LiDAR Bathymetry for Nautical Charting
Sub Theme : Coastal and Marine Mapping
Cdr. Sanjeev Sharma, IN (Retd.)
Head Hydrography
IIC Technologies Ltd, Hyderabad, AP – 500 034 (India)
E-mail – sanjeev.sharma@iictechnologies .com
Abstract - Safety Of Life at Sea (SOLAS) is the
overarching principle for all offshore operations
and one of the major contributing factors to
SOLAS is quality navigational charts that
are comprehensive and adhere to International
Hydrographic Organisation (IHO) established
standards of accuracy. However, a chart is only
as good as the data that is used to compile it and
one of the more challenging areas for
hydrographic surveyors is the coastal/shallow
areas where it can be very time consuming,
difficult and sometimes impossible to obtain
data using traditional acoustic techniques. The
main weapon in the hydrographer’s toolbox is
the sonar, whether it is a multibeam, singlebeam
or sidescan. Even with these tools we cannot
always get data in the shoalest area due to a
variety of factors including the vessel’s draught,
dangerous terrain etc.
scanning, pulsed laser beam operated from an
aircraft. It is also known as Airborne LiDAR
Hydrography (ALH) when used primarily for
nautical charting. It not only provides the
capability to survey faster but also enables the
hydrographer to reach dangerous areas
inaccessible by boat.
Though not considered a replacement for
acoustic surveys, ALB can be used in
conjunction with traditional acoustic surveys
for better area coverage and charting the
unreachable areas. The paper discusses LiDAR
bathymetry concept, its advantages and
limitations, operational methodologies, meeting
the accuracy challenges with reference to a
combined acoustic and LiDAR survey that IIC
Technologies has recently undertaken.
I. INTRODUCTION.
Advancement and development in laser
technology, electronics and computing systems
over the past few decades has resulted in
development of systems capable of conducting
bathymetric surveys using platforms other than
traditional boats/ ships.
Airborne Laser (or LiDAR) Bathymetry
(ALB) is a technique for measuring the depths
of relatively shallow, coastal waters using a
Airborne Laser (or LiDAR) Bathymetry
(ALB) is a technique for measuring the depths of
relatively shallow, coastal waters using a scanning,
pulsed laser beam operated from an aircraft. It is
also known as Airborne LiDAR Hydrography
(ALH) when used primarily for nautical charting.
It can be effectively used for varied hydrographic
applications such as nautical charting, repetitive
port and harbour surveys, rivers/ lakes/ canals/
inland waterways charting surveys, coastal zone
management and environmental assessment
surveys. Apart from providing the capability to
survey faster it also enables the surveyor to reach
inaccessible areas by traditional boats and
simultaneous coverage of land and sea areas.
II. HISTORY OF LIDAR.
Airborne Laser Hydrography (ALH) concept
grew out of efforts in mid-sixties to use the Laser
technology, primarily for Submarine detection.
The seminal paper confirming the ability to
perform near shore bathymetry was written by
Hickmann and Hogg based on work done at the
Syracuse University Research Center, USA. The
first generation airborne hydrographic LiDAR
systems were successfully tested by US, Canada
and Australia in early 1970s. Joint efforts by
NASA, NOAA and US Navy led to production of a
scanning Airborne Oceanographic LiDAR (AOL)
for hydrography. Successful testing of AOL was
conducted in 1977. In 1980 the experience with the
AOL led to the HALS (Hydrographic Airborne
Laser Sounder) program sponsored by the US
Navy, the Defense Mapping Agency (DMA) and
NASA. Other second generation systems were
built and tested in Canada, Australia and in Soviet
Union.
In 1986, the LARSEN 500 system was built by
Optech, sponsored by the Canadian Hydrographic
Service (CHS) and based on the surveys performed
in Northwest territories, it became the world‟s first
operational ALH system.
Successful testing of Australian WRELEADS
II in 1980s lead to construction of its operational
version called LADS for the Royal Australian
Navy.
In 1990s, systems that became operational were
LADS for Royal Australian Navy, SHOALS
(Scanning Hydrographic Operational Airborne
LiDAR Survey) for US Army Corps of Engineers
(USACE) and Hawkeye ALH system in Sweden.
LADS is flown in a dedicated Fokker F-27
fixed wing aircraft, SHOALS is operated from
NOAA Bell 212 helicopters and various recent
platforms, the Hawkeye systems operated from
different types of helicopters, whilst the Canadian
LARSEN-500 system continued to perform in
several fixed wing aircrafts.
Few new systems in development/ validation
stages, worth mentioning, are CZMIL (Coastal
Zone Mapping and Imaging LiDAR) presently in
validation phase and Airborne Laser Terrain
Mapper (ALTM) Aquarius, both from Optech and
Chiroptera from Airborne Hydrography AB
(AHAB) for very shallow applications (< 10m)
III. ALB CONCEPT
The general technique of laser bathymetry
involves use of a pulsed laser transmitter with
green and Infrared (IR) beams. The green laser is
used for bottom detection since this wavelength
can penetrate typical coastal waters with least
attenuation. The IR laser penetrates very little and
is used for sea surface detection. Depending on the
system design, IR beam may be nearly collimated
and scanned collinearly with the green beam, or it
may be broader and constrained at nadir. Red
energy generated in the water from green-excited
Raman backscatter (the incident 532 nm green
laser frequency is altered at the air/water interface,
shifting it to red spectrum) immediately beneath
the air/ water interface may also be used as a
surface return when its arrival time is properly
corrected to the interface. The transmitted laser
pulses are partially reflected from the water surface
and from the sea bottom back to the airborne
receiver. In effect, distances to the sea surface and
bottom can be calculated by measuring the times of
flight of the pulses to those locations and knowing
the speed of light in air and water. Water depths
are determined from the resulting time differences
and corrected for known errors such as electronic
and atmospheric delays.
The receiver consists of a telescope, various
optical filters and field-of-view controls, light
detectors, amplifiers, analog surface detection
logic, and analog-to-digital converters (a digitizer).
The receiver, system control logic and tape storage
are all operated under computer command.
Because of the complexity of the environment and
of the interactions of the LiDAR beam with the
environment, it has not been possible to calculate
all depths with high accuracy and reliability in real
time. Approximate depths are calculated in the air
for quality control, but precise depths, involving
more-detailed calculations and a limited amount of
manual intervention for difficult cases, are
determined via post-flight processing of stored
waveforms. A schematic representation of the ALB
concept is depicted in Fig. 1.
crystal) pulsed dual frequency laser to output
simultaneous pulses at 532 nm (green beam) and
1064 nm (IR beam). As discussed earlier, the IR
beam is used to detect the water surface whilst the
green beam penetrates water and is used to detect
the seafloor. A moving average window is used to
create a mean water surface, in order to remove the
wave action and heave so that tides can be applied.
Distances to the sea surface and seafloor are
calculated from the times of the laser pulses, using
the speed of light in air and water. System
orientation while airborne is typically provided by
the Applanix POS AV Inertial Movement Unit
(IMU) or equivalent.
The bathymetric LiDAR system also
incorporates digital RGB cameras which acquire
one 24-bit or better colour photo per second. The
POS AV, combined with and the relatively low
flying heights, helps the camera to produce a pixel
resolution of 8-30cm (depending upon flying
height and actual system). This digital camera
imagery forms an important QC tool as it is used
during processing to detect elements interfering
with the laser (vessel traffic, Sun glint, turbidity
etc.) and to assist in the identification of
bathymetric features and aids to navigation.
IV. ALB ADVANTAGES.
The reasons for pursuing such an innovative
and expensive technology, arise form the following
requirements:-
Fig.. 1.
The LiDAR bathymetry concept (Photo courtesy - Canadian
Hydrographic Service (CHS))
All systems use a solid state Nd-YAG
(Neodymium doped – Yytrium Aluminum Garnet
1) Perform surveys within small operational
window
2) To complete survey quickly
3) Rapid mobility
4) Fairly large survey areas
5) Survey where it would be difficult or
impossible to use traditional water borne
techniques of surveying.
6) Mobility to perform rapid assessments of
seasonal change.
7) Better swath coverage.
LiDAR
systems with relatively fixed swaths independent
of depth are efficient for surveying in shallow
waters to IHO Order 1 accuracy. MBES systems,
whose swath widths are reducing with a decrease
in depth, are more efficient in deeper waters and
areas where higher order of accuracies are
envisaged. Fig. 2 represents a graphic comparison
of LiDAR and sonar operations in shallow waters.
altitude. Larger scanner nadir angles would mean
unacceptably larger timing errors in the surface and
bottom returns due to extreme geometry. A typical
survey flight mission is of about 5 – 6 hrs
depending on the type of aircraft in use and the
weather conditions. For a 6 hour mission with a
65% online fraction, about 70 km2 can be
surveyed. Higher coverage rates can also be
achieved by having a wider swath width coupled
with a faster air craft speed, this would have an
adverse effect on the sounding density.
VI. LIMITATIONS
Fig.. 2. A comparative depiction of LiDAR and Multi-beam swath coverage.
(Photo courtesy – IIC Academy)
8) Simultaneously survey water and terrestrial
areas. ALB provides unique survey opportunities
and capabilities in shallow waters and across the
land/water boundary which are worth even if they
are expensive. A few examples could be beach
gradient and coastal engineering surveys.
V. AIRCRAFT OPERATIONS
Most of the aircrafts operate at an altitude
of 300 – 500 m with an average speed over ground
of 130 kn. The maximum scanner nadir angles are
up to 20° which corresponds to survey swaths with
widths roughly equal to one half of aircraft
A.
Water Clarity.
It is the most
significant limitation for Airborne Laser
bathymetry systems. For a typical eyesafe
operation, the max surveyable depths range from
50 m in very clean waters to less than 10 m in very
murky near shore waters. Surveying may not be
possible for extreme murky waters (due to
turbidity, water discharge from industrial plants,
muddy water, algal growth etc.). As a rule of
thumb, successful operations can be expected for
depths between 2 – 3 times the Secchi disk
reading. However, it is a broad estimate and cannot
be relied 100% for aircraft operations unless you
run a test flight in the survey area. The Secchi
depth is not a very good predictor of performance
as the Secchi disk readings can vary daily at the
same location. This is because its relationship to
the proper optical parameter, the attenuation
coefficient, varies with the scattering to absorption
ratio. The water property which most nearly
dictates the received bottom return pulse energy in
an ALB system is the diffuse attenuation
coefficient (K) at the Green laser wavelength (Fig.
3).
altitude, the resultant reflections can reduce the
quality of the surface and seabed return. In extreme
shallow waters (less than 0.5 m), the shallow water
algorithm does not work well and will drift in and
out thus causing variation in the depths. This is due
to the fact that surface and seabed return are so
close on the waveform that it is beyond the
system‟s ability to discriminate between the two
returns.
Fig. 3. Jerlov curves. Greater the K Value, dirtier is the water column.
(Jerlov, N.G., 1976)
B.
Sun Glint.
Sun glint is a known issue
with bathymetric LiDAR and line direction is
planned to reduce its impact. Sun glint increases
the amount of noise in the data which becomes
more significant with weak bottom returns from
deeper areas combine with areas of high turbidity
C.
Small Object detection.
Another
limitation of ALB systems is that it cannot prove,
beyond doubt, that a navigation channel is free of
small objects of the order of 1 m3. The problem lies
in the fact that it is almost impossible to resolve the
small target return in the presence of the much
stronger and immediately following bottom return.
The detection probability for such small objects
can be increased by increasing the survey density,
but this would not be a fool proof method and is
also not economically a viable option. In general, it
can be said that, objects with larger surface areas
and smaller heights are well detected, as are
objects with smaller areas and larger heights.
D.
Detection in extreme Shallow waters.Very
Shallow water can cause extreme glint even at low
E.
Operational limitations.
Weather
plays an important role in successful ALB data
collection. It not only restricts the aircraft‟s
operational window, but weather conditions such
as low cloud, fog and rain can also severely affect
the LiDAR performance. Another factor limiting
the operational data collection window is the
permissions and clearances from the Air Traffic
Control (ATC) to operate the aircraft due to
prevalent traffic at airports.
F.
Logistic considerations.
One of the
most important factor limiting ALB surveys is the
plethora of logistic considerations and meticulous
planning for aircraft operations. An aircraft not
only needs daily permissions to fly, but also needs
to carry out routine planned maintenance schedule,
effective ground support infrastructure, daily
refueling, restricted availability of power supply
(all equipment has to be run on the aircraft
generator supply, no additional generator can be
installed) and above all aircrew rest as per FAA
safety recommendations.
VII. PLANNING AN ALB SURVEY
A.
Environmental Risk study. Climatology
is an important criterion to choose the season of
survey. Hence it is most important to conduct an
environment risk study, even if it is a statistic
figure and depends on the hazards of the weather.
The risk can be minimized by extending work over
large areas, presenting different weather patterns.
The weather prediction reports must be exploited
for the operation of the aircraft and the laser
sensor. The important parameters to be checked are
wind, sea state, precipitation, heat & humidity and
cloud cover. Tidal predictions allow one to know
not only the height and time of the High Water/
Low Water, It also gives an indication on the
stream coefficients during Spring/ Neap tides and
can help in decision making for surveying either
during Low water or High water to get good
coverage in very shallow waters.
B.
Line Planning.
An ALB survey line
planning, unlike MBES survey lines, is
independent of the depth. The line spacing is of
fixed width depending on the density and coverage
requirements, something akin to single beam
surveys. However, the line planning is guided by
the topography and wind conditions in the survey
area. It will be prudent to plan survey lines in the
direction of wind and along the coastline. ALB
survey lines should not be planned perpendicular to
the depth contours if there is an abrupt change
(steep change) in bathymetry, as the depth
recording algorithms take some fractions of
seconds for providing a depth solution from
shallow to deep waters.
C.
Ground truthing.
ALB ground truthing
is akin to the calibration patch tests for MBES
surveys. It goes up a notch further as an essential
element in proving that the survey is not only
progressing as per the required specifications but it
also provides mechanism for monitoring and
identifying system changes over a period of time.
Ground truthing over water requires MBES
support. For this, a suitable patch with flat seabed
or gently shelving seabed is required. For an
efficient sea control patch the MBES itself has to
be calibrated and relied upon for accuracy
compliance. The sea control patch site should not
be chosen at the extreme limit of LiDAR depth.
The calibration flights are then run daily over the
chosen calibration patch to ensure system
compliance and satisfactory performance.
VIII.
IIC EXPERIENCE
IIC technologies along with its partner
Pelydryn Ltd, UK, recently completed an ALB
survey in the Red Sea for General Commission for
Survey (GCS), Kingdom of Saudi Arabia (KSA).
The survey was conducted using a Hawkeye IIb
bathymetric LiDAR sensor (Fig. 4a) fitted on a
Caravan aircraft (Cessna) operated by CAE Ltd
(Fig. 4b)
Fig. 4a. Hawkeye IIb LiDAR sensor
Fig. 4a. Caravan Aircraft (Cessna)
A.
Survey planning.
The requirement for LiDAR in this project,
was to survey depths less than 30 m. Based on the
initial assessment from existing nautical charts, the
survey area (Fig. 5a) was divided into 8 blocks
(Fig. 5b). Following an initial period of operations
by the MBES, the potential for a large number of
uncharted shoals was deemed to present a hazard
to operations. Subsequently, an additional block
was planned for survey to detect any uncharted
shoal / reef (Fig. 5c)
Fig. 5b. Survey Area
Fig. 5a. Caravan Aircraft (Cessna)
Fig. 5c. Additional Survey Block (In Red)
B.
Challenges during the conduct of survey.
1) Meteorological Impact.
Weather
during the early part of the year had little impact
on operations with 3 ½ days being lost to
sandstorms (figure 6) and inclement winds. The
major problem was loss of visibility with 10km of
visibility dropping to less than a km in minutes.
3) Extreme Shallow waters.
Designed to
operate across the land/sea interface the system
generally can detect depths up to 0.5m. In most
areas the range of tide allows either a deeper return
at a higher state of tide or a topographic detection
at low water. The tidal range experienced in
survey area was no more than 0.5m during the
survey, which meant that the shallow fringing reef
plates (figure 8) would be difficult to survey.
Fig. 6. Sandstorms
Fig. 8. Extreme Shallow Waters
2) Sun Glint.
Sun glint (Fig. 7) was a
significant factor whilst surveying deeper areas
later in the period, when the sun angle remained
higher than 70 degrees for longer periods during
the day. Subsequently, flying sorties were timed
around the sun‟s elevation to reduce its impact.
4) Land Reclamation and Dredging activities.
Dredgers working in and around Jeddah
bay (Fig. 9a) undertaking land reclamation &
channel clearance and vessels operating from the
port (figure 9b) posed a serious block in the
progress of LiDAR survey.
Fig. 7. Sun Glint
Fig. 9a. Dredgers operating for channel clearance
The turbidity in water as a result of these
activities, reduced depth penetration in the area
already suffering from sediments and shipping
activities. Such areas, though flown twice, had to
be covered again by MBES.
Fig. 11b. Sink Holes in waveform format
This is a result of a number of reasons:
Fig. 9b. Vessels operating in port
5) Coverage in deep reef pockets / sink holes.
The reef structures consist mainly of
limestone and this is prone to erosion and collapse
and this evident in the large number of sink holes
(Fig. 11a, 11b) that were encountered on the reefs.
In a large number of cases no coverage of the
seabed at the bottom of the sinkhole could be
achieved.
The seabed lies beyond the extinction depth
of the system.
ii) The sinkhole size and orientation is such
that the system is unable to detect the
seabed but can detect the steep sides due to
masking from the shallower reef plate and
shoals that may be within the sinkhole or
channel (Fig. 11c).
i)
Fig. 11c. Graphic depicting seabed detection in sink holes
Fig. 11a. Sink Holes
6) Complex Geomorphology. There are two
recognised reef types present in the Red Sea. The
first type called the fringing reef is generally
connected to the coast or surrounds islands. This
reef type is evident all along the coastline and on
offlying islands. In between these reefs a second
reef type is present. Called a Platform or patch
reef they are scattered in the calm, shallow waters
between the mainland and edge of the continental
shelf often rising from great depths to form
isolated coral pinnacles. They often tend to be
more broken up and do not always present a
defined barrier to the sea and these have been
encountered in the area surveyed covering
extensive areas.
Fig. 12. Complex Geomorphology
The reef structures are complicated (figure 12) and
provide unique challenges for hydrography. The
reef flat is often in very shallow water and provides
additional challenges to LiDAR systems because
of the shallow depths encountered. Reef crests are
easily visible and often form small islands. As the
water gets deeper the fore reef and shelf can extend
for significant distances before the reef edge and
drop-off is encountered. Alternatively on the
seaward side of fringing and patch reefs they can
abruptly drop off in a near vertical wall. Within
the reef flats and fore reef it is common to find
isolated coral heads, gullies and highly irregular
seabed due to the breakdown of dead coral to form
boulders and new growth on and around it.
C. The Accuracy Challenge.
The accuracy standards, accepted for
hydrography are established by the International
Hydrographic Organization (IHO) in Monaco and
disseminated in Special Publication 44. The
accuracies achieved for the present survey fall in
IHO Order 1 (1b) category. Table I summarizes the
Horizontal and vertical uncertainties computed
from the survey error budget and a-posteriory
computed uncertainties of each equipment.
TABLE 1
ACCURACY STANDARDS ACHIEVED
AS AGAINST IHO ORDER 1B ACCURACY STANDARDS
Depth
(m)
Horizontal Uncertainty
IHO
Specified
Achieved
for Order
1b
Vertical Uncertainty
IHO
specified for
order 1b
Achieved
5
5.25
2.318978691
0.504207299
0.237321466
10
15
20
25
30
35
5.5
5.75
6.0
6.25
6.5
6.75
2.546759573
2.886726157
3.304432315
3.774154128
4.278795072
4.807371069
0.516623654
0.536679606
0.563560112
0.596343022
0.634113554
0.676036242
0.28017122
0.339784201
0.408893202
0.483442813
0.561269287
0.641180496
is envisaged. ALB systems can be used safely, for
efficient conduct of survey by MBES systems, by
delineating the dangerous areas and features that
might imperil the survey boats. If used wisely, the
ALB systems can provide great cost savings whilst
ensuring good coverage benefits and timely
completion of surveys.
ACKNOWLEDGEMENTS
-
General Commission for Survey, Kingdom of Saudi Arabia
Véronique Jégat, IIC Academy
REFERENCES
[1] G. C. Guenther, A. G. Cunningham, P. E. LaRocque, and D. J. Reid,
“Meeting the accuracy challenge in Airborne LiDAR Bathymetry,” in
Proc. EARSel, Dresden, Germany, 2000.
[2] Hickman G.D. and Hogg, J.E., 1969. Application of an airborne pulsed
laser for near-shore bathymetric measurements, Remote Sens. of Env.,
1, Elsevier, New York, 47-58.
[3] Guenther, G.C., 1985. Airborne laser hydrography: System design and
performance factors, NOAA Professional Paper Series, National Ocean
Service 1, National Oceanic and Atmospheric Administration,
Rockville, MD, 385 pp.
[4] Guenther, G.C., 1989. Airborne laser hydrography to chart shallow
coastal waters, Sea Technology, March, Vol. 30, No. 3, 55-59.
[5] Jerlov, N.G., 1976. Marine Optics, Elsevier Scientific Pub. Co.,
Amsterdam, 231 pp
[6] Presentation “Bathymetric LiDAR for Hydrographic Charting” by Dr.
Hilario C. Lamotte , CHS
[7] Website http://www.optech.ca
[8] Website http://www.airbornehydro.com
ABOUT THE AUTHOR
IX. CONCLUSION.
Though not considered a replacement for
acoustic surveys, ALB can be used in conjunction
with traditional acoustic surveys for better area
coverage and charting the unreachable areas.
LiDAR systems with relatively fixed swaths
independent of depth are efficient for surveying in
shallow waters to IHO Order 1 accuracy. MBES
systems, whose swath widths are reducing with a
decrease in depth, are more efficient in deeper
waters and areas where higher order of accuracies
Cdr. Sanjeev Sharma, IN (Retd.) is the Head of
Hydrography at IIC Technologies. With over 14 years
of professional experience in planning and executing
hydrographic surveys of national and international
importance, the author is an IHO accredited Cat „A‟
hydrographic surveyor and holds a Master‟s degree in
Hydrographic surveying.