Sampling Location

Why is the exact location of the sampling site important?

Water quality data are not useful if the sampling location is unknown, incorrect, or mismatched. It is important to know the location of the site where samples of water or bottom sediment samples are taken for analysis. The location provides information that makes it possible for other samples to be taken at the same place at a later time, to make comparisons, and for others to find the site.

It is possible to return to the general area of a sampling station in a lake by using dead reckoning. This is done by keeping track of how fast the boat moves, the time it is moving a given speed, and the direction or directions traveled. If the wind is blowing, the waves are large, a current is present or any combination of these factors, the accuracy of knowing where the vessel is located is reduced. Returning to a given point on the lake by using this method can be only approximate.

What is latitude and longitude?

Navigational charts use latitude and longitude coordinates to mark positions. Latitude and longitude are indicated by degrees, minutes and seconds. Each degree has 60 minutes ( ' ); each minute has 60 seconds ( " ). Latitude is measured in degrees north or south relative to the equator. The equator is 0° latitude and the poles are 90° N and 90° S latitudes. Longitude is measured in degrees east and west of the Prime Meridian which is an imaginary line running from the North Pole to the South Pole through Greenwich, England.

The Vessel Data Sheets have blank spaces for latitude and longitude for each sampling station. Typical readings for W. G. Jackson trips in the Muskegon area are 43° 12' to 43° 23' N latitude and 86° 15' to 86° 24' W longitude. Sampling stations for the D. J. Angus trips in the Grand Haven area are generally between 43°02' to 43°06' N latitude and 86°10' to 86°18' W longitude.

What is a Global Positioning System?

A more recent advance has been the Global Positioning System (GPS). This is satellite navigation and positioning system that can be accessed by a relatively inexpensive GPS receiver that can be used anywhere in the world. Developed by the U.S. Department of Defense, the first satellites for the system were deployed in 1978.

GPS consists of 24 satellites orbiting the earth, five ground stations, and GPS receivers. Ground stations monitor satellites in "known" positions and triangulation is used to determine position. The distances between the GPS receiver and a satellite are determined by a timing-signal process where the signal's travel time multiplied by the speed of light equals distance. Each satellite continuously transmits a unique high frequency radio timing signal sequence or a binary code. Signal travel time is determined by the difference between the GPS receiver's internal signal generation and the arrival of the satellite's signal. Four satellite ranges are used to calculate a three-dimensional position with accuracy of 25 to 100 meters (82 to 328 feet). Differential GPS (DGPS) employs a base station that increases the accuracy to 1 to 5 meters (3 to 16 feet). Global positioning units often measure in just degrees, minutes, and fractions of minutes for latitude and longitude. These units also measure altitude.

What is RADAR?

One important piece of information that the GPS does not provide is the presence of other boats or the shore that could present danger to the boat and its occupants. Another device using radio waves provides information about objects on or above the surface of the water near the vessel. It is called RADAR. This is an acronym taken from the phrase RAdio Direction And Range.

Very high frequency radio waves can be reflected in the same way light is reflected from a mirror. RADAR sends out a short pulse of radio waves in a very narrow beam from an antenna located above the pilot house. The narrow beam rotates in a horizontal plane several times per minute. The radio signal will travel out into space unless it hits an object. If some of the radio waves hit an object, they will be reflected from the object back to the RADAR set where they are received, amplified, and then used to create a picture on the face of a cathode ray tube (CRT). The picture is a view looking down from above. It shows the location of the vessel you are on in the center of the screen. Objects from which the RADAR signal is reflected are indicated as bright spots on the CRT.

Since it takes time for the RADAR signal to leave the antenna, travel to an object, and then reflect back to the antenna, a simple relationship between time and distance is established. This is shown on the CRT with the bright spots representing other boats or the shore at a distance from the center of the screen corresponding to the actual distance between the boat you are on and objects in its vicinity. The person using the RADAR set can select various ranges or distances by pushing the range switches. Selection of a range will cause bright concentric circles to show up around the center of the CRT screen. The concentric circles are called Range Markers. Comparison between the bright spot representing an object and the concentric circles makes it possible to determine the distance between the vessel and the object. The angle between the direction the vessel is headed and the surrounding objects can also be seen on the screen.

Consequently, the name RADAR represents what the instrument can do. It uses radio waves (RA), to find the direction (D) between the vessel and an object, and the distance or range (R) to that object. Whenever the vessel is used, the RADAR is turned on and a range is selected to give the Captain a good view of objects in the vicinity of the vessel. By watching the picture on the CRT screen, the Captain can note the presence of other boats, piers, and the shoreline. He can select other ranges to look for objects close to or far from the vessel. He can navigate by comparing the location of the vessel within the banks of the river or along the shoreline. The RADAR, combined with the GPS and depth finder, allows the Captain to know where obstacles on the surface are located, where the vessel is located, and how deep the water is below the vessel keel.

What other kinds of vessels navigate the lakes?

The ports on Lake Michigan are visited or are home for a variety of commercial and government vessels. The largest ships are freighters that are designed to carry bulk cargoes such as coal, iron ore, stone, and grain. Ocean bulk freighters are about 500 to 730 feet in length and have one main cabin area. Lake bulk freighters or straight-deck bulk carriers are 600 to 800 feet in length with cabin areas at the bow and stern. Self-unloader freighters are the largest vessels ranging up to 1,000 feet long. Other vessels include Coast Guard cutters with distinctive bows that can cut ice, dredges that are barges with cranes, harbor tugs, and fish tugs.

Vessel fleets can be identified by the flag flown at the stern of the ship, port of registry painted across the stern, the hull colors, and smoke stack markings. For instance, the stack insignia of the U.S. Army Corps of Engineers has a silver castle on a red field bounded by narrow silver bands on a black stack. Ninth District Coast Guard boats have orange and black stacks with the Coast Guard insignia. There are over 800 ships from 60 countries that visit the Great Lakes.

How does the weather affect navigation?

Especially on Lake Michigan, there are often small craft warnings when small vessels should stay off the "Big Lake". The greater the wind velocity, the higher the waves. Wave height is directly related to the amount of energy possessed by the wave, which depends on the wind speed, length of time the wind blows over water, and the fetch or distance over deep water that the wind blows. Wavelength is the distance between successive waves. If the steepness of a wave reaches a ratio of the wavelength divided by 7 (L/7), the wave becomes unstable resulting in whitecaps.

Wind speed and direction can be determined by use of an anemometer. The anemometer is an apparatus with four cups on a rotating axis that turn in response to the wind. A cable leads to a recorder that measures the wind speed. A wind vane indicates the direction of the wind. The arrow on a wine vane points into the wind and shows the direction from which the wind is blowing. For instance, northwest winds (onshore breeze) often bring in storms across Lake Michigan and east winds (offshore breeze) can move near-shore water to offshore locations. Sky observations are clues to weather.

The three basic types of clouds are cirrus, cumulus, and stratus. Cirrus clouds are wispy high clouds composed of ice crystals. Towering, billowing, or puffy cumulus clouds are found in both fair weather and approaching thunderstorms. Stratus clouds are low layers of thick, gray clouds typical of overcast days. They are often associated with rainy weather that lasts all day.

There is a weather station at the AWRI Muskegon shoreline facility. The sensor channels at the weather stations gather information about wind direction, wind speed, barometric pressure, relative humidity, temperature, and rainfall. Interesting comparisons can be made between conditions onshore and out in the lake. Real-time weather information for the Muskegon area can be found at the NOAA/GLERL website.