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Diving Deeper Shorts: Episode 8 (January 27, 2011) - Nautical Charts

HOST: Today on Diving Deeper Shorts, we revisit our previous interview on nautical charts with Tom Loeper from NOAA’s Office of Coast Survey.  

Let’s listen in.

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.

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: 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.

That’s all for today’s Diving Deeper Shorts, where we highlight a few minutes of your favorite Diving Deeper episodes.

Want to learn more? Go to and select the March 2009 podcast archive to listen to the full interview with Tom Loeper on nautical charts.

You can catch the next episode of Diving Deeper on February 10.