By Eleanor Gee

A new marine navigation chart survey of the waters of Ha‘apai and Nuku‘alofa, has discovered uncharted marine volcanoes that are big enough to sink a cruise ship.

Tonga's marine charts - among the oldest marine charts still in use - are being updated using airborne laser and an unmanned submarine.

The navigation chart survey of the waters of Ha’apai and Nuku’alofa, some marked in the 18^th century by Captain Cook's voyages, is being updated by a hydrographic team, as part of the Pacific Regional Navigation Initiative project aimed at increasing navigation safety.

Rhys Davies, Senior Hydrographic Surveyor of iXblue and Project Manager, told Matangi Tonga they are conducting a hydrographic survey of the two Tongan areas, as well as collecting a lot of information useful for marine scientists, especially geologists.

“We are collecting the best metric sonar data to update the marine charts, some of the oldest charts that I’ve ever come across. A lot of them are still in fathoms.”

A fathom is a unit for measuring the depth of water that is equal to 6 feet or 1.8 metres.

Rhys said that Captain Cook used fathoms to measure the sea depth, which was done by tying knots on a rope and using lead as weights. They would lower it into the sea and measure the depth by the knots.

Small submarine - DriX

For this project, the team is using a iXblue unmanned surface vessel which looks like a small submarine called the DriX, which is the first of its kind. It is able to go to places that are too hazardous for humans and to carry out work at a fraction of the cost of a manned vessel. The vessel can operate in fully autonomous, semi-autonomous or direct remote control. 

“Essentially it does the same thing as the boat but it doesn’t have people on board. It has the same survey equipment as we’ve got and then it just works in tandem with us. We’re actually collecting data faster than what we normally would on the vessel.”

“So basically, once we load in the [survey] lines, it goes away and does it. It measures the depth. That’s all we’re really doing is measuring the depth over a wide spot and what we end up with is 3D data set of the entire seafloor.”

The project is part of a wider project that combines satellite symmetry.

“They've actually flown some with an airborne laser so they’ve got down to about 20 metres and then we’re doing from 20 metres to 400 metres depth.”

He said this is the first project he has come across where it integrates all three.

“Normally we just do stuff with the boats but now we’re using satellites, planes, and drones.”

Volcanic features

The team has already discovered high mounds of volcanic features in Ha’apai not recorded on the old charts.

“We’ve come across a whole lot of volcanic features that come out of 400 metres of water right up to 20 metres of water. It’s pretty amazing. Nobody knew they were there,” said Rhys.

“If a cruise ship came in and ran over some of those things out there they would be in a bit of trouble because they are not on the chart.”

“We’ve already submitted three of what they call H notes to the New Zealand Government for them to publish that sort of information right now, just to let everybody know that there are some dangers out there that are not on the chart.”

NAVAREA

Tonga falls under the NAVAREA [2] looked after by the New Zealand Government who is responsible for charting it. The world’s oceans are divided into NAVAREAs which are geographical sea areas that various governments are responsible for navigation and weather warnings.

Rhys said people don’t realize New Zealand has a big area of charting responsibility.

“It’s the largest in the world, because it’s so remote. But we are doing a lot more work around the Pacific which is great.”

Rhys said he would like to survey the whole of Tonga but is limited with the available budget and funding. The team have been in Tonga since November and will complete this project next week.

The project is funded by the New Zealand Government’s Land Information New Zealand (LINZ) and the Tongan Government, and implemented by iXblue, a hydrographic company.