Chapt-30.pdf

CHAPTER 29

HYDROGRAPHY

2900. Introduction

nautical chart, whether in digital or paper form, from the

initial planning of a hydrographic survey to the final print-

ing. It is important to note that digital charts are no more

accurate than the paper charts and other sources from which

they are produced. “Digital” does not mean “more accu-

rate,” for in most cases the digitized data comes from the

same sources that the paper charts use.

With this information, the mariner can better

understand the information presented on charts, evaluate

hydrographic information which comes to his attention, and

report discrepancies in a form that will be most useful to

charting agencies.

Hydrography is the science of measurement and

description of the features which affect marine navigation,

including water depths, shorelines, tides, currents, bottom

types, and undersea obstructions. Cartography transforms

the scientific data collected by hydrographers into data

usable by the mariner, and is the final step in a long process

which leads from raw data to a usable chart.

The mariner, in addition to being the primary user of

hydrographic data, is also an important source of data used

in the production and correction of nautical charts. This

chapter discusses the processes involved in producing a

BASICS OF HYDROGRAPHIC SURVEYING

2901. Planning the Survey

Mariners . Tidal information is thoroughly reviewed and

tide gauge locations chosen. Local vertical control data is

reviewed to see if it meets the expected accuracy standards

so the tide gauges can be linked to the vertical datum used

for the survey. Horizontal control is reviewed to check for

accuracy and discrepancies and to determine sites for local

positioning systems to be used in the survey.

Line spacing refers to the distance between tracks to

be run by the survey vessel. It is chosen to provide the best

coverage of the area using the equipment available. Line

spacing is a function of the depth of water, the sound

footprint of the collection equipment to be used, and the

complexity of the bottom. Once line spacing is chosen, the

hydrographer can compute the total miles of survey track

to be run and have an idea of the time required for the

survey, factoring in the expected weather and other

possible delays. The scale of the survey, orientation to the

shorelines in the area, and the method of positioning

determine line spacing. Planned tracks are laid out so that

there will be no gaps between sound lines and sufficient

overlaps between individual survey areas.

Wider lines are run at right angles to the primary

survey development to verify data repeatability. These are

called cross check lines .

Other tasks to be completed with the survey include

bottom sampling, seabed coring, production of sonar

pictures of the seabed, gravity and magnetic measurements

(on deep ocean surveys), and sound velocity measurements

in the water column.

The basic sources of data used to produce nautical

charts are hydrographic surveys. Much additional

information is included, but the survey is central to the

compilation of a chart. A survey begins long before actual

data collection starts. Some elements which must be

decided are:

• Exact area of the survey.

• Type of survey (reconnaissance or standard), scaled

to meet standards of charts to be produced.

• Scope of the survey (short or long term).

• Platforms available (ships, launches, aircraft, leased

vessels, cooperative agreements).

• Support work required (aerial or satellite

photography, geodetics, tides).

• Limiting factors (budget, politics, geographic or

operational constraints, positioning system

limitations, logistics).

Once these issues are decided, all information

available in the survey area is reviewed. This includes aerial

photography, satellite data, topographic maps, existing

nautical charts, geodetic information, tidal information, and

anything else affecting the survey. The survey planners

then compile sound velocity information, climatology,

water clarity data, any past survey data, and information

from light lists, Sailing Directions , and Notices to

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HYDROGRAPHY

2902. Echo Sounders in Hydrographic Surveying

order to correct the soundings during processing.

Tides are accurately measured during the entire survey

so that all soundings can be corrected for tide height and

thus reduced to the chosen vertical datum. Tide corrections

eliminate the effect of the tides on the charted waters and

ensure that the soundings portrayed on the chart are the

minimum available to the mariner at the sounding datum.

Observed, not predicted, tides are used to account for both

astronomically and meteorologically induced water level

changes during the survey.

Echo sounders were developed in the early 1920s, and

compute the depth of water by measuring the time it takes

for a pulse of sound to travel from the source to the sea

bottom and return. A device called a transducer converts

electrical energy into sound energy and vice versa. For basic

hydrographic surveying, the transducer is mounted

permanently in the bottom of the survey vessel, which then

follows the planned trackline, generating soundings along

the track.

The major difference between different types of echo

sounders is in the frequencies they use. Transducers can be

classified according to their beam width, frequency, and

power rating. The sound radiates from the transducer in a

cone, with about 50% actually reaching to sea bottom. Beam

width is determined by the frequency of the pulse and the size

of the transducer. In general, lower frequencies produce a

wider beam, and at a given frequency, a smaller transducer

will produce a wider beam. Lower frequencies also penetrate

deeper into the water, but have less resolution in depth.

Higher frequencies have greater resolution in depth, but less

range, so the choice is a trade-off. Higher frequencies also

require a smaller transducer. A typical low frequency

transducer operates at 12 kHz and a high frequency one at 200

kHz.

2903. Collecting Survey Data

While sounding data is being collected along the planned

tracklines by the survey vessel(s), a variety of other related

activities are taking place. A large-scale boat sheet is

produced with many thousands of individual soundings

plotted. A complete navigation journal is kept of the survey

vessel’s position, course and speed. Side-scan sonar may be

deployed to investigate individual features and identify rocks,

wrecks, and other dangers. Divers may also be sent down to

investigate unusual objects. Time is the single parameter

which links the ship’s position with the various echograms,

sonograms, journals, and boat sheets that make up the

hydrographic data package.

2904. Processing Hydrographic Data

The formula for depth determined by an echo sounder is:

--------------KD r

During processing, echogram data and navigational

data are combined with tidal data and vessel/equipment

corrections to produce reduced soundings . This reduced

data is combined on a plot of the vessel’s actual track with

the boat sheet data to produce a smooth sheet . A contour

overlay is usually made to test the logic of all the data

shown. All anomolous depths are rechecked in either the

survey records or in the field. If necessary, sonar data are

then overlayed to analyze individual features as related to

depths. It may take dozens of smooth sheets to cover the area

of a complete survey. The smooth sheets are then ready for

cartographers, who will choose representative soundings

manually or using automated systems from thousands

shown, to produce a nautical chart. Documentation of the

process is such that any individual sounding on any chart

can be traced back to its original uncorrected value. See

Figure 2904.

The process is increasingly computerized, such that all

the data from an entire survey can be collected and reduced

to a selected set of soundings ready for incorporation into an

electronic chart, without manual processes of any kind. Only

the more advanced maritime nations have this capability,

but less developed nations often borrow advanced technolo-

gy from them under cooperative hydrographic agreements.

where D is depth from the water surface, V is the average

velocity of sound in the water column, T is round-trip time

for the pulse, K is the system index constant, and D r is the

depth of the transducer below the surface (which may not be

the same as vessel draft). V, D r , and T can be only generally

determined, and K must be determined from periodic

calibration. In addition, T depends on the distinctiveness of

the echo, which may vary according to whether the sea

bottom is hard or soft. V will vary according to the density of

the water, which is determined by salinity, temperature, and

pressure, and may vary both in terms of area and time. In

practice, average sound velocity is usually measured on site

and the same value used for an entire survey unless variations

in water mass are expected. Such variations could occur in

areas of major currents or river outflows. While V is a vital

factor in deep water surveys, it is normal practice to reflect

the echo sounder signal off a plate suspended under the ship

at typical depths for the survey areas in shallow waters. The

K parameter, or index constant, refers to electrical or

mechanical delays in the circuitry, and also contains any

constant correction due to the change in sound velocity

between the upper layers of water and the average used for

the whole project. Further, vessel speed is factored in and

corrections are computed for settlement and squat, which

affect transducer depth. Vessel roll, pitch, and heave are also

accounted for. Finally, the observed tidal data is recorded in

2905. Automated Hydrographic Surveying

The evolution of echo sounders has followed the same

HYDROGRAPHY

411

Figure 2904. The process of hydrographic surveying.

412

HYDROGRAPHY

pattern of technological innovation seen in other areas. In

the 1940s low frequency/wide beam sounders were

developed for ships to cover larger ocean areas in less time

with some loss of resolution. Boats used smaller sounders

which usually required visual monitoring of the depth.

Later, narrow beam sounders gave ship systems better

resolution using higher frequencies, but with a

corresponding loss of area. These were then combined into

dual-frequency systems. All echo sounders, however, used

a single transducer, which limited surveys to single lines of

soundings. For boat equipment, automatic recording

became standard.

The last three decades have seen the development of

multiple-transducer, multiple-frequency sounding systems

which are able to scan a wide area of seabed. Two general types

are in use. Open waters are best surveyed using an array of

transducers spread out athwartships across the hull of the survey

vessel. They may also be deployed from an array towed behind

the vessel at some depth to eliminate corrections for vessel

heave, roll, and pitch. Typically, as many as 16 separate

transducers are arrayed, sweeping an arc of 90

of this system is fixed by the distance between the two outermost

transducers and is not dependent on water depth.

Airborne Laser Hydrography (ALH) uses laser light

to conduct hydrographic surveys from aircraft. It is particu-

larly suitable in areas of complex hydrography containing

numerous rocks, shoals, and obstructions dangerous to sur-

vey vessels. The technology has developed and matured

since the 1970’s, and in some areas of the world up to 50%

of the hydrographic surveying is done with lasers. Survey

rates of some 65 square km per hour are possible, at about a

quarter of the cost of comparable vessel surveys. Data densi-

ty is variable, ranging down to some 1-2 meters square, and

depths from one half to over 70 meters have been successful-

ly surveyed.

The technology uses laser light generators mounted in

the bottom of a fixed or rotary wing aircraft. Two colors are

used, one which reflects off the surface of the sea and back to

the aircraft, and a different color which penetrates to the sea-

bed before reflecting back to the aircraft. The difference in

the time of reception of the two beams is a function of the wa-

ter depth. This data is correlated with position data obtained

from GPS, adjusted for tides, and added to a bathymetric da-

tabase from which subsets of data are drawn for compilation

of nautical charts.

Obviously water clarity has a great deal to do with the

success of ALH, but even in most areas of murky water, sea-

sonal or meteorological variations often allow sufficient

penetration of the laser to conduct surveys. Some 80% of the

earth’s shallow waters are suitable for ALH.

In addition to hydrographic uses, ALH data finds appli-

cation in coastal resource management, maritime

boundaries, environmental studies, submarine pipeline con-

struction, and oil and gas exploration.

HYDROGRAPHIC REPORTS

2906. Chart Accuracies

This is in addition to the caution the mariner must exercise

to be sure that his navigation system and chart are on the same

datum. The potential danger to the mariner increases with

digital charts because by zooming in, he can increase the chart

scale beyond what can be supported by the source data. The

constant and automatic update of the vessel’s position on the

chart display can give the navigator a false sense of security,

causing him to rely on the accuracy of a chart when the source

data from which the chart was compiled cannot support the

scale of the chart displayed.

The chart resulting from a hydrographic survey can be no

more accurate than that survey; the survey’s accuracy, in turn,

is limited by the positioning system used. For many older

charts, the positioning system controlling data collection

involved using two sextants to measure horizontal angles

between surveyed points established ashore. The accuracy of

this method, and to a lesser extent the accuracy of modern,

shore based electronic positioning methods, deteriorates

rapidly with distance. In the past this often determined the

maximum scale which could be considered for the final chart.

With the advent of the Global Positioning System (GPS) and

enhancements such as DGPS and WAAS, the mariner can

often now navigate with greater accuracy than could the

hydrographic surveyor who collected the chart’s source data.

Therefore, one must exercise care not to take shoal areas or

other hazards closer aboard than necessary because they may

not be exactly where they are charted. This is especially true in

less-travelled waters.

2907. Navigational and Oceanographic Information

Mariners at sea, because of their professional skills and

location, represent a unique data collection capability

unobtainable by any government agency. Provision of high

quality navigational and oceanographic information by

government agencies requires active participation by mariners

in data collection and reporting. Examples of the type of

information required are reports of obstructions, shoals or

. The area

covered by these swath survey systems is thus a function of

water depth. See Figure 2905 . In shallow water, track lines must

be much closer together than in deep water. This is fine with

hydrographers, because shallow waters need more closely

spaced data to provide an accurate portrayal of the bottom on

charts. The second type of multiple beam system uses an array

of vertical beam transducers rigged out on poles abeam the

survey vessel with transducers spaced to give overlapping

coverage for the general water depth. This is an excellent config-

uration for very shallow water, providing very densely spaced

soundings from which an accurate picture of the bottom can be

made for harbor and small craft charts. The width of the swath

HYDROGRAPHY

413

Figure 2905. Swath versus single-transducer surveys.

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