How do we make nautical charts?

A nautical chart is a map of the sea. Just as a map helps us navigate on land, a nautical chart helps those traveling on the ocean get where they’re going safely and efficiently.

In 2017, $1.6 trillion worth of goods moved through U.S. ports. With all that traffic, it’s important that those navigating through our ports and along our coastlines have the information they need about the shape of the shoreline and seafloor, water depths, potential hazards in the water, buoys, anchorages, and other features.

Federal laws say most commercial vessels must have nautical charts while traveling in U.S. waters. NOAA's Office of Coast Survey makes and updates all charts of U.S. coastal waters, the Great Lakes, and waters surrounding U.S. territories. So how do these important resources get made and updated? It’s a continuous process that involves many people from different disciplines and organizations.

Data, Data, Data

It all starts with collecting the data. Historically, chart makers measured water depths with simple methods, like tying a lead weight to a line. Today, NOAA uses advanced technologies to study bodies of water and locate dangerous hazards to navigation, such as shoals or shipwrecks.

This data is gathered by hydrographers — people who study the physical characteristics of bodies of water — at the Office of Coast Survey, oceanographers at NOAA’s Center for Operational Oceanographic Products and Services, professionals at other government agencies and international organizations, and even private citizens.

Through sonar, LIDAR, and aerial photography, experts capture some of the data that goes into charts. That includes data about the shape of the coast and seafloor, depth of the water offshore, and features in the water that might hinder navigation.

With sonar, hydrographers bounce sound off the seafloor or underwater objects to determine their features like a dolphin using echolocation to “see” its surroundings. LIDAR works similarly, except it uses light instead of sound. Hydrographers use LIDAR to map features of the seafloor as well as the coast. Pilots also collect information about features of the coast from the air with aerial photography.

Often, humans operating instruments on ships and airplanes collect this data. But, increasingly, NOAA makes use of unmanned systems — robots that collect data where it might be expensive, dangerous, or otherwise impractical to send a human. Data collected with unmanned systems can be better quality, and it can be collected more safely, efficiently, and inexpensively.

Mapping it Out

Once we have the data, we have to turn it into a chart. This is not as straightforward a task as it might seem at first. The data we collect about the ocean and coasts describes a 3D world, but a chart is a 2D representation.

Making a flat chart of some section of our Earth requires making a projection. Imagine a spherical globe. Now, wrap a rectangular piece of paper around it, so it forms a cylinder. If there were a light shining from the center of our globe, the features on the globe would be projected onto our piece of paper. If we unroll the paper, we have a rectangular map. Map projections are actually created through mathematical calculation, but this is a way of imagining what happens when a map projection is created.

NOAA has a team of cartographers who are experts in interpreting data after it is collected.  They are responsible for formatting, interpreting, controlling quality, and making sure that it is easy to understand as charts are made ready for use.  

No map projection is a perfect picture of our world. Each projection method introduces its own distortions, so cartographers have to pick the right projection for the job. For charts, that's usually Mercator. Lines of latitude and longitude on the Mercator projection meet at right angles, and any constant compass heading can be drawn on a chart with a straight line. That’s why mariners have used Mercator charts since the 16th Century.

Sea Change

But the story doesn’t end there. Our ocean and coasts are always changing. Storms and other natural processes change harbor depths and the shape of coastlines. New channels, harbors, piers, wharves, marinas, pipelines, communications cables, bridges, or buoys get added. Recommended travel routes change. All of these updates have to be put on the charts.

And, of course, we’re always working to improve the quality of our charts. Many charts are based on data collected in the very early days of surveying, when data was collected with methods that are now outdated. These charts are accurate, but incomplete. With today’s technology, NOAA is working to complete and update charts for these underserved areas, such as Alaska and the Arctic.

NOAA releases updates to its charts weekly. You can visit the Weekly Updates page to view these updates. You can also read more about how to find any of our charts.