ADVANCES IN MULTIBEAM BACKSCATTER DATA PROCESSING AND THE
DELIVERY OF DATA PRODUCTS IN A GIS ENVIRONMENT VIA THE INTERNET
Bill Gilmour – Thales GeoSolutions (Pacific), USA
Doug Lockhart – Thales GeoSolutions (Pacific), USA
David Millar – Thales GeoSolutions (Pacific), USA
1.
ABSTRACT
Thales Geosolutions (Pacific) Inc. has been successfully
conducting fisheries habitat mapping for government
agencies for a number of years, and has pioneered new
data acquisition, processing and presentation techniques
associated with increasingly high volumes of data.
The presentation will show the latest techniques for the
processing of multibeam backscatter mosaic images,
including collection of Footprint Time Series data for
each ping and beam, which has resulted in higher
quality and higher resolution data being available for
habitat characterization.
A result of the new technologies has been a magnitude
increase in the volume of raw data collected and in the
resultant products, with one of the products being a very
high-resolution geo-referenced image of the seabed.
These images can easily exceed 1 GB in size,
depending upon the resolution selected and the size of
the area presented. Thales GeoSolutions (Pacific) is
able to serve this imagery over the Internet, thus
providing the remote user with access to the image and
even going so far as to provide remote interpretation
and classification of benthic habitats. This is possible
through the use of ESRI’s ArcGIS on the client side and
ESRI’s ArcIMS on the server side or with a standard
internet browser on the client side and ESRI’s ArcIMS
on the server side.
Either solution is possible,
depending upon the sophistication of the user and the
tools available on the customer’s site.
In one case, the remote user can connect to an IMS
service running on Thales GeoSolutions’ server from
ArcGIS and load the imagery as if it were hosted on
their local machine.
GIS layers presenting the
interpreted classifications and habitats can be created
and saved locally. In the other scenario, the remote user
can visit Thales GeoSolutions’ “Chartroom.com”
website and interact with the imagery via standard webbrowser. There the user can use IMS tools to remotely
interpret classifications and habitats, creating GIS layers
that are stored on the server side and can subsequently
be added to hard copy plots or served later over the
Internet.
2.
INTRODUCTION
There is a growing demand for improved mapping of
the ocean for scientific and engineering purposes. There
is also considerable pressure on government budgets
and hence on collaboration between government
agencies to allow joint projects and multiple use of data.
This has resulted in some interesting pilot projects.
Thales GeoSolutions (Pacific) has been involved in
several of these projects and in parallel has been
developing new software and techniques to provide the
user with high-resolution seabed images in their GIS
environments.
Recognition of the superior data and coverage provided
by multibeam echosounders
(MBES) has been
widespread and is increasing rapidly. In addition to the
significant value of ensonification of a swath, as
compared to a profile of bathymetric data, the acoustic
data have been frequently presented as “pseudo side
scan sonar”. Improved technology and increased
bandwidth from the sonar head since the pioneering
days of Simrad in the 1980’s have provided the
opportunity to develop enhanced acoustic imagery by
manipulation of the backscatter information within each
MBES beam. This work has resulted in improved
MBES data products using new acquisition and
processing techniques.
During 2001, Thales GeoSolutions (Pacific) conducted
extensive MBES operations in US waters of the Pacific
Ocean. Working closely with Reson, Inc. and Triton
Elics, Inc., we have jointly added the capability to
record the raw backscatter data from each beam for
each ping of the Reson 8000 series systems. We refer to
this as “Snippet” or our terminology is Footprint Time
Series (FTS).
The FTS data produced by a Reson system is raw. Each
beam has a time series for each ping. The data are not
altered by the system in anyway. Simrad MBES also
log backscatter data in a format that contains a distinct
time series for each beam. The Simrad backscatter,
however, is compensated for power, gain and grazing
angle. In this respect, Simrad does not log raw
backscatter.
During 2001, initial products were “stitched
together“ from each beam, within the Reson system,
providing a high-resolution image that was exported and
processed as a side scan sonar record. In 2002,
continued developments have enabled processing of the
full captured FTS resulting in further improvement in
image quality
techniques.
and
more
automatic
processing
This paper will provide some background information
on multibeam backscatter data acquisition and
processing, then explore some of the technologies that
enable these datasets to be accessed in GIS formats via
the Internet, showing some provisional products from
the new processing techniques.
3. TECHNOLOGIES
Figure 1 Derivation of pseudo side scan sonar imagery
3.1 Mulibeam / Backscatter
Multibeam backscatter data are available in several
forms from the Reson 8000 series of multibeam
echosounders – Beam intensity, pseudo side scan, and
Footprint Time Series. Data volume changes with each
format and sounder, but is generally in the range of
2,000 points per swath in the older pseudo side scan
format to 20,000 points per swath in the latest FTS
datasets.
FTS records have considerable overlap
between beams.
Pseudo side scan imagery is derived from sampling
across the full width of the transmit beam at each time
interval “t” indicated by the horizontal arrow. The
vertical arrows indicate data output for each time
interval to the waterfall display, as shown in Figure 2.
In general, the advantages of the multibeam backscatter
over side scan sonar information are derived from the
following:




precise co-registration of the backscatter with the
multibeam bathymetry data set,
improved signal-to-noise compared to conventional
imaging sonar,
increased resolution (smaller pixel size) for the
final products, and
Increased survey speed and shorter line turns due to
no towed sensors.
These data characteristics are discussed in more detail
below. In addition, examples of various data formats
have been shown to illustrate the results.
3.1.1 Multibeam backscatter sampling
Side scan sonar data are acquired by forming a large
beam on either side of the towfish. Time series data are
sampled across these beams by summing all of the
returns from any giving time into one pixel. Some
multibeam systems have emulated this data through
digital beam forming, referring to the resultant image as
“pseudo side scan sonar”. An example of pseudo side
scan imagery is presented in Figure 1.
The diagram graphically illustrates the single fanshaped beam pattern with the broad time-arcs that sum
into each time sample. The imagery example in Figure 2
shows typical pseudo-side scan-sonar results over a
rocky outcrop near Kodiak, Alaska.
Figure 2 Screen-grab of pseudo side scan sonar image
from waterfall display
Multibeam backscatter is acquired by sampling an
individual time series for each beam in the system.
Sampling only occurs in the region of the bottom detect
rather than throughout the water column (time-arc).
Multiple beams may have samples acquired at the same
time due to seafloor relief. However, those samples will
be only assigned as backscatter returns for their
respective individual beam footprint. As a result, the
signal-to-noise ratio of the backscatter record is greatly
improved.
This is demonstrated in the diagram in Figure 3 and the
resulting image improvement for the same portion of the
seabed off Kodiak Island illustrated in Figure 4.
Figure 3 Derivation of multibeam backscatter imagery
Multibeam backscatter imagery is derived from
sampling across the narrow width of each received
beam at each time interval “t” indicated by the
horizontal arrow. The vertical arrows indicate data
output for each time interval to the waterfall display, as
shown in Figure 4.
In its raw form, the boundaries between the individual
beams can be seen in a waterfall display as shown in
Figure 5. Once processed, in either the multibeam
firmware or post-processing software, the data become
seamless and are indistinguishable from side-scan sonar
data as shown in Figure 6. The stitching of the raw FTS
data packets into an image is done by statistically
combining adjacent FTS, which partially overlap each
other.
Figure 5 Screen-grab image of raw backscatter returns
(top panel)
Figure 6 Screen-grab image of raw data shown in Figure
5 when reformatted into seafloor image (bottom panel)
The backscatter time series from an individual beam is
being referred to as a Snippet by Reson or as a
Footprint Time Series (FTS) by Triton Elics. This data
packet represents the acoustic backscatter in its most
basic raw form. Sonar power, gain, pulse width,
spreading loss, etc are logged along with the snippets so
that they can be normalized and reduced to a calibrated
standard. Current processing development is focused
on this data and is pointed toward robust sediment
classification.
In some ways, FTS data is processed much like
sounding data. The position of each backscatter sample
is placed on the seafloor using the full 3-D motion of
the sonar head and the seafloor terrain. Refraction
corrections are used to refine the position. Grazing
angle corrections are taken from the clean bathymetry
model.
3.1.2 Multibeam backscatter geo-referencing
Registration of the backscatter data with the acrosstrack bathymetry is based on the intersection of the slant
range with the digital seafloor profile. Therefore, the
backscatter value is placed at the correct depth on an
irregular seafloor. More specifically, the final data
product will yield the image pixels precisely located in
3-dimensions, a capability developed by Thales
GeoSolutions (Pacific) to support NOAA-sponsored
fisheries habitat and tsunami regional surveys
conducted over the past two years.
Figure 4 Screen-grab of multibeam backscatter image
from waterfall display
The MBES collects a series of backscatter records
across-track for each ping. These backscatter data are
mosaicked on the terrain as noted above using pixel size
no greater than 0.1% of the water depth per pixel (i.e.
5cm pixels in 50 meters water depth). The placing of
imagery on terrain results in more accurate placement of
the acoustic data. Importantly, the hull-mounted
transducer offers significant improvements in the
positioning of the sonar beams compared to towed
sensors.
In deeper water, the resolution of some side scan sonar
systems will be better than current multibeam systems
due to the frequency required and also footprint size.
Positioning of the FTS data will be more accurate than
the side scan mosaic.
In shallow water, the systems look similar, as illustrated
in section 4. Positioning on the FTS mosaic will be
more accurate then the side scan mosaic. The FTS
mosaic will also be free of surface return and water
column noise.
3.2 Geographic Information Systems (GIS)
Commercial GIS systems provide a range of data
management functions, including the graphical
presentation of geographical data, spatial analysis
functions, and charting. Due to the spatial nature of
marine survey data, it is most efficiently used in a
geographic information system. Raster imagery of the
seabed and the vector layers of interpretation that are
often created from this imagery are best presented,
manipulated, analyzed and distributed within a GIS
environment. The new distributed GIS technologies that
are currently available from ESRI provide mechanisms
for sharing marine survey data both within a local
workgroup or a more widely distributed audience over
the Internet. ESRI’s ArcGIS product line provides a
scalable set of software with the same underlying
architecture and user interface, as illustrated in Figure 7.
This single integrated platform for geographic data
creation, management and analysis dramatically
increases both usability and interoperability. One of the
greatest strengths of ArcGIS to the fisheries habitat
mapping application is the software’s capability to
integrate Internet map and data services.
A key aspect of Thales GeoSolutions’ development
strategy is the company’s close working relationship
with ESRI, the chosen vendor of GIS technologies for
spatial data management solutions at Thales
GeoSolutions (Pacific). Thales GeoSolutions (Pacific)
is a consultant and developer business partner with
ESRI.
Figure 7 ArcGIS scalable architecture (diagram taken
from ESRI’s web site)
3.3 Image Compression
The viewing and distribution of large raster images has
historically been a challenge for both GIS systems and
GIS operators. The inclusion of large raster images
within desktop and especially web-based GIS
environments has often degraded system performance
so significantly that such imagery could not be used or
at a minimum, its use required very clever management.
While high-resolution aerial photographs and satellite
imagery have traditionally been problems for terrestrial
GIS applications, high-resolution seabed imagery has
similar issues on the marine GIS side. In recent years
however, significant improvements have been made in
both the compression and distribution of raster imagery.
Thales GeoSolutions uses a combination of two
commercially available and competing image
compression technologies in its marine GIS solutions.
The choice on a particular application depends on many
factors, including image size, user requirements, user
constraints and desired solution type (desk-top or
Internet). The company will use MrSID by Lizardtech
ECW by ER Mapper, and or IWS by ER Mapper
depending upon the requirements of the project.
All products are very similar in their function. They
encode large geo-referenced image files into smaller
sized, high quality images that maintain the geometric
accuracy of the original image. When used on maps,
satellite imagery, aerial photographs and seabed
imagery, a compression often exceeding 95% is
possible.
Lizardtech’s MrSID product is a powerful wavelet
based image encoder, viewer and file format that has
been designed specifically for the GIS industry to
permit true portability of large raster imagery. MrSID,
an acronym for Multi-resolution Seamless Image
Database, permits instantaneous viewing and
manipulation of imagery both locally and over networks,
without sacrificing image quality.
ER Mapper’s ER Mapper product uses the powerful,
open standard Enhanced Compressed Wavelet (ECW)
image technology to compress images. ECW images
can be read locally or served from an Image Web
Server. In addition, full map geocoding information is
recorded so imagery is always registered. ECW
provides an adjustable compression ratio and offers
interactive real-time panning and zooming. Input files
can be of any size or resolution.
ER Mapper also offers a product that serves ECW
images over the Internet. That product is called Image
Web Server (IWS). It is significantly different from
other servers in that it is specifically designed for
providing large amounts of image data over a network
using in expensive web serving hardware. Image Web
Server enables a Microsoft Web Server to send imagery
of unlimited size over the Internet directly into a users
web browser or application. With this technology, it is
possible to send and receive high-resolution seabed
imagery of any size and resolution across the Web with
standard desktop computer equipment and a standard
web browser. A Java plug-in is required however.
3.4 The Internet and Internet-Based Mapping
Web servers and service-based, Internet map servers
have advanced enough to now permit the publishing of
fisheries habitat related data in conjunction with highquality maps and imagery on the Internet. These
technologies thus enable the distribution of recently
acquired seabed imagery data to restricted groups, semirestricted groups or the general public. What is more,
these remote groups can use this data to interpret
fisheries habitats and if desired, even post the fisheries
habitat interpretations as vector layers back to the
Internet map servers for publication to the same
restricted groups, semi-restricted groups, or general
public. It is now possible to provide this information to
both technical and non-technical experts in other
disciplines, who can use it for a wide variety of
purposes, including permitting programs, collaborative
fisheries habitat zoning and so on. Thales GeoSolutions
has applied this technology to fisheries habitat mapping
projects over the past year. The system utilizes ESRI’s
ArcIMS software to bring the data to the Internet and to
deliver maps and imagery through the Web. Either
Mr.SID or ER Mapper technologies handle
compression of the very large image files associated
with the seabed imagery acquired on these programs.
4.1 Data Products
The examples presented below are from the same
portion of our Kodiak test range. The area is 4 km East
to West and 4 km North to South. Depths range from
140 m in the north corner to 13 m near the center.
Figure 8 shows color stratification of multibeam
bathymetry data combined with sun-illumination
shading. The shading shows added relief detail and
texture, both of which are valuable for most user groups.
This is a fairly common style of data presentation.
Multibeam backscatter imagery for the same area as
Figure 8 is shown in the data example in Figure 9. This
data set is valuable for geological mapping, habitat
assessments and for engineering applications. The
lighter patterns in the image are sedimentary deposits
and the darkest patterns are rocks. These seafloor
features would not be delineated by multibeam
bathymetry data alone.
4. APPLICATION TO HABITAT MAPPING
Side scan sonar and multibeam echosounder imagery
are very similar, particularly in shallow water, as
illustrated with the waterfall displays from a 240 KHz
system shown in Figure 8.
Figure 8 MBES bathymetry in color augmented by
shaded relief (North is up).
Figure 8 Screen-grabs of multibeam backscatter image
from waterfall display.
In deeper water, the aspect ratio of the echosounder
changes, but the ability to differentiate between
different sediment types is maintained, and in
conjunction with the co-registered bathymetry remains a
powerful tool.
Typical file sizes for multibeam bathymetry are on the
order of 1 byte per pixel. So, a bathymetry DTM of a 1
km square box, binned at 1 meter, will occupy roughly
1 MB. A backscatter mosaic for this same area will
likely require 10 MB or more. Larger areas and higher
resolution will increase storage requirements. Image
file sizes of well over 1GB are common.
Multibeam bathymetry and backscatter data were
collected simultaneously, so the co-registration between
the two data sets is precise. This allows for excellent
integration of both types of information. Further, the
low-noise FTS data supports a higher image resolution;
in these examples the pixel size is 25cm over the full
depth range.
associated with incorrect geometry. The result is a
clearer and more easily interpretable image.
Figure 11 Stitched backscatter image (left) versus full
FTS data (right).
Figure 9 Multibeam backscatter image of seafloor
showing patterns of differing geological composition of
the seafloor. (North is up)
The perspective view of the Kodiak test area in Figure
10 illustrates draped backscatter imagery, which
provides the user with a powerful visualization tool.
The precise co-registration makes this image relatively
noise-free and unambiguous. This degree of agreement
between bathymetric surfaces and seabed backscatter
imagery is not possible with typical towed sonar
systems.
Figure 10 Backscatter imagery draped over bathymetry
(North is to the left; this is the same area shown in
Figures 8 and 9).
The final example in Figure 11 shows the result of
processing the full FTS data and comparing it against a
‘stitched’ backscatter image in water depths up to 940m.
Both images are mosaicked at the same resolution.
However, the improved signal to noise ratio of the FTS
is apparent in the clarity of the features. The precise
pixel position of the FTS also helps reduce noise
4.2 Web Delivery / Access
Thales GeoSolutions has been providing web-based
GIS and data management services to the marine
community since 1999. As a value-add to the marine
survey data acquisition and processing programs that
the company normally provides, Thales GeoSolutions
has been able to provide “deliverables” to the customer
in an expedited manner using the Internet. Where “time
is of the essence” or where collaboration amongst
widely separated groups is involved, such an approach
makes a great deal of sense.
Most recently, Thales GeoSolutions has added the
capability to serve very high-resolution seabed imagery
to its portfolio. Using the technologies described
previously, the company is now able to serve the raster
imagery that results from a marine survey for fisheries
habitat or other purposes. This would not have been
possible, even a couple of years ago. While such an
approach still only makes sense for Intranet or Internet
applications where high bandwidth is available, as the
availability of bandwidth continues to increase and the
costs of increased bandwidth continue to decrease, such
solutions will inevitably become more and more popular.
As it stands now, Thales GeoSolutions is able to take
the seabed imagery that results from a multibeam
backscatter survey and post it on the Internet. The
company operates a commercial web site,
http://www.chartroom.com that provides various webbased GIS and data management services to customers.
At this location, a remote user can access imagery that
has been made available from their local ArcGIS
application. As mentioned previously, one of the
greatest strengths of ArcGIS is to integrate map and
data services. With ArcGIS, a user is able to add a data
source that is not necessarily “local”, meaning that that
user can connect to and extract data from another
computer. In this case, it can be seabed imagery data
residing at a secure location on Thales GeoSolutions’
“chartroom.com” site.
The remote user can
immediately access seabed imagery data that has been
collected by Thales GeoSolutions and integrate it into
their GIS environment. There is no longer the need to
have a dedicated copy of this data sent to the remote
user, though that option is always available if it makes
sense. This approach makes a great deal of sense for
technical users that are GIS savvy and are already
running ArcGIS within their organization. Figure 12
shows the home page to “chartroom.com” portal.
Figure 13 Example image from www.chartroom.com
4.3 Remote Interpretation
Given the above, it is easy to see how significant benefit
can be obtained from combining these technologies to
serve the marine survey community and beneficiaries of
marine survey data. All of these technologies combined
with the increased availability and decreased costs of
bandwidth should make such approaches to data serving
and data management more prevalent.
Figure 12 Home page to www.chartroom.com
An alternative approach for delivery of and access to
this data is also available and provided by Thales
GeoSolutions. The latter approach is geared more
towards less technical users or users that are not running
ArcGIS within their organizations. In this scenario,
Thales GeoSolutions is able to serve the seabed imagery
data within an ArcIMS service that is accessible to
remote users via secure log in and password at the
“chartroom.com”.
Stand-alone, extremely highresolution imagery is served using ER Mapper’s IWS
technology. Additionally, imagery of slightly less
resolution is available within a map / GIS environment
using Mr.SID’s image compression technology.
Regardless, the remote user is again able to access and
if required operate on the data. An example of the
imagery served by “chartroom.com” is shown in Figure
13.
A case study that will be looked at in more detail here is
the example of remote interpretation of fisheries
habitats. Thales GeoSolutions sees this as a very
achievable and beneficial service that can be provided
to existing customers using existing commercially
available technologies.
In this scenario, Thales GeoSolutions would acquire
and process multibeam backscatter data to produce
high-resolution geo-referenced imagery of the seabed
for the area of interest. As discussed previously, this
imagery would be of relatively inexpensive to acquire
and would be of relatively high quality. It would
certainly be of high enough quality to conduct fisheries
habitat studies and analysis.
With this imagery
available, it could then be compressed and posted on
“Chartroom.com”.
The compression methodology
would ultimately depend upon how the data is to be
served. Two possibilities exist. Mr.SID compression
could be used to produce a Mr.SID file of the image or
images that could be accessed by authorized remote
users, running ArcGIS at their desk-top computer. With
this approach, the remote user would have a full GIS
environment running locally and would simply
“connect” to the seabed imagery available at
“Chartroom.com”. This data could be integrated as an
additional layer within the remote user’s local GIS
environment and full GIS functionality would be
available.
At this point, the remote user could
“digitize” over the seabed imagery to produce an
interpretation of the fisheries habitats contained within
the survey area. This vector layer could be saved
locally or could itself be offered to other remote users
operating on ArcGIS. Once again, this approach makes
the most sense for those remote users who are GIS
savvy and are running ArcGIS applications within their
organization.
An alternative scenario for the remote interpretation of
fisheries habitat data would involve the inclusion of this
data within an ArcIMS site served from
“Chartroom.com”. In this scenario, the remote user
does not need to know anything about GIS and does not
need a local version of ArcGIS to perform the remote
interpretation. With this approach, the remote user
could access the data from any standard web browser.
If viewing only is required, the HTML viewer would
suffice and no special plug-in would be required. If
interactivity and remote interpretation were required,
then the ArcIMS Java client would be used, thus
requiring a Java plug-in. Thales GeoSolutions would,
in advance on the server side, set up an HTML service,
a Java service or both, depending upon the requirements.
Regardless, the remote user would access the service
via a secure login name and password at
“Chartroom.com”. Once connected, the remote user
would be able to see the seabed imagery and any other
data layers that may have been included in the site. The
user could move about the data (pan, zoom) and have
access to some basic GIS functionality such as
identification of features, query building and buffering.
Advanced functionality including posting of comments
and editing of vector layers is also possible. This means
that a remote user can actually perform fisheries habitat
interpretation by creating and editing vector layers
using a web browser only. No special knowledge or
software on the client side is required. The interpreted
habitat can be visible to others with access to the site. It
should be noted that an original vector layer is never
changed on the server side unless the server
administrator makes the change. This means that vector
layers can be edited and those edits presented, but the
original underlying vector layer remains as it were in
the beginning, until the administrator is instructed to
accept and post a change.
5. SUMMARY
Considerable advances in computer technology, GIS
Internet technology, image compression algorithms and
increased
Internet
bandwidth
have
enabled
manipulation of large high resolution images via the
Web. Further advances in these areas including
holographic memory/storage and wireless networking
will make future working practical in remote offices or
from home, accessing remote government or
commercial databases.
New developments in multi-beam data systems
including multiple pings in the water column and
collection of full water column data will allow greater
data density and hence improved seabed imagery and
also provide advanced biomass evaluation from a single
data source. Significant and necessary development of
automated processing techniques for multibeam
bathymetric and backscatter data are currently
underway to enable these highly representative images
of the seafloor that you need for habitat characterization
to be provided cost-effectively.
6. REFERENCES
Millar, David; Gilmour, Bill; and Caswell, David,
Using the Internet to Manage and Distribute
Geospatial Submarine Cable Data, ICPC Plenary,
Copenhagen, Denmark, May 2000.
Lockhart, Doug; Saade, Edward and Wilson, Jerry; New
Developments in Multi-beam Backscatter Data
Collection and Processing, Shallow Water Surveying,
Portsmouth, New Hampshire, October 2001.