Meg Stewart: Data Acquisition on a Small Boat: Tips and Tricks, July 14, 2019

NOAA Teacher at Sea

Meg Stewart

Aboard NOAA Ship Fairweather

July 8 – 19, 2019

Mission: Cape Newenham Hydrographic Survey

Geographic Area of Cruise: Bering Sea and Bristol Bay, Alaska

Date: July 14, 2019

Weather Data from the Bridge
Latitude: 58° 36.7 N
Longitude: 162° 02.5 W
Wind: 9 knots SE
Barometer: 1005.0 mb
Visibility: 10 nautical miles
Temperature: 61° F or 15.5° C
Weather: Overcast with fog, no precipitation

Fairweather in fog
The other day while on a survey launch, we came up on the Ship Fairweather as fog was rolling in.

Science and Technology Log

Launch preparation
A launch getting ready to survey. The setup process takes some time and all of the preparation is necessary for accuracy in the data.
Heave, pitch, roll, and yaw describe the movements of a boat (or a plane). An inertial measurement unit reads those discrete movements. Source: wikipedia

In the last post I talked about hydrographic surveying, the software used and the multibeam echosounder on the survey boats (called launches). The software is setup in the cabin by the hydrographer in charge. It takes a good five minutes to get an accurate read from the GPS (global positioning system) receiver. Then it takes time for the IMU (inertial measurement unit) to respond and start to read the boat’s heave, pitch, roll, yaw, and heading values. 

The hydrographer in charge (standing) is showing the hydrographer in training (seated) how to setup the day’s survey project using the echosounder software.
launch data storage
The four Fairweather launches have the same, high-end technology in their cabins used to collect data from the multibeam echosounder, CTD sensor, a sound speed system, and a positioning and altitude system.

Often, the launch drives in a circle eight in order for the positioning receivers to be “seen” by the satellites, as a  stationary object is more difficult to detect than one that is moving. Setting up the day’s project using the multibeam echosounder software also takes some time but all the steps need to be done properly and to the correct specifications prior to starting the sounder. If not, the locational data will be wildly off and the depths inaccurate.

Another task that must be done from the launch before starting to transect is to test the salinity and water temperature using a CTD probe, which is called a cast. I mentioned this in a previous post. CTD stands for conductivity, temperature and depth. In the general area where the launch will survey, the CTD drops slowly to the bottom of the seafloor, collecting data that will be fed into the hydrographic program. Salinity and temperature at different depths will slightly change the rate at which sound travels in water. Again, the CTD process makes the location and depths as accurate as possible and must be done.

Meg casts CTD probe
Casting the CTD probe into the survey location to get conductivity, temperature and depth readings.

Usually, the chief hydrographer sets the defined area to be transected for the day and this is usually a polygon. The launch will sweep with the multibeam echosounder the outside lines and then scan at parallel set distances between the lines, either in a roughly north-south direction or a roughly east-west direction. For this particular hydrographic project, coverage of survey lines can be spaced at about 400 meters apart or greater apart depending on the depth. Recall that the nautical chart of Bristol Bay from the last post showed soundings dotting the area. Solid bathymetric coverage is not always needed on these projects. The Cape Newenham area has proven to have gradually varying depths and is mostly quite flat so free from obvious obstructions like large boulders and sunken ships. 

Once the technology setup is complete in the cabin, the hydrographer shares the map window with the coxswain (the person in charge of steering or navigating the boat). The hydrographer sets the points and the lines so that the coxswain knows where to direct the launch. And by direct, I mean the coxswain uses compass direction and boat speed to get from place to place for the survey. And the hydrographer in charge turns the echosounder on and off when the launch is in position or out of position.

The coxswain navigates the survey line set by the hydrographer in charge.

Because the transects run parallel to each other and are equally spaced apart, the hydrographers call this technique “mowing the lawn,” (see video below) for they are essentially mowing the surface of the ocean while the multibeam echosounder is collecting soundings of the surface of the seafloor.

A video of someone mowing a lawn on a riding lawnmower

A day out on a launch will go from about 8:30am to about 4:30pm but sometimes an hour or so later. If the Alaskan weather is cooperating, the hydrographers want to do as much as they can while out on the launch. Once surveying is complete for the day, the hydrographer in charge has to close up and save the project. Then data get transferred to the larger workstations and shared drive on the Fairweather.

Meg on launch
Every day on the launch, at least on this leg, has been great with perfect weather. And today, the added bonus for me was the phenomenal geology as we surveyed right along the shore.

Personal Log

I’ve taken loads of photos and video while at sea. I have tried to post just those pictures that help explain what I’ve been trying to say in the text. I haven’t posted any video on here as the internet on the ship is very weak. These next photos are a tour of different parts of the NOAA Ship Fairweather.

  • view of the bridge 1
  • view of the bridge 2
  • view of the bridge 3
  • barometer

The above slide show gives an idea of what the bridge is like. The ship is steered from the bridge. All the navigational instruments and weather devices, among other tools, are found on the bridge.

emergency billet
These emergency billets are for me, TAS Stewart, Meg, and it’s posted on my door. For each emergency situation, Fire, Abandon Ship, or Man Overboard, there is a bell sound and the location on the ship where I am to muster. Life at sea is all about being ready for anything.
This is the mess (where we eat. And eat. And eat!) The food is fantastic but I’ve gained some pounds for sure.
Ice cream spot
Maybe this is why. Sometimes the Ice Cream Spot looks like this. Ha!
The galley
The galley
Laundry machines available and detergent is supplied. No need to bring all your clothes. Also, sheets and towels are supplied.
Stairs are called ladders on a ship. Makes sense to me – they’re often pretty steep. You must always hold a rail.
The Lounge
The Lounge
DVD collection
DVD collection of over 500 films
Lounge full of people
Yes, so this is the lounge and there can be meetings in here, training, movies, games, puzzles, quiet space, etc.
DVD in stateroom
Or, you can pop a DVD into a player in the Lounge, go back to your stateroom and watch. Or fall asleep. This is the original Blade Runner (which I never saw) and which I didn’t care for.
Finer things
The good folks of Ship Fairweather like to have a nice time every now and again, so they set up evenings, about once a leg, to have Finer Things. People come by, bring fine cheeses, fine chocolates, fine almonds, fine fig jelly, and fine maple sugar candy from Rhinebeck, NY, and have a fine time. And a disco ball.

Did You Know?

Inertial Measurement Units (IMU) technology that is so important for accurate hydrographic survey mapping was developed by the U.S. military. IMUs were used in the development of guided missiles, unmanned aerial vehicles (and now drones), battlefield reconnaissance, and target practice.

Quote of the Day

“A ship in port is safe, but that’s not what ships are built for.” – Grace Hopper


Leave a Reply