2.1: Spatial Data in the Oceans- Bathymetry, Latitude, and Longitude

People have been navigating the oceans for thousands of years, for exploration, travel, acquiring food, and transporting goods. To do so requires some form of map and the ability to tell direction. Therefore, various systems of navigation have been around for centuries.

Early Pacific Islanders used stick charts, where shells indicated islands, and bent sticks represented wave and current patterns around the islands (Figure \(\PageIndex{2}\)).

 

Figure \(\PageIndex{1}\): A Micronesian navigational chart from the Marshall Islands, made of wood, sennit fiber and cowrie shells (By Cullen328 – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/inde...curid=46844500).

 

The first Western civilization known to have developed navigation at sea were the Phoenicians, about 4,000 years ago (c. 2000 B.C.E.). Phoenician sailors accomplished navigation by using primitive charts and observations of the sun and stars to determine directions. They explored the Mediterranean and Red Seas, and even circumnavigated Africa in 590 B.C.E.

Ptolemy was a Greek-Egyptian writer, mathematician and scientist living in Alexandria, Egypt. He produced maps of the known world in around 150 AD, including the locations of major cities, and the first known use of lines of latitude and longitude (Figure \(\PageIndex{2}\)). This early coordinate system forms the basis of geolocation techniques still in use today.

 

Figure \(\PageIndex{2}\): A 15th century world map based on Ptolemy’s Geography (Public domain via Wikimedia Commons).

 

Before we learn more about the ocean's features, and the processes that shape them, let's go over a few key terms used to describe the ocean in both horizontal and vertical space. 

 

Latitude and Longitude

We use our cell phones or our car’s navigation system for directions to an address, but how does the phone know its location? What if we are out on the ocean where there are no roads or street signs, how do oceanographers know where we are on the vast blue ocean with no land in sight? Global positioning systems, otherwise known as GPS, are used to find locations on Earth from satellites. Multiple satellites are used to triangulate our position using a grid system overlaid onto the surface of the Earth called Latitude and Longitude.

Any point on Earth can be defined by the intersection of its lines of latitude and longitude. Latitude is measured as the angle from the equator, to the Earth’s center, to your position on the Earth’s surface (Figure \(\PageIndex{3}\)). It is expressed as degrees north or south of the equator (0^o), with the poles at a latitude of 90^o. Thus the poles are referred to as high latitude, while the equatorial region is considered low latitude. Lines of equal latitude are always the same distance apart, and so they are called parallels of latitude; they never converge. However, the parallels of latitude do get shorter as they approach the poles.

 

Figure \(\PageIndex{3}\) The latitude of a point on the Earth’s surface is determined by the angle (ø) between the point and the equator, passing through Earth’s center (Peter Mercator [Public domain], via Wikimedia Commons).

One degree of latitude is divided into 60 minutes (‘). One minute of latitude equals one nautical mile, which is equal to 1.15 land miles (1.85 km). Each minute of latitude is further divided into 60 seconds (“). So traditionally, positions have been expressed as degrees/minutes/seconds, e.g. 36^o 15′ 32″ N. However, with modern digital technology, positions are increasingly expressed as decimals, such as 36^o 15.25 N’, or 36.2597^o N.

Longitude measures the distance east or west of an imaginary reference point, the prime meridian (0^o), which is now defined as the line passing through Greenwich, England (although throughout history the prime meridian has also been located in Rome, Copenhagen, Paris, Philadelphia, the Canary Islands, and Jerusalem; unlike the equator, the prime meridian’s location is fairly arbitrary). Your longitude represents the angle east or west between your location, the center of the Earth, and the prime meridian (Figure \(\PageIndex{4}\)).

 

Figure \(\PageIndex{4}\) Longitude is determined as the angle (λ) between the prime meridian and your position (Peter Mercator [Public domain], via Wikimedia Commons).

As you move east and west from the prime meridian, eventually you reach 180^o E and W on the opposite side of the globe from Greenwich. This point is the International Date Line. Lines of longitude are called meridians of longitude, or great circles. All circles of longitude are the same length, and are not parallel like lines of latitude; they converge as they near the poles. Therefore, while one minute of latitude always equals one nautical mile, the length of one minute of longitude will decline from the equator to to poles, where it will ultimately decline to zero.

Using the figure below, click on the ? to help review some of these terms in the context of a figure.

 

Today we use GPS (Global Positioning System) technology to determine latitude and longitude, and even the smallest smart phones and smart watches can use GPS to calculate position. GPS works through a system of orbiting satellites that constantly emit signals containing the time and their position. A GPS receiver receives these signals from multiple satellites, and triangulates the signals to calculate position. The system needs 24 satellites to be functional at one time; as of 2015, the system consisted of about 32 operational satellites, able to give a position with an accuracy of 9 meters (30 feet) or less.

 

Review your knowledge by dragging and dropping in the interactive figure below.

 

Ocean Bathymetry 

Now we know a bit about how to characterize where we are in the ocean, but we've only scratched the surface (quite literally!) Did you know that the ocean contains the world's tallest mountain (Mauna Kea)? And, at its deepest depth, the ocean is deeper than Mt. Everest is tall - if you were to place Mt. Everest at the bottom of the Mariana Trench, the mountain's peak would still be over a mile under the ocean's surface. In the ocean, we call differences in depth bathymetry. See the figure below to understand the difference between topography and bathymetry (Figure \(\PageIndex{5}\)).

Figure \(\PageIndex{5}\)) Topography is used to describe land elevation, while bathymetry is used to describe ocean depth. Topography and Bathymetry by Jono Hey, Sketchplanations is licensed under CC-BY-NC 4.0.

 

To map the ocean floor we need to know the depth at a number of places. These measurements were first made through soundings, where a weighted line (lead line) was let out by hand until it touched the bottom, and the depth could be recorded from the length of the line (Figure \(\PageIndex{6}\)). This technique led to the fathom as a unit of depth; as sailors hauled in the sounding line they would stretch it out to cover their arm span. The average arm span of a sailor was about six feet, so one fathom equals six feet, and the sailors could simply count the number of “arm spans” as they pulled in the line.

 

Figure \(\PageIndex{6}\)) Lead line survey from a catamaran hull in Alaska, 1942 (http://celebrating200years.noaa.gov/...hy/image7.html).

 

This technique had a number of drawbacks, and was usually limited to shallower water. It was very time consuming, and only gave depth data for a single point, so many individual soundings were needed to map an area. It could also be error-prone; in deep water it could be difficult to determine when the weight hit the bottom as the weight of the line itself could cause the line to keep sinking, and currents could deflect the line away from vertical, thus overestimating the depth. In later years, winches and heavy steel cables were used for deeper water, but this did not solve all of the problems inherent in the sounding method, and also added the constraint of excessive weight of the equipment.

In the 19th century, a number of modifications were made to this simple design. In 1802 the British clockmaker Edward Massey invented a mechanical device that was attached to the sounding line; as the device sank, a rotor turned a dial which locked in place when the line hit bottom (Figure \(\PageIndex{7}\)). The line could then be reeled in and the depth read from the dial. In 1853 American sailor John Mercer Brooke developed a cannonball weight attached to a twine reel. The cannonball was dropped over the side and allowed to free-fall to the bottom; by timing the fall rate (the rate at which the twine unspooled) and noting when the rate changed as the cannonball hit the bottom, the water depth could be calculated. When it hit bottom, the cannonball was released and the line could be hauled back in, bringing with it a sample of mud in the iron bar that held the cannonball, thus confirming that the bottom had been reached.

 

Figure \(\PageIndex{7}\) Massey’s sounding machine (Public domain, via Wikimedia Commons).

 

After the Titanic disaster in 1912, there was an effort to develop better methods of detecting icebergs from a ship. This led to the development of sonar (SOund Navigation And Ranging) technology, which was soon applied to mapping bathymetry. A sonar device called an echosounder sends out a pulse of sound, then listens for the returning echo. The timing of the returning echo is used to calculate depth. We know that the speed of sound in water is approximately 1500 m/s. Since the returning echo traveled to the bottom and back, the water depth corresponds to half the time it takes for an echo to return, multiplied by the speed of sound in water (Figure \(\PageIndex{8}\)):

depth=1/2*(two-way travel time)*(speed of sound in water)

 

Figure \(\PageIndex{8}\) Measuring depth using an echosounder (Public domain via Wikimedia Commons).

 

Echosounders allowed a fast, continuous record of bathymetry under a moving ship. However, they only give the depth directly under a ship’s path. Today, high resolution seafloor maps are made through multibeam or side scan sonar, either from a ship or from a towed transmitter (Figure \(\PageIndex{9}\)). Multibeam sonar produces a fan-shaped acoustic field allowing a much a wider area (>10 km wide) to be mapped simultaneously.

 

Figure \(\PageIndex{9}\) Multibeam sonar (NOAA).

 

Large-scale mapping of the ocean floor is also carried out by satellites (originally SEASAT, then GEOSAT, now the Jason satellites) which use radio waves to measure the height of the sea surface (radar altimetry). The sea surface is not flat; gravity causes it to be slightly higher over elevated features on the ocean floor, and slightly lower over trenches and other depressions. Satellites send out radio waves, and similar to an echosounder, can use the returning waves to detect differences in sea surface height down to 3-6 cm (Figure \(\PageIndex{10}\)). These differences in sea surface heights allow us to determine the topography under the surface. Unlike the old lead line technology, where hundreds of soundings were necessary to map a small area, each taking an hour or more to complete, the current satellites can map over 90% of Earth’s ice-free sea surface every 10 days!

 

Figure \(\PageIndex{10}\) Radar altimetry (left) and a map of the seafloor produced by radar altimetry satellites (right) (NOAA).

---