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| Captain Cook and his vessel Endeavour. |
Despite great advances in navigational aids (e.g. GPS), in many places in the world there remains great uncertainty as to exactly where dangerous reefs and shoals are in respect to a navigating vessel. Although the vessel might know (via GPS) where it is within a few metres with respect to the World Geodetic System Ellipsoid (WGS84), this is of little help if the chart on which the course is being plotted has a local offset of several hundred metres. And it doesn’t make a bit of difference if one is using an old paper chart, or a modern electronic one, because the latter is very likely based on the same old data, either way. Even worse, in remote areas there may very well be shoals and rocks that remain undiscovered, and the chart may suffer not only from a linear offset, but be distorted in some arbitrary way. This is because, even today, the foundation information for many charts of remote areas was laid by seafarers of a bygone age, when ships were driven by sail, positions were determined by sextant and chronometer and depths sounded by a lead line.
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| Figure 1a. Where in the world is Tonga? |
It may surprise you to learn that there are parts of the world, often in remote tropical seas, where the charts have not been significantly updated or revised, only a few details amended, since the pioneering work of talented surveyors such as Captain James Cook and other explorers in the 18th Century. How is this possible? The answer lies in the considerable funding and technological capacity that are required to accurately survey an area with modern instruments, balanced against the limited resources available in remote and thinly-populated areas. The remote islands of the Ha’apai group in the Kingdom of Tonga represent one such case.
The Kingdom of Tonga is a Polynesian archipelago, scattered over 700,000 km2 of the southern Pacific Ocean. Most people have no idea where it is. If it helps, think of Tonga as being about 2/3 of the way from Hawaii to New Zealand. Still struggling to visualize it? Take a look at Fig. 1a, which gives the big picture. Talking of a big picture, did you realize that, viewed from just the right angle from space, the earth appears almost completely covered by the Pacific Ocean?
Tonga has Fiji to its west, Samoa to the northeast, Niue to the east and New Zealand far to the southwest. With only 100,000 inhabitants, spread over 170 islands strewn along a line of over 800 km, that’s a lot of ground to survey. Tonga essentially consists of three main parts; the cluster of islands of Vava’u in the north, the Ha’apai group in the middle and the largest island, Tongatapu, in the south, which hosts the kingdom’s capital, Nuku’alofa (Fig. 1b).
Tonga was nicknamed “The Friendly Islands” by Captain James Cook because of the congenial reception accorded him on his first visit in 1773. He arrived at the time of a major festival, and he and his crew were invited to the festivities. According to the chronicler William Mariner (who arrived a few years later, on a vessel whose crew were all killed, apart from him) the Tongan chiefs decided to murder Cook and his men so they could loot his ship, and the invitation to eat and drink was a ruse to take them off-guard. The plan stalled when the chiefs argued about how, when and where the ambush should be launched, so Captain Cook and his crew returned safely to their ship none the wiser. In 18th century Tonga, apparently dangerous things lay just beneath the surface, and perhaps, concerning the charts from that era, they still do.
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Figure 1b. A close-up of Fig. 1a, showing the
Tongan archipelago. |
Tonga lies on the Tonga-Kermadec ridge, a geologically active region formed by the collision of tectonic plates in the Southern Pacific, with a dramatic mix of coral reefs and active volcanoes, including sub-surface. Try charting a sea where brand new islands appear from time to time, such as the one that broke surface in 2015 near Hunga, between Ha’apai and Tongatapu, now some 4 km long and 100 m high. You know that your charts are of doubtful accuracy when you see two islands drawn on the chart, but three clearly visible a few miles off the starboard beam, as happened to Sailing Yacht (S/Y) Jocara this September.
And what, you might ask, was S/Y Jocara doing in Tonga, and more to the point, what has it to do with Ocean Engineering? The answer is that, curiously, in a way we have come full-circle since the days of Captain Cook and his original surveys, conducted from sailing vessels of old. Breakthroughs in charting and surveying technologies, such as airborne LIDAR and multi-beam sonar deployed from Autonomous Surface Vessels, have brought the cost of accurate high-resolution surveying of such large and complex areas into an affordable range. But LIDAR and Sonar depths must be corrected for the tides, to reduce them to a common datum (referred to as Sounding Datum) that closely correlates with the Lowest Astronomical Tide. GPS benchmarks ashore are also needed to fix this datum so that it can be recovered in future surveys, and also to allow referencing of the depths and tide heights to the WGS-84 Ellipsoid and then onto other useful land-based datums. These GPS benchmarks serve the dual purpose of increasing the density of Tonga’s national geodetic network and providing sparse high-quality control points in the most remote locations.
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S/Y Jocara, under sail between stations, with
iXblue’s GPS buoy secured on deck. |
Enter S/Y Jocara, which the survey company iXblue chartered to support a team to establish such a GPS reference network and install tide gauges at four locations in the Ha’apai island chain and one to the south, at Eua Island. Choosing S/Y Jocara was an unconventional move. Oceanographic exploration and surveys are normally conducted from much larger, commercial motor vessels. But while a motor-driven ocean-going research vessel needs to be relatively large (to accommodate fuel and engines), typically costing tens of thousands of US Dollars per day, a blue-water sailing vessel such as S/Y Jocara can be chartered for a fraction of the cost, and yet be a perfectly good platform for lightweight exploration and ocean engineering work in remote places, e.g. installing tide gauges and landing ashore to survey GPS reference points. While relatively small (18 m in length with a 4m beam), S/Y Jocara is full-ocean capable with an autonomous endurance between refueling/resupplying spanning many weeks, if not months. S/Y Jocara is also equipped with an instrument-grade pure sine wave inverter to provide mains AC power and copious USB recharging sockets so everyone can keep their personal devices, laptops and scientific recording equipment up and running. In summary, S/Y Jocara offers a cost-competitive solution to small expeditionary survey teams who need access to remote islands, even for extended periods. Finally, it just so happens that S/Y Jocara is owned and operated by Dr. John Potter, an IEEE Fellow and a member of the OES AdCom, who has spent the better part of 2018 working alongside his son, Casper, in New Zealand refitting S/Y Jocara in preparation for supporting just this kind of remote surveying and research effort.
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The survey team hard at work, setting
up a GPS receiver. |
On this occasion, a 4-man surveying team was formed by iXblue under a contract from Land Information New Zealand (LINZ) funded by the Ministry of Foreign Affairs and Trade (MFAT) under the Pacific Regional Navigation Initiative (PRNI). The PRNI has NZ$7.2m to invest over a 6-year programme to improve maritime safety, including hydrographic charts, in the Pacific. Their focus is on the countries for which New Zealand is the Primary Charting Authority (Tonga, Tokelau, Niue, Samoa and the Cook Islands). Obviously, it is not possible to survey every part of such a vast area, nor is it possible to detect and accurately describe every hazard, and inevitably some will remain uncharted. Nevertheless, the PRNI works in close partnership with LINZ and the Pacific Community (SPC), with the aim to enable countries to establish points of contact who can coordinate Maritime Safety Information, forming a key link in getting Notices to Mariners (warning of hazards) communicated effectively. As part of this effort, nautical charts are being updated and converted to Electronic Navigation Charts following high-resolution hydrographic surveys. These new charts will empower compliance with international maritime regulations and provide for safer maritime transport, in addition to facilitating increased tourism by a variety of smaller sail and motor vessels. PRNI is also delivering capacity building and training to Pacific island officials, one of whom was on board S/Y Jocara as part of the survey team.
Jocara began her charter in North Island, New Zealand, by taking on board 400 kg of equipment for the expedition, which she then ferried up to Nuku’alofa, the capital of Tonga, in a 7-day ocean transit of 1,100 n.m. The survey team then flew in and joined Jocara in Nuku’alofa, from where she headed out to the island of Eua to establish the first station. Each station required Jocara to stand by for 2-3 days while three GPS receivers, accurately positioned directly over the installed GPS benchmarks, collected sufficient data to establish, via post-processing, a GPS reference point and for the tide gauges to get a preliminary estimate of tidal components and phases. Then the team would recover the GPS receivers, re-deploy the tide gauges to collect a longer-term record, and we’d move on to the next site. Each site might typically be separated by 30-60 n.m. from its nearest neighbours, with the five sites distributed through the Tongan archipelago so as to provide the best network coverage.
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Figure 2. Preliminary LIDAR bathymetry (not to be
used for navigation) (a) superimposed on an
existing chart that has had a 350 m corrective
offset applied, (b) superimposed on an existing
chart, with no offset correction. |
Fig. 2a shows a preliminary result (not to be used for navigation), overlaying LIDAR bathymetric data (shown as a colour scale) onto an existing chart. The colour is intuitively graded from purple (very shallow) through red, orange, yellow and green (deeper). LIDAR is most useful in providing bathymetric coverage over inshore areas, which may not be deep enough for a vessel to safely navigate. At depths greater than 20-40 m (depending on water clarity) the shallow-water LIDAR system employed (a Leica Chiroptera 4X) becomes increasingly less able to detect the reflected laser beam from the seabed, and eventually cannot determine the depth. This is where iXblue’s Autonomous Surface Vessel, with its multi-beam sonar, takes over. So the colour depths overlaid in Fig. 2a only extend to intermediate depth water, the deepest areas remaining white as the LIDAR data overlay becomes transparent. The example in Fig. 2a shows the island of Ha’afeva, in the central region of the Ha’apai, with numerous smaller islets scattered to the south and west, and an extensive reef to the west.
At first glance, the LIDAR bathymetry seems to agree remarkably well with the older chart, whose depths are marked in fathoms. The shape of the islands, and of the shallow-water bathymetric contours, are in close agreement. It seems that Capt. Cook did a pretty good job, with his limited technology. And so he did, at least in describing the general shape and arrangement of the islands and reefs.
The most startling, and alarming, feature of this overlay is that, in order to align the LIDAR bathymetry up with the chart, a 350 m offset had to be applied to the chart. That is, the current chart has a local positional error of some 350 m. The equivalent overlay, but without the 350 m chart offset, is shown in Fig. 2b.
If a vessel is negotiating a narrow pass into an anchorage, Fig. 2b shows that this can make the difference between being squarely in the middle of the channel, and being wrecked on a reef. It is this chart offset, reflecting the difficulty of precise positioning in Latitude and Longitude in the days of Captain Cook, that makes an apparently detailed and mostly accurate chart an invitation to disaster, if the local chart offset is unknown. This is enough to deter unfamiliar mariners and pose a real threat to vessels venturing into the area without local knowledge. It is thus vital, but often poorly-understood, that the local offset, if any, is carefully estimated before entrusting the navigation of a vessel to GPS.
Even so, this ‘offset’ is only a local linear approximation to a more complex distortion of the chart’s features, so needs to be determined separately for different areas covered by the chart. Jocara routinely estimates the local chart offset by aligning radar returns from islands, and breakers over reefs, with the electronic chart. We are fortunate to be able to directly overlay radar returns onto the electronic chart on the 12” colour screen of our electronic navigation system. By applying East-West and North-South shifts, and perhaps also a rotation, radar features and expected returns from elements shown on the chart can often be reconciled and the differences minimized to provide a good estimate of the local chart offset. Jocara can then proceed with caution, and always with a sharp lookout for the changing colours of shallow water. Even so, it has happened more than once that a lookout on the bowsprit has seen a coral head appear out of nowhere, directly ahead, the coral polyps clearly visible, only to slip harmlessly beneath the keel a few seconds later, while the helmsman in the cockpit believes himself to be in deep water with no dangers indicated for miles.
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| S/Y Jocara at anchor at one of the stations in Ha’apai. |
The westernmost site we were called on to survey was on the extinct volcanic island of Tofua, with its towering steep-sided cliffs guarding its entire shoreline. Captain Bligh certainly found Tofua inhospitable, having been cast adrift by Fletcher Christian with nothing more than a few rations, plus a quadrant and compass to navigate by (no charts or chronometer). Bligh (who had served under Captain Cook) stopped at Tofua, but one of his men was killed by the locals, so he ended up sailing another 3,500 n.m. west to reach Timor in Indonesia, regarded to this day as one of the most accomplished feats of open water navigation.
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A fine young yellowfin tuna—sashimi
and sushi on the menu tonight! |
At each station, our local surveyor, from the Ministry of Lands and Natural Resources, was of invaluable help in providing liaison, often being called upon to explain why these odd characters with their yellow tripods and brightly-coloured safety gear, carrying strange equipment, should be left alone to drill into rocks and concrete jetties, cementing in their GPS reference marks. Sometimes its hard to explain to a hard-working man, laboring under the weight of a pig or two, that it is so important not only to not bump into, but not even to touch, a sensitive tripod with its GPS receiver, surveyed to an accuracy of millimeters rather than metres.
As our work drew to a close, Jocara sailed back south to Nuku’alofa, revisiting some of the sites along the way to check on the tide gauges, and eventually discharged her survey team back on to dry land for their flights home. The team, having previously been accustomed to working on grimy workboats smelling of diesel, left with a new experience of surveying from a compact, wind-powered alternative surveying support vessel, finding it a novel but very pleasant alternative. The food wasn’t too bad either, often supplemented by the fresh catch of the day, including sashimi and sushi (Jocara is furnished with Japanese rice wine, sushi nori dried seaweed sheets, the best wasabi and soy sauce and bamboo mats to roll the sushi in).
iXblue are now engaged in conducting the multi-beam sonar part of the work, using their in-house designed 8 m Autonomous Surface Vessel, supported by a larger (but much more expensive) motor vessel. Once all the data is in, and the data analysis complete, we can all look forward to seeing the first genuinely new charts of the Ha’apai island chain, with unprecedented accuracy and resolution, courtesy of iXblue, LINZ, MFAT and PRNI. But lest we be tempted to rest on our laurels, there’s a lot more to do, as Fig. 1a illustrates!
Plans are to continue to use S/Y Jocara for expeditions, research, adventure and eco-tourism anywhere in the world, given the time and water under her keel to get there.
https://www.facebook.com/JocaraOceanExpeditions http://www.jocara.net. |
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