric height difference between the GPS receiver and the geodetic BM, and F is the height from the GPS antenna to the transducer. Tide (T) is given by

FhbaT GPS −∆−+= 2.4 Fundamentals of Ocean Tides

Oceanic tides result from the gravitational pull of the moon, the sun, and the planets and from local meteorological disturbances. The tide is the alternating rise and fall in sea level (or water surface of a tidal lake of creek) produced by the gravitational force of the moon and the sun. Other non-astronomical factors, such as a meteorological forces, bathymetry, coastline configuration also play an important role in shaping the tide (NOS, 1976). The planes of the moon’s orbit around the earth and the earth orbits around the sun are nearly parallel. It takes 24 lunar hours or solar hours for the earth to expose the same point to the moon or the sun. During this time interval, the bulge of tidal forces will pass twice through this point.

Field Measurements

Many hydrographic projects require preliminary analysis of site condition related to the shoreline and tides. Tidal waters in estuaries have a different behavior than tides at coastal tide stations where the coastal shoreline if fairly uncomplicated. However there are a few general measurement strategies that apply equally to field measurements for tidal studies at the open coastal waters tide stations and for tide ion estuaries

Measuring water levels at coastal stations is fairly straightforward. The equipment includes a graduated staff although more sophisticated instruments are generally used nowadays. There is a great variety of meters which purport to measure currents, especially tidal currents. Data from these monitoring stations are collected and synthesized for a mathematical model that present the tide height in terms of its constituents.

The purpose in considering the observed tide as a sum of constituent components is that it

allows prediction of tides. If the amplitude and phase lag of any significant constituents are known for a given location, that information and knowledge of certain astronomic factors allow the prediction of both the amplitude and time of the tides at that location for years in advance. The

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relative elevation (h) of the water level (i.e., tide) at any time (t) may be represented harmonically by the following formula

∑=

−++⋅⋅+=N

iiii VtTaHfHh

100 )],()()(cos[),(),(),( λϕκµλϕλϕλϕ (5.1)

where H0(φ, λ): mean height of water level above prediction datum Hi(φ, λ): mean amplitude of constituent i ai(T) : speed of constituent i fi : factor of reducing Hi (φ, λ) to prediction year V0 + µ : equilibrium argument of constituent i at time t κ(φ, λ) :phase angle of constituent i t : time reckoned from beginning of prediction year

In equation (5.1), the speed ai(T) of each constituent is known as a function of the period T; and the factors f and (V0 + µ) are available from astronomic tables. The mean sea level, H0, is site-specific.

2.4.1 Geophysical Effects on Water Depth

The relative elevation of water levels are effected by many gravitational effects as described above and also by geophysical processes inside the earth. The earth is not a rigid non-deformable rock body but rather it is a visco-elastic planet. Its rheological properties and chemical composition allows its outer crustal shell to deform under various changes of pressure and temperature. A few of the external and internal forces (loading) that deforms the earth surface and hence the relative elevation of surface water levels include

• Atmospheric (tidal) loading • Solid Earth Tides • Post Glacial Rebound • Crustal Tectonics • Surface (and Mantle) Mass Wasting

Atmospheric tides manifest themselves by a periodic variation in the barometric pressure over a give point. The apparent amplitude of this oscillation is caused not by the tidal forces but by the diurnal heating and cooling of the atmosphere. The amplitude of the true atmospheric tide is on the order of 0.03 mbars, barely above the limits of resolution of a standard barometer. Such a slight variation in pressure is easily masked by the much stronger atmospheric disturbances which are almost always present in mid-latitudes. However, it is relatively easier to measure atmospheric tide along equatorial regions because along the equator space weather and atmospheric perturbations are fewer than at low to mid latitude regions.

Solid body tides (i.e., crustal tides) are extremely minute in amplitude, but thanks to the development of ultrasensitive instrument and GPS they may now be measured to a surprising degree of accuracy. Everybody is used to the ocean tide. The pull of the Moon and the Sun on the ocean causes cyclic variations in local sea level that can exceed 10 meters in some places. What is less well known is that the Earth’s solid outer surface itself also responds to luni-solar gravitational attraction.

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The solid earth tide (body tide) often reaches +/- 20 cm, and can exceed 30 cm. While ocean tides can be easily measured relative to the solid Earth, solid earth tides are easily measured only with satellite systems or sensitive gravimeters. The component of the force perpendicular to the surface of the earth affects g by about 0.0002 percent (%) while the tilt in the horizontal induced by the tidal stresses causes a deviation from the vertical of the order of 0.03″ of arc. Crustal tides are disturbed by atmospheric storms and oceanic tides, among other things, so that the instruments devised to measure them must be housed at the bottom of a mine shaft in order to escape the vibrations and temperature changes, preferably at a site removed a fair distance from the ocean.

Post-glacial rebound (sometimes called glacial isostatic adjustment (GIA)) is the rise of land masses that were depressed by the huge weight of ice sheets during the last glacial period, through a process known as isostasy. It affects, the Great Lakes of Canada and the United States, northern Europe especially Scotland, Fennoscandia and northern Denmark, parts of Siberia, Canada and in the southern hemisphere such as Patagonia (southern part of Chile and Argentina), and Antarctica.

2.5 Methods to Establish Local Datums for Engineering Projects

A. Extension from Established or Published Tidal Datum

This is a simple method of extending a known tidal datum from a NOS tide station located near a project site. This method can be used if the project site is along an open shoreline and less than 1 mile from an NOS tide station located in the same water body, with MHW published on either NGVD29 or NAVD88. Here it is assumed that all the tidal characteristics at the primary station applied equally at the subordinate station B. Extrapolation using Very Short Tide Studies

Short-term study methods consist of simultaneous tide observations at the project site (subordinate station) and at a NOS control tide station. Short-term tide studies use as few as three tide cycles to determine a local MHW. Three effective methods can be employed for extrapolating MHW with short tide studies:

1. Height difference method;

)( CCPP MHWHWHWMHW −−= where PMHW computed mean high water at subordinate tide station; PHW the observed high water at subordinate tide station; CHW observed high water at the control tide station; and PUBL

CHW is the published (NOS) mean high water at control tide station.

2. Range ratio method;

C

PC

PCCP TR

TRMRTAMTLMHW

2⋅+∆+= ,

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where CMTL is the published mean tide level at the control station, PCTA∆ is the difference between

the peak tide amplitudes at the subordinate station P and at the control station (i.e., CP

PC TATATA −=∆ ), PTA is the observed peak tide amplitude (i.e., halfway point between low and

high water), and the observed tide ranges CTL and PTR are the high water minus the low water at the control and subordinate stations, respectively.

3. Amplitude ratio method.

C

PCP TA

TATRTR ⋅=

where CTR is the observed tidal range (high water minus low water) at the control tide stations;

PTR is the computed range of tide at the subordinate tide stations P, and PTA and CTA are the observed tidal amplitudes at the control and subordinate tide stations, respectively. C. Interpolation using Very Short Tide Studies

The interpolation method can be used if a project site is located between two NOS tide stations on the same water body. The tide stations may be separated by several miles if the elevation of MHW and the mean range of tide at both tide stations have similar values. A linear interpolated value of MHW at the project site is computed by utilizing published (by NOS) MHW elevations on either NGVD29 or NAVD88.

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3. Depth Determination

3.1 Conventional (Manual) Methods The simplest approach uses a winch-powered Tagline for range measurements and a Lead Line Chain, with a weight attached, for depth measurement. The tag line is anchored on the shore. Another conventional method of depth (bathymetry) determination involves the use of range pole, small floating platform (e.g., raft, boat, etc.), and transit. Figure 3.1 illustrates the basic set up using a prism-mounted range pole.

3.2 Acoustic Depth Determination Method Acoustic methods on water depths determination requires specific knowledge of the physical oceanography

1. Underwater acoustics 2. Vessel Attitude and Heave

measurements Measurement Calibration

1. Sound velocity Profiler to measure the velocity versus depth (z) through the water column

2. electronic instrument (CTD) to measure conductivity , temperature, and depth (z)

3. Other complex methods beyond the scope of this workshop

Underwater range measurements are obtained from SOund Navigation And Ranging (SONAR)

SEDIMENT

Water Surface

Mudline

Figure 3.1: Conventional Bathymetric Surveying

BM

Prism-mounted Range pole TSI

Figure 3.2: Survey Boat with Acoustic Measuring Equipment

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The SONAR equation, given as the signal of the return echo energy (EE) level, is described as

DTBSDINLTLSLEE SSSS −+−−−= )(2 σσσ where

SLS is the source level

TLσ is the signal transmission loss

NLσ is the noise level

DIσ is the source directivity index BSS is the backscatter coefficient, and

DTS is coefficient for the signal detection threshold. The source level SLS is the acoustic signal intensity of planar wave. The signal transmission loss, TLσ , is due to absorption, spherical spreading, and scattering by suspended particulates, bubbles, and debris etc.,). The power (Π ) of the acoustic pulse is equal to the intensity (I) multiplied by the area (A) of the ensonified region. The equation is given as;

2211 AIAI ×=×=Π where 2

11 RA ×Ω= and 222 RA ×Ω= and Ω is the solid angle.

Principles of Single Beam Echo Sounding (SBES)

The transducer transmits acoustic energy into the water in the form of a vertically oriented beam. Reflected energy (i.e., echo) is sensed by the transducer. The measured depth (z) is given as;

ctmz ×∆⋅=21)( ,

where c is the velocity of sound in water column t∆ is the time interval (in seconds) between pulse transmission and echo reception. (See Figure 3.2)

Sources of Errors on Depth Measurements

1) Bottom slop (and roughness). The quantity (dz) depends on beam width and slope 2) Sound velocity variation (spatio-temporal variation) 3) Time measurement

4) Attitude of vessel (where 2

,, φθθθ >YPR )

5) Draft: transducer draft depends on coverage load during the survey (fuel and water consumption during survey will results in variation in measurement) is a function of the float area at sea surface

6) Recording errors – analogue versus digital

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Depth Reduction equation is given as;

THVSVVDC CCCdD ++++= δ0 , where

CD is the final charted depth

0d is the uncorrected depth observation

VDδ is the change in vessel draft

SVC is the sound velocity correction

HVC is the total vessel heave correction

TC is the tidal height correction Tidal reduction for the vertical datum (co-tidal model or weighted average of the tide Gauge (TG) measurement

SEDIMENT

Instantaneous Sea Surface Topography

Transducer

Figure 3.2: Acoustic Beam Characteristics

2φ

Z

Footprint

DW

Ensonified Area

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Coverage/Footprint

Coverage is defined as the amount of points (point density) per square meters of the ensonified areas. It refers to the size of the area covered by the footprint on a flat surface. Figure xxx illustrates the for a single beam

The seafloor coverage (ensonified area) is circular over a flat and horizontal sea bed. Over inclined or sloping bathymetry, the shape of the ensonified area will resemble more of an ellipse than a circle the steeper the seabed gradient. The area within the beam is given by the footprint size (Dw). The diameter of the footprint is

×⋅=

2tan2 φzDW

Coverage of the seafloor is a function of several factors including 1) dimension of the ensonified area (footprint) 2) beam spacing across track 3) ping rate 4) vessel speed 5) vessel attitude (i.e., pitch, roll, and yaw) For multi beam echo sounding (MBES) the swath width (Dw) is defined as

∆×⋅=

2tan2 θzDW

where θ∆ is angular coverage between the outer beams.

Corrections to echo sounding include the velocity of sound correction, dynamic draft correction, tide or water level reduction, instrument correction. Figure 3.3 shows the relationship of these corrections 3.3 Real-time Hydrographic Mapping

Real-time mapping require the expert time synchronization of depth measuring systems and

the three-dimensional positioning systems. In addition, a critical element in dynamic mapping is the correct integration of various technologies. Figure 3.3 illustrates the various corrections that require successful real-time dynamic hydrographic mapping.

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SEDIMENT

Instantaneous Sea Surface Topography

Sound Velocity Correction

Datum

Char

t Dep

th

Elap

sed

Tim

e D

epth

Inst

r. Co

rrn

Act

ual D

epth

Tide Reduction/Corrn

S1

S2

S3

GPSh∆ BM

Figure 3.3: Acoustic Hydrographic Survey

Dynamic Transducer Draft Corrn

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4. Field Operations and Survey Procedures

• Field operations and survey procedures • Data Processing

4.1 Field Operations

Two common methods include manual (conventional) and automated. Manual depth measurement techniques may use a lead line for depth and tagline for position. A sounding pole with a prism may also be used with a TSI to obtain bottom elevations. Automated depth measurement systems involve streaming positioning data from a GPS or robotic TSI, depth data from an electronic echosounder, and motion data from an MRU into a computer to be integrated and recorded.

4.1.1 Conventional Surveys

Manual hydrographic surveys can use a small marine vessel (e.g., less than 20 ft boat Boat). A minimum of two-man crew is needed to perform this survey. A robotic TSI or RTK gps setup can reduce this survey to a one-man operation, however, safety must be considered if working alone on the water.

The advantage of this conventional approach is that it is cheaper to obtain the equipment and much simpler to calibrate. It also provides direct measurement of the bottom. The disadvantages are that this method can be error prone, labor-intensive, time consuming, and yield limited coverage. Project specifications typically dictate the method to be used to obtain a hydrographic survey. As with any survey, the purpose of the hydrographic survey to be performed is a strong consideration in the planning process. Field reconnaissance and survey design are always required for medium and large scale surveys.

4.1.2 Equipment Calibration

Equipment calibration is a critical element of hydrographic surveying. From the smallest and simplest platforms to large surveying ships, it is critical that all equipment is properly calibrated. Calibration includes the measurement of offsets between all peripheral equipment. Each vessel has its own coordinate system. The center of mass is typically used as the origin. The relative location of singlebeam and multibeam transducers, the inertial motion unit, gps antennas, any prisms mounted must be measured and accounted for prior to collecting data. Typically the keel is used as

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one axis, a line 90 degrees to the keel passing through the center of mass is used as the second axis. The third axis is vertical while the vessel is at rest. It is ideal to survey the vessel while it is out of the water and on blocks. This allows careful measurements to be taken without the vessel rising or falling due to tide, or moving horizontally due to current or wind. Once all offset measurements are completed, further calibration is still needed. The BAR CHECK is a mandatory calibration procedure that should be performed prior to any critical survey as well as on a regular basis. This applies to both Single and Multi Beam surveys. There are different methods for completing a bar check. If a velocity probe is available, a velocity cast is taken. A bar or plate is then suspended in the water a known distance below the transducer. Correcting for draft, the distance reported by the echosounder is compared to the known distance. The velocity probe should also be tested by taking readings in distilled water of a known temperature and comparing the readings to theoretical readings. Latency tests are performed with single beam systems to measure the difference in time between positions and depths. PATCH tests are performed on multibeam systems to calibrate vessel pitch, roll, yaw, and latency measurements. If real time kinematic GPS is used for tide determination at the vessel, the values must be compared with an independent measurement such as a tidestaff. 4.2 Data Processing Conventional surveys require post processing of total station (distance, Azimuth, and vertical angle) data to compute “time-stamped” intersections (x, y, Depth) points. Electronic (acoustic) methods require automated data processing. Complex data analysis includes tidal correction, the application of the depth reduction equation, removing artifacts from the dataset, setting beam angles, sorting parameters, and various other parameters. Data processing converts data from a raw state into various user defined products.

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Figure 3: Ellipsoid-Geoid Separation Correction Bathymetric chart (contour) lines are generated by a suitable interpolation method using the depths at the grid nodes. Industry standard software include HYPACK etc. The main features of hydrographic data processing and mapping softwares should include

1. Survey Preparation Module that allow • Loading of background charts • Selection of geodetic parameters (ellipsoid datum, Projection, datum Transformation) • Planning and designing ship lines • Hardware and vessel setup controls

2. Data Collections (Survey) Module • On-the-Fly display of profiles • Display vessel parameters (tracking etc) • Data entries (tide, draft, sound velocity corrections etc)

3. Post Processing and Mapping Module • Data thinning and editing • Logging events in survey • Contouring (generate TIN model) • Areas and volumes Computations

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4. 3 Survey Coverage and Resolution

The spacing between stations is project and site specific. For example, a 50-ft nominal grid spacing interval spacing may be insufficient to map the structural or morphological details of an unexpected submerged structure or dumping mound. Higher spatial resolution sounding may be required in the vicinity of the feature to be mapped. Single beam line spacing may vary from 10 feet to over 1000 feet. Lines run in the opposite direction may also be required to fully define an area or for QC purposes. Multibeam line spacing may vary due to depth, beam angle, and overlap required. For example, if 45 degree either side of nadir is used, the coverage will be 2x the water depth.

Voyage (“Taglines”) Tracks

COAST LINE

Figure 4.1: Survey Design and Field Campaign for Single beam

Ensonified Seafloor Area

Lan

e W

idth

Survey VesselSurvey Grid

SS

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Vessel Tracks

COAST LINE

Figure 4.1: Survey Design and Field Campaign for Sweep Multibeam Surveys

Ensonified Seafloor Area

Lan

e W

idth

Vessel Direction

Sounding Overlap

Ping rate

S

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