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Official Agencies of
Hydrographic Surveying
The International Hydrographic
Organization (IHO)
The International Hydrographic Organization
(IHO)
Consisting of a collaboration of roughly 87
countries, the IHO is the inter-governmental
organization which represents hydrographic
interests throughout the globe.
A primary goal of the IHO is to ensure that the
world’s seas, oceans and other significant navigable
water regions are adequately surveyed and
charted. The IHO establishes international sets of
standards for hydrographic surveying, and
coordinates the efforts of the various national
hydrographic survey offices.
Observer Status at the UN
The IHO has “observer status” at the United
Nations, which means it has non-member
privileges, giving them the ability to participate in
the UN's activities.
The IHO is the principally recognized authority on
hydrographic surveying and nautical charting
within the United Nations. When referring to
hydrography and nautical charting in Conventions
and similar Instruments, it is the IHO standards and
specifications that are typically referenced.
History of the IHO
The IHO was first established in 1921, as the
International Hydrographic Bureau (IHB). In 1970
the present title of IHO was adopted as part of a
new international “Convention on the IHO.”
Throughout the 19th century, a number of
maritime nations established individual
hydrographic agencies as a way of improving the
navigational abilities of naval and merchant
vessels, by providing nautical publications, charts,
and other navigational services. However, there
were considerable differences in hydrographic
procedures, charts, and publications. In 1889, an
International Maritime Conference was held,
proposing the establishment of a "permanent
international commission."
Similar proposals were suggested at the sessions of
the International Congress of Navigation in 1908,
and the International Maritime Conference in
1912. In 1919, hydrographers from England and
France collaborated, taking the necessary steps to
convene an international conference of
hydrographers selecting London as the most
suitable locale for the conference.
1919 Conference
In July of 1919, the First International Conference
opened, attended by the hydrographers of 24
nations. The objective of the conference, which still
stands today, was:
"To consider the advisability of all maritime nations
adopting similar methods in preparation,
construction, and production of their charts and all
hydrographic publications; of rendering the results
in the most convenient form to enable them to be
readily used; of instituting a prompt system of
mutual exchange of hydrographic information
between all countries; and of providing an
opportunity to consultations and discussions to be
carried out on hydrographic subjects generally by
the hydrographic experts of the world."
As a result of this 1919 Conference, the permanent
organization was formed and statutes for its
operations were prepared.
Page 2: Office of Coast Survey
US Coast Survey
The “US Coast Survey” was the official title of the
governmental agency tasked with creating nautical
charts for the US. Established in 1807 by Thomas
Jefferson, it is among the oldest scientific
organizations still in operation within the US
government.
In 1878, the name was changed to that of the “US
Coast and Geodetic Survey (C&GS),” and later, in

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are usually mounted on aircraft and provide
seamless, contiguous coverage between land and sea. Bathymetric LIDAR
This can be used to acquire imaging in areaswith complex and rugged shorelines where surface
vessels cannot operate efficiently or safely because
of rocks, kelp or breaking surf. Some of the
inhospitable areas include Alaska, the North
Atlantic Coast and the Caribbean Isles. LIDAR
systems will be covered in more depth, later in this course, as well.Marine Magnetometry
Marine magnetometry surveys are performed
using a specially modified magnetometerwhich is
housed within a tow-behind shell known as a
towfish.
The magnetometer is dragged behind the vessel
using a tether line which doubles as a data
transmission cable. Magnetometers come in a variety of designs, used for a variety of specialized applications.
Marine DC Resistive Imaging
Direct current (DC) electrical resistivity is a
geophysical surveying technique that has a long track record for characterizing subsurface
conditions.
Marine electrical resistivity imaging (ERI) has the precision capabilities of DC electrical resistivity
methods, combined with the data collection speed
of traditional EM techniques. By towing an
electrode array behind a survey vessel and
continuously recording data, the marine resistivity
system can record many times the line distance of
a traditional electrical resistivity land system.
Applications include characterizing sediment types,
identifying zones of underwater groundwater seep
and discharge, as well as the mapping of geological
structures.
Georadar
(GPR) Ground penetrating radar or “georadar” is a
geophysics seismic method based on the use of focused radar energy which penetrates the ground to image subsurface conditions.
This system can only be used in fresh water
applications and is not suited for use in brackish or
saltwater environments.
Satellite-based Radar Altimetry
Satellite measurements of sea floor features are based on gravitational bulges in sea surface caused
by underwater topography change. The surface of
the ocean bulges outward and inward representing
the topography of the ocean floor.
The bumps, too small to be seen, can be measured
accurately by a radar altimeter aboard a satellite.
Satellites are also used to measure bathymetry.
Satellite radar maps deep-sea topography by
detecting the subtle variations in sea level caused
by the gravitational pull of undersea mountains,
ridges, and other masses.
On average, sea level is higher over mountains and ridges than over abyssal plains and trenches.

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surveying, and safer navigation in some uncharted
regions.
These 312 Fathometers displayed deep-water
depth soundings with a continuously rotating white
light. The fathometer technician would read depth
measurements by monitoring the position of the
light when an echo was heard in a set of
headphones.
The 312 Fathometer differed from previous
systems, in that it could still be operated while the
survey ship was moving. This technique was
eventually improved upon with a red light method,
where a rotating neon tube flashed adjacent to the
depth scale when the echo returned.
Dorset Fathometer with Transceiver
In 1933, the Dorsey fathometer was developed,
which incorporated a transmitter plus a receiver
into a singular unit which came to be known as a
“transceiver”. The Dorsey fathometer had an
operating range of 3 to 900 feet, and by 1939,
could record depths automatically, using a graph-
recording instrument.
Other Improvements in Fathometers
In 1940, the portable 808 fathometer was released,
which was equipped with a graphic recording
device. This became the standard device for
shallow to intermediate surveying until the mid-
60’s.
In the mid-60s, with the advent of digital
technology, computerized data collection systems
for hydrographic surveying equipment became the
standard. In the 70’s, the accuracy of echo
sounding devices were increased with the
development of single-beam frequency systems,
and by the 80’s, were enhanced by the deployment
of dual frequency beams.
These systems combined a narrow high-frequency
beam for precision, and a wider low-frequency
beam to provide broader sampling of the
surrounding area.
Modern Day Hydrographic Survey
Methods
Modern Tools for Hydrographic Surveying
A number of revolutionary technologies have been
advanced in the latter half of the 20th century that
changed the fundamentals of hydrographic
surveying, and how we view the seafloor.
New technologies such as side scan sonar offered a
means of obtaining high quality underwater
imaging, improving the ability to identify long lost,
submerged shipwrecks (see image below), or
viewing subtle obstructions and protrusions.
Other new marine
surveying technologies
include: single and
multibeam sonar,
LIDAR, Magnetometry,
DC Resistive Imaging,
radar altimetry, and
ground penetrating
radar. These topics will
be discussed in further
detail in chapters 3 and 4 of this course.
Side Scan SONAR
Side scan sonar is a type of sonar imaging device
that efficiently
and clearly
creates an image
of large regions
of the sea bed.
(The image to the
left) show a
diagram of a side
scan sonar’s
potential imaging
path. This type of
sonar system will
be discussed in depth, later in this course.
LIDAR
LIDAR (or Light Detection And Ranging) technology,
measures the elevation or depth by analyzing the
pulses of laser light reflecting off an object. LIDAR
survey systems which perform bathymetric surveys

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Marindin and Ogden
From 1919 to 1942, the USC&GS Marindin and
USC&GS Ogden conducted wire-drag surveys as a
joint operation.
Hilgard and Wainwright
From 1942 to 1967, the USC&GS employed two
ships, the Hilgard and her sister ship the
Wainwright, which conducted wire-drag
hydrographic survey operations together along the
US East Coast until 1967, when they were then
replaced by the USC&GS Rude (ASV90, and S590)
and Heck (ASV91, and S591).
Research ships of this type were known as auxiliary
survey vessels or “ASV” while under the authority
of the USC&GS, then later to be designated as
survey vessels or “S”, following the 1970 change of
agency, falling under the new authority of NOAA.
Heck and Rude
Commissioned in 1967, the Heck and her sister ship
the Rude, (see image below) like the previous
paired vessels, were specifically designed for
conducting
wire-drag
survey
operations.
These ships
worked
together
under a single
command
conducting wire-drag surveys, clearing large swaths
between them with a submerged wire.
During their commissions, however, electronic and
acoustic technologies were developed that allowed
a single ship to perform the same work as two
wire-drag vessels, using side-scan sonar or multi-
beam sonar.
As a result, Heck and Rude began to operate
independently in 1989, employing this improved
technology, thus ending the days of these wire
drag pairings.
Page 6: Early Methods – Echo Sounders and
Fathometers
1930s - Single beam echo sounders and
fathometers
In the 1930s, single-beam echo sounder systems
were developed and subsequently implemented
for use in
hydrographic
surveying
practice. Echo
sounding was
based on
technological
concepts
published in
1904 by Norwegian Inventor Hans Sundt Berggraf,
and patented by German inventor Alexander
Behm, in 1913. These devices used sound to
measure the distance to the sea floor directly
beneath a survey vessel.
By running a series of lines at a specified spacing,
single beam echo sounders and fathometers
greatly increased the speed of the surveying
process by allowing a greater number of data
points to be collected. Even so, this method still left
gaps in the quantitative depth data in between the
survey lines.
In addition to single beam echo sounder devices,
there are echo sounders that are capable of
receiving multiple return "pings". These systems
known as multi-beam echo sounders are covered in
depth in chapter 4 of this course.
Fathometers
From the combination of “fathom” and “meter,”
fathometers were a type of echo sounder. In 1925,
the Submarine Signal Company of Boston
developed the first fathometer, under the product
title “312 fathometer,” for charting the water
depth while a ship was moving.
The fathometer proved to be more precise and
easier to use than previous sounding methods,
making it an essential device for hydrographic

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Sounding Machines
Several sounding machines were patented in the
late 19th century, using reels or drums with a
handle, to deploy the wire
and measure output.
Fashioned after the lead
line method of depth
measurement, they were
invented out of a need to
increase the accuracy and
speed of deeper water
soundings.
Accumulators
The heave effects on the sounding lines were
compensated for, by devices called
“accumulators.” They were attached at intervals
along the length of the sounding line and could
stretch up to lengths of 17 feet, with a maximum
exertion of 70 pounds force, in order to somewhat
equalize the strain and prevent snapping of the line
due to vessel roll, bottom snags, or other abrupt
motions.
Page 5: Early Methods – Wire Drag Surveys
1900s - Weighted Wire-drag Surveys
In 1904, the weighted wire-drag method (see
image below) of surveying was implemented,
whereas a wire would be attached in between two
vessels, and dragged between the two points.
The wire was set at a given depth using a system of
weights and buoys. When this rig encountered an
obstruction, it would become tight, forming a "V"
shape, thus revealing the depth and position of
submerged rocks and other obstructions. The wire
drag system was the most reliable means of
detecting and recording the location of submerged
obstacles at the time. Wire drag data was used
mainly to supplement the existing hydrographic
survey data used when preparing navigational
charts.
This system was used for a good part of the 20th
century, until electronic and acoustic technologies
arrived that allowed a single ship to do the same
work as two wire-drag vessels, using side-scan
sonar or multibeam sonar.
Wire Drag Operation
The wire drag operation operated at a speed of
around 2 knots. Each vessel was independently
controlled to maintain position along a pre-plotted
course. One of the vessels, the “guide ship,”
maintained a planned speed along the course
without regard to its position relative to the sister
vessel. The sister vessel would adjust its position,
to remain abeam of the guide ship, speeding up or
slowing down slightly as needed.
The buoys which suspend the drag wire were under
constant observation for any “hang,” (which was an
indication of a submerged object.) When a hang
occurred, it was necessary to determine as
accurately as possible the coordinates of the
obstacle, be it a wreck, shoal, or other form of
debris.
Location of the hang was determined by recording
the locations of each vessel and the bearing of each
vessel to the apex of the “V” which marked the
location of the hang. Scuba divers were then
dispatched to swim along the ground wire until the
snag was located. The obstruction was then
investigated and its exact depth measured by
divers using a lead line.
Use of Sister Ships
There were a limited number of ships which
specifically performed these wire drag operations
for the US Coast and Geodetic Survey, and later
NOAA. Until the late 80’s these ships were
commissioned jointly as “sister ships”.

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The “Fathom”
In the early days of
hydrographic surveying, a
fathom was used to
measure water depth. A
fathom is a linear unit of
measurement, equal to 6
feet.
“Mark Twain”
In modern literature, Samuel
Clemens took his pen name from
his early employment as a
leadsman on the Mississippi River.
The leadsman was the person
who used a sounding pole or lead
line to measure the river depths
to ensure the boat was staying
away from the shallow banks. To
"mark twain" referred to marking
a depth of the water of two
fathoms, or 12 feet.
Early Hydrographic Surveying
Methods – Sounding Poles and Lead Lines
Prior to 1900s – Poles and Lines
Shallow water surveys – In early hydrographic
practice, shallow water surveys consisted of depth
measurements conducted by use of sounding
poles.
Deep water surveys – Sounding Lines
In greater depths, a sounding line (or lead line) was
used to make measurements of river channel,
estuarial and oceanic navigational pathways.
Positions were determined by three-point sextant
fixes to mapped reference points. A more
sophisticated means of measuring deep water at
this time was through the use of sounding
machines, which were simply reel or drum based
line devices.
Sounding poles
A sounding pole (see image below) is simply a long
rod, (usually made of wood, or aluminum in
present day use), that is used to measure
navigational channel depths. Though mainly used
to measure water depth, they may be used to
measure the depths of boreholes, snow piles, or
loose soil stockpiles as well.
A typical length of a sounding pole is about 20 feet,
marked in one inch increments. They might have a
circular plate at one end to ensure that the location
being measured is somewhat level and normalized,
and to prevent inaccuracies due to the pole sinking
too deep into the
muck or silt bottom.
The pole technician
would drop the pole
until there is adequate
resistance at the soil
bed, and then a
measurement is taken
at that depth. Multiple
drops are made within close proximity to verify
that a consistent reading was taken.
Though rudimentary equipment such as sounding
poles have since been replaced by SONAR, LIDAR
and other bathymetric methodologies, they are still
sold by some
survey supply
companies, and
may be used in
areas where
large amounts of
vegetation or
thick mud would
tend to distort
the electronic
returns of
modern
equipment.
Lead Lines
Lead line systems consisted of ropes or lines, with
depth markings and
lead weights attached
(see image). The lead
weights were usually
conical in shape,
weighing around 12-
14 lbs., and
occasionally had a
small amount of wax
or lard on the base to
collect a small sample
of the surface sedimentary deposits. This
rudimentary but effective system could reportedly
provide reliable soil samples to depths of 100
fathoms or 600 ft.
The line was lowered
until it hit bottom,
then read manually in
a tedious, painstaking
process. These initial
depth soundings were
fairly accurate.
However due to the time involved per reading,
there were a limited number of sounding
measurements relative to the area being surveyed,
which inevitably left gaps in coverage between the
individual soundings.
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Types of Professionals at
the USCS
 mathematicians
 cartographers
 geodesists
 meteorologists
 hydrographers
 topographers
 sailors and laborers
 administrators
 to verify that navigational channels are
clear and accurate
 port and harbor maintenance (dredging),
expansion, and re-design
 coastal engineering and geomorphological
evaluations
 beach erosion and replenishment studies
 coastal zone management
 offshore resources development
 oil and gas exploration
 volumetric studies
 erosion and siltation studies
 pre- and post-dredge evaluations
 river crossing profiles and evaluations
 aquatic vegetation design
 flood studies
 to locate and verify the integrity of
submerged cabling and piping
 for determining fisheries habitat and
understanding marine geologic processes
 to determine seabed and riverbed materials
(i.e. sand, mud, rock)
 preliminary bed analysis for anchoring,
construction of marine structures, and
pipeline and cable routing
 disaster response following storm events;
looking for changes in depth or debris in
navigational channels
History of Hydrographic Surveying
Origins
The history of hydrographic and bathymetric
surveying likely dates back to the origins of sailing.
As long as people have sailed the oceans, there has
been the need to chart pathways for safe travel to
avoid vessel groundings, and to document regional
and seasonal oceanic currents and meteorological
patterns.
US Coast Survey
Establishment of the US Coast Survey – 1807
In the US, the first officially sanctioned
hydrographic surveying office began in 1807, with
the establishment of the “US Coast Survey”, which
is one of the oldest scientific agencies in the US.
This agency was established when then President,
Thomas Jefferson and Congress authorized a
survey to be taken of coasts of the newly formed
“United States of America."
First Head of the US Coast Survey
The first person to officially head the US Coast
Survey was Ferdinand Rudolph Hassler, who
assembled a number of skilled professionals from
various scientific communities, to survey and chart
out the coastlines of the US.
The first official US survey - 1834
The first official hydrographic survey performed in
the US was conducted along the southern shore of
Long Island in 1834.
The first official US nautical chart – 1839
In 1839, the US government produced its first
nautical chart. However, many privately
commissioned charts of the Americas were known
to exist prior to this date.
Navigational chart of the Potomac estuary

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08/07/2026

Comprehensive Guide to Hydrographic
Surveying

Hydrography is the science of measuring and describing the topographical
features and the entire aquatic environment as a whole, beneath the
surface of water bodies. These subsurface features affect bridge scour,
flood mitigation, erosion control and siltation transport, maritime
navigation, marine construction, dredging, offshore oil exploration and
offshore oil drilling, among many other activities.
Hydrographic surveying does not strictly apply to coastal and oceanic
regions of the US. Hydrographic surveying has many applications inland.
Many environmental and civil engineering project in and around bodies
and channels of water (streams and rivers, lakes and ponds, wetlands) can
all benefit from hydrographic surveys and analysis.
Topics
 Intro and overview of hydrographic surveying
 History and early methods of measurement
 Sounding poles, lead lines, wire drags
 Echo sounders and fathometers
 Agencies which perform hydrographic surveys: office of coast
survey, NOAA, USGS, COE, IHO, NOO
 Marine magnetometry
 DC resistivity imaging
 Radar altimetry
 GPR for marine applications
 SONAR devices: side scan, single beam, multi beam sonar
 Light Pulse (LIDAR) devices
 US maritime limits and boundaries
 Sources of existing bathymetric datasets
 Types of tidal datums (sounding and chart) and tidal cycles
 AUV, ROV and UAS technologies
 Crowdsourced bathymetric surveying
 Fisheries acoustics

Bathymetry
The term "bathymetry," originally
nautical in origin, referred to the
ocean's depth relative to the sea
level (free surface), although it
has since come to refer to the
“submarine topography,” or the
depths and shapes of the
underwater earthen terrain.

Hydrography is a branch of physical oceanography
or applied science which deals with the measuring
and description of the configuration of the bottoms
and adjacent land areas of oceans, lakes, rivers,
harbors, and other water forms.
What are Hydrographic Surveys?
A hydrographic survey deals with measuring and
description of features of a water area and the bed
configuration; however, it may also encompass a
wide variety of other objectives such as the
measuring of tides, currents, gravity, Earth
magnetism, and for determining the physical and
chemical properties of water.
Bathymetric Surveying
The label of hydrographic surveying is often
synonymously applied to bathymetric surveying.
However, the
latter actually
refers to the
study and
measurement of
the bottom or
“bed” of a
navigable water
body (whether it
is an ocean, river,
lake or other body or channel of water.) In other
words it deals with the topography of the solid
earth, beneath a body of water.
Below are various types of hydrographic surveying
methods.
Hydrographic Surveys for Safe Navigation
There are numerous reasons to conduct a
hydrographic survey, but the most common
objective is for compiling nautical data along
navigational channels, with an emphasis on the
measurement and profiling of the subsurface
physical features that may affect safe navigation
and travel by marine vessels.
The image above shows a dredging operation
consisting of a clamshell bucket dredge on a barge
Documenting the Geomorphic Changes
Routine hydrographic surveys are essential to
documenting the transportation of silt and
sedimentation, in order to unsure safe lanes of
shipping in rivers and along coastlines.
Natural currents and storm events such as
hurricanes tends to shift the silt on the sea or river
beds, potentially accumulating in previously
dredged channels, causing dangerous conditions or
obstructions.
Locating Submerged Obstacles
In addition to the surveying of geomorphic
changes, hydrographic surveys may be utilized to
locate submerged vessels and other hazardous
debris along river and sea beds.
A hydrographic survey can be used to facilitate
offshore oil and gas exploration, drilling operations,
marine construction, dredging, installation of
subsea cables and more.
Other Applications
Other applications for hydrographic surveying:
 to generate the data needed to create and
update nautical charts

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