Sea-Floor Mapping

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Welcome to GISWeekly! This week we feature a story on sea-floor mapping in developing countries. Another story entitled “Where is GIS when we need it?” is about a New Mexico community whose residents have lost thousands of dollars in flood taxes that they didn't need to be paying-as a result of outdated survey information.

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Industry News

Sea-Floor Mapping

In developing nations, environmental management institutions are challenged to find a software solution to develop a low cost, reliable sea-floor monitoring system. Mapping and monitoring marine environments is an important requirement for coral reef preservation programs, deep-sea research, salvage, marine archaeology, aquaculture and other exploration and environmental management purposes. Marine scientists from the University of Oslo settled on Windmill data collection software, which would allow a GPS and sonar to be plugged into a laptop computer.

According to Trond Kvernevik, Research Fellow - Marine Spatial Ecology, Marine Biology and Limnology Section, Department of Biology at the University of Oslo, Windmill Software is a great alternative for simple mapping for conservation purposes. “Since conservation goes hand-in-hand with basic scientific research, the fact that Windmill can log lots of additional data besides the basic seafloor data is its greatest advantage.” Kvernevik said there are dozens of solutions around today that can automatically log GPS and sonar data (Garmin's hand-held GPSMAP76s, for example, can receive sonar data and log XYZ data on a tiny removable memory card). And there are lots of software around that can log similar data. ”The reason Windmill was chosen was that (1) it was free of charge, (2) it was user friendly (3) it has capacity for receiving loads of *other* types of data than the XYZ data, via additional COM ports and (4) Windmill provided excellent user support even for the free-of-charge software. That still costs more than basic sonar + GPS with Windmill, and doesn't let you log other interesting data.

“Together with Malaysian Marine Park authorities we've mapped three marine parks in Malaysia (Payar, Perhentian and Redang Marine Parks) using Windmill for data acquisition. We've been using the free-of-charge version. Costs are thus restricted to hardware, boat usage and so forth. The cost of even the commercial version of Windmill software (about GBP 50 or so last time I checked) is small compared to alternative software because most alternatives come along with data acquisition boards, i.e. are bundled with hardware, which tends to be expensive.”

Although the scientists chose Windmill for a very specific task, the software soon proved that it was capable of doing more. The system can be expanded by adding more COM ports and peripheral instruments such as thermometers, salinometers and current meters. Using such setups, 3D-images of the sea-floor with superimposed maps of water conditions can be produced very quickly, using instruments already available at most institutions. Using simple synchronization techniques, video camera evidence can also be incorporated.

“The GPS gives positioning for the boat, where the antenna is mounted. GPS positions can not be measured directly under water. However, the location of the bottom soundings, made by sonar, is straight below the boat for a vertical-mounted single-beam sonar. Hence, the GPS gives X and Y coordinates in the horizontal plane and the sonar a Z coordinate (depth). And XYZ data (= 3 dimensions) is all that's needed to map a surface such as the seafloor,” said Kvernevik.

“Windmill is used in shallow coastal environments. The acoustic beam is still reasonably narrow by the time it reaches the seafloor, and it is located directly below the boat even if the boat has a bit of roll/pitch/heave due to waves. In deep water, the cross-section of the acoustic signal from a cheap sonar will be very wide. Thus in deep water the actual whereabouts of the depth reading relative to the GPS antenna on the surface is NOT exactly known. You will get data and could make some sort of map, but it'll be full of errors. The position offset per depth sounding could be tens of meters, and if the seafloor has complex topography at that place, the depth measurements itself could vary substantially depending on the boat's rolling movements. Thus, to be clear about this, fairly precise mapping with a basic sonar + GPS using Windmill (or any other equivalent software) is restricted to water of say, shallower than 30 meters and under calm conditions. The precision constraint is however not Windmill - it is the mapping hardware!! To get up and running with high-standard mapping for the purpose of hydrographic chart production, etc, you'll be looking at an investment of about GBP150-200,000.”

As a GIS tool, Windmill is a powerful data acquisition tool that can be adapted for multivariable environmental survey, and can be used for ground truthing for remote sensing data derived from satellite images, aerial photos etc. Once Windmill acquires the data it can be imported into any major mapping software on the market. Mapping sessions of small areas have been completed in less than 20 minutes. Useful mapping speeds are generally within the range of 5-15 knots.

The marine biologists at University of Oslo concentrate on developing replicable and reliable fieldwork and analysis procedures. Acquiring the data is one thing, but procedures by which automatically acquired data can be related to GIS information are essential and come under the heading of data processing, quality control, relational database building, interdisciplinary analyses, etc. The mapping becomes more than measurements of positions and depths, but rather encompasses mathematical modeling of local tides, error analyses, multiple consistency tests of seabed classification accuracy among independent groups of video reviewers, testing and revisions of simple and detailed classification schemes.

Their Malaysian counterparts have now successfully adopted high-quality and cost-effective marine survey methods for management and conservation. EcoScience Consultants who were involved in the process has the capacity to work with local government agencies using largely existing official manpower to implement large-scale and very cost saving projects in integrated coastal zone planning, and is able to assist both research groups and governments in implementation of research and conservation projects.

The usage of Windmill for scientific applications within marine surveys is explained in the paper:
Kvernevik, T.I., Mohd Akhir, M.Z, & Studholme, J. (2002). "A low-cost procedure for automatic seafloor mapping, with particular reference to coral reef conservation in developing nations." Hydrobiologia vol 474
(2002) pp 67-69.

Perhentian Marine Park, east coast Peninsular Malaysia was mapped in a joint effort by the University of Oslo, EcoScience Consultants Malaysia ( Email Contact) and Marine Parks of Terengganu, courtesy Head of Marine Parks in Terengganu, Ab Rahim Gor Yaman ( Email Contact), who provided manpower, boats and other field logistics. For information contact Trond Kvernevik, Email Contact
This image with classifications of coral types is based on video recordings and painstaking review and bottom classifications by Trond Kvernevik and Mohd Zambri Mohd Akhir of EcoScience Consultants Malaysia ( Email Contact) in May 2001. That image shows just three broad categories of sites with a very clear dominance by branching, massive or tabulate coral. (end of caption)

EcoScience Consultants Malaysia's dataset, and, soon, the posters on display at the new Visitor's Centre in Perhentian Marine Park, comprises tens of thousands of manual seabed evaluations based on video recordings, which are posted on the maps. About a dozen staff of the Marine Parks of Malaysia has done a tremendous job of video recording and classifying the reefs of Perhentian and those of two additional marine parks (Redang, also in Terengganu, and Payar, in Kedah) over the course of a two-year collaboration. Thus, to add the fourth dimension, namely ecological information about the seafloor to drape over the seafloor model, ecologists still have to do some work!

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