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Diving Deeper : Episode 5 (Mar. 23, 2009) —
What is a Nautical Chart?


HOST: Welcome to Diving Deeper where we interview National Ocean Service scientists on the ocean topics and information that are important to you! I’m your host, Kate Nielsen.

Today’s question is…What is a Nautical Chart?

Nautical charts contain information about the shape of the coast, the depths of the water and the general configuration of the bottom of the sea floor. Nautical charts also show locations of obstacles to navigation, the rise and fall of the tides, and locations of navigation aids. Nautical charts make safe and efficient marine transportation possible.
To help us dive a little deeper into this question, we will talk with Tom Loeper about nautical charts – what they are, how they are developed, and why they are so important. Tom is the Chief for the Coast Pilot Branch in the Office of Coast Survey. Hi, Tom, welcome to our show.
TOM LOEPER: Thank you Kate for inviting me here to talk more about nautical charting.

HOST: First can you explain to us the difference between a map and a nautical chart?

TOM LOEPER: Kate, that’s a good question. There are many differences between a map and a nautical chart. A map is focused more on what is on the land where a nautical chart shows what is under, in, on, and around the water. Nautical charts help mariners travel safely on the water where maps are focused more on helping people travel from place to place on land. Other differences are that nautical charts are working documents. Mariners add course lines, they add turning points and way points. They are legal documents that can be used in a court. Maps can be many sizes and formats like a road atlas for example. The paper charts used on many ships are variable in size and they can be relatively large – some as big as three feet by four feet. Regulated vessels are required to keep charts and publications updated using weekly Local Notice to Mariners.

HOST: Tom, you mentioned the Local Notice to Mariners in your last response. Can you explain to us a little bit more about what this is?

TOM LOEPER: A Local Notice to Mariners are weekly corrections to nautical charts that are published by each Coast Guard District. Mariners apply the corrections to their charts on a regular basis to keep them up-to-date. New chart editions are also announced in the Local Notice to Mariners.

HOST: I think what most of us are familiar with are the little numbers we see on nautical charts. What do these numbers mean?

TOM LOEPER: The numbers you see on a nautical chart represent soundings. Soundings are water depth measurements and they tell the user how deep the water is in that particular area in either feet or fathoms. A fathom is a nautical unit of measurement. There are six feet to a fathom. On a chart, sounding data with the same values are usually connected with a line known as a depth curve, similar to the topographic lines or surface features that you see on a map.

HOST: Tom, besides the depth readings that help with safe navigation, what other information is available on a nautical chart? 

TOM LOEPER: Charts include locations of obstacles or dangers to navigation such as coral reefs, rocks, wrecks, and shoals. Other data on nautical charts includes when the document was developed, the edition of the chart, projection and scale of the chart, the compass rose as well as latitudes and longitudes. You may also find pipelines and submerged cables, lighthouses and buoys, and channels and tunnels.

HOST: Tom, it sounds like there is a lot of information available on nautical charts to show what is in, under, and around the water, again rather different than a map. How long does it take to develop a nautical chart?

TOM LOEPER: Well, Kate, the time it takes to develop a new nautical chart varies greatly and it depends on the priority of the job and the intensity of the activity in the area. For instance, if there is a need for a new nautical chart in an area that has good current survey data, it may be done in as little as six to 12 months. If you have a very remote area say the north slope of Alaska, it may take several years because of the amount of survey work that needs to be done. Another consideration is the length of the survey season. The survey season in Alaska is only a few months each year so it may take several years to collect the necessary data while the Gulf of Mexico can pretty much be surveyed any time of year.

HOST: Is it easier to update existing nautical charts?

TOM LOEPER: Kate, well yes it is, but updates to existing charts can still be time and labor intensive. A good time estimate to compile a new chart edition is three to four weeks. Ports with high shipping activity such as the Port of Long Beach or New York Harbor may be updated as frequently as two or three times per year because of the intensity of traffic and the high value of cargo while some of the charts in Alaska may only be updated once every 10-12 years.

HOST: Where does the data come from to develop nautical charts?

TOM LOEPER: Data comes from several sources to develop a nautical chart. Information on things like aids to navigations, such as light characteristics or repositioning of buoys, typically comes from local Coast Guard Districts. We may receive some updates from a Port Authority or local boating group like the Power Squadron or the U.S. Coast Guard Auxiliary. The U.S. Army Corps of Engineers provides a huge volume of data to NOAA on dredging projects in channels and waterways. NOAA also receives update information from commercial mariners and the recreational boating community. In addition, the Office of Coast Survey does survey work with private contractors and our own fleet of survey vessels which includes the Rainier and Fairweather, which are in the Pacific, and the Thomas Jefferson and Bay Hydro II in the Atlantic.

HOST: Tom, is data collection different or how do you handle updating nautical charts after severe events such as a hurricane?
TOM LOEPER: Hurricane response is a little bit different because the idea is to just get into the port and get it back into operation as soon as possible. We have six Navigation Response Teams scattered around the country and a portable system that can be mounted on any vessel like a Coast Guard or Navy boat or a tugboat. For hurricane response, the response teams work to quickly survey an area and locate obstructions for removal like sunken barges, shipping containers, and other debris that could be a danger to navigation.  During the response to Hurricane Katrina, there was also a Navy SEAL Team working directly with a response team to immediately clear obstructions that were located. If the Navy SEAL could not remove the obstruction, they marked them for heavy lift units later on.

HOST: With all of these various ways to get new and updated data, why is it that nautical charts can become outdated?

Kate, this is a question that we are often asked. The coastal waters of the U.S. are in a constant state of natural and manmade change. Some changes may include newly dredged or re-routed channels; aids to navigation are established, they’re moved, or they’re removed altogether; new wrecks and obstructions are discovered; and new berthing facilities and other structures may be built along the shoreline and in our harbors. In order for the mariner to transit safely, it is imperative that these changes be reflected on nautical charts as soon as practical.

The Office of Coast Survey maintains charts along the East, Gulf, and West Coasts, Alaska, and the Great Lakes as well as islands in the Caribbean and Western Pacific Ocean. We chart over 95,000 miles of coastline and over 3.4 million square nautical miles of U.S. waters. We have a relatively small staff of cartographers and only four survey vessels plus some contract surveyors – it’s a really, really big job!

HOST: Tom, besides the shipping, fishing, and boating communities, who else uses nautical charts?

TOM LOEPER: NOAA is focused on the needs of the commercial mariner to promote safe and efficient navigation.  Many recreational boaters use our products as well. In addition, coastal managers, city planners, scientists, environmentalists, developers, and others use nautical charts for a variety of purposes.

As far as I know, the United States is the only country in the world that provides electronic charting data for free. Because of this, many businesses take our data and incorporate it into their products and they sell them to the recreational or commercial market. They add value to our data to make it more useful to their customers – it’s really a win-win situation for everyone.

HOST: Tom, it’s great that this valuable data is available for people to use in so many ways and for free like you’ve said. What are some of the other uses of charting data besides the most important use for safe navigation?

TOM LOEPER: Kate, nautical charts show the limits of international boundaries and fishing limits, they are used for offshore mineral development and oil exploration, they are used to determine where to place mooring buoys and safe anchorage sites, they are used to determine where to deposit dredge materials called spoils, and they are used to plan building projects that extend into the water such as piers and marinas. In addition, nautical charts can support conservation and preservation efforts by determining the limits of several types of underwater preserves like national marine sanctuaries, national estuarine research reserves, and marine protected areas.

HOST: Are nautical charts only available in paper format?

TOM LOEPER: Nautical charts are available in two basic formats – paper and electronic.  Our two most popular paper products are the traditionally printed charts and print-on-demand product. The print-on-demand chart is a cooperative effort we have with a private printer. We keep our chart images in a state of continual maintenance - the printer downloads our images, they include some additional value-added information, and then they sell them through their sales agents. Commercial mariners like this product since they don’t have to update it before they use them – the charts have all the latest information included so mariners save time and money. Over half our charts are sold by print on demand.

On the electronic side, we offer Raster Navigational Charts, or RNCs, which are full-color digital images of our entire suite – basically, they are scans of our paper charts.  Electronic Navigation Charts, or ENCs, are the newest and most powerful electronic charting product we offer. Think of an ENC as an image generated from a database file.  These charts are available for free download from the Office of Coast Survey Web site and they are updated on a regular basis.

Regulated vessels are also required to have other supporting marine publications on board including tide tables and something near and dear to my heart, the Coast Pilot. The Coast Pilot is a series of nine books arranged geographically and they are a companion document to the nautical chart – it is a text supplement to the chart. The Coast Pilot is available as a hard copy book or they are available for free download in a number of formats from our Web site.

There is much more information on the Office of Coast Survey Web site about all of our products and how to download or purchase the products that I mentioned today.

HOST: Tom, can you tell me a little bit more about historic charts?

TOM LOEPER: Well, the Office of Coast Survey maintains an online historical archive of all printed versions of past nautical charts some dating back as far as the early 1800s. Right now we have over 21,000 scanned images available to the public and we are processing an additional 9,000 images which should be added to the site in the next few months. We are adding new images to the collection all the time and the hope is to have about 40,000 images available to the public in the next two or three years.

HOST: Tom, that’s great news. What is the role of the National Ocean Service in developing nautical charts?

TOM LOEPER: The mandate to create nautical charts of the nation's coasts dates back to 1807, when President Thomas Jefferson ordered a survey of our nation's coasts. The Organic Act of 1807 authorized the newly formed coastal survey agency to construct and maintain the nation's nautical charts. The agency, then known as the U.S. Coast Survey, is the oldest scientific organization in the federal government. It has been a part of the National Ocean Service since 1970, when the National Oceanic and Atmospheric Administration, or NOAA, was created.

Our mandate is to promote safe navigation in U.S. waters. The Office of Coast Survey is responsible for providing nautical charts for the nation with a suite of over 1,000 charts encompassing the coasts of the U.S., the Great Lakes, and the U.S. territories.

HOST: Thanks Tom. I think we covered a lot of information today on nautical charts and I appreciate your time and examples to help us understand the difference between a nautical chart and a map as well as more on how nautical charts are developed. Do you have any final comments for our listeners today?

TOM LOEPER: Thanks Kate for having me here today to talk a little bit about the Office of Coast Survey and nautical charts. I would like to mention to everyone that we have staff members from the Office of Coast Survey attend national and regional boat shows throughout the year to connect more with local users. We look forward to meeting some of your listeners in person at these boat shows and answering any questions they may have about safe and efficient marine commerce.

HOST: Thanks Tom for joining us on today’s episode of Diving Deeper to discuss nautical charts, how they are developed, and why they are important. To learn more about nautical charts, please visit the Office of Coast Survey Web site at

That’s all for this week’s show, please tune in on April 6th for our next episode on tides.